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Created page with "Category:Python_ops435Category:rchan = <font color='red'>Still under Construction - Please do not use</font> = = LAB OBJECTIVES = :In this lab, you will learn how to i..."
[[Category:Python_ops435]][[Category:rchan]]
= <font color='red'>Still under Construction - Please do not use</font> =
= LAB OBJECTIVES =
:In this lab, you will learn how to identify and create different types of '''functions''', study the properties of '''list objects''' and learn how to manipulate them using their built-in methods, and finally we will explore how '''loop''' can be used to reduce code repetition.
:'''<u>Objectives</u>'''
:Write Python code in order to:
:*'''Create and use built-in and user created functions''' - using the '''def''' and '''import''' keywords
:*'''Obtaining properties and manipulating list object''' - using the built-in function: len(), min(), max(), and list object methods: copy(), sort(), append(), and remove()
:*'''Looping through lists'''. Looping (iteration) is the method of using loop to process items in a list object one at a time using the same code to reduce code repetition in your Python script. This will result in a better, more efficient and effective program execution.
<br><br>
= INVESTIGATION 1: CREATING THE SIMPLE FUNCTIONS =
:"In the context of computer programming, a '''function''' is a named sequence of statements that perform a computation" -- Think Python 2nd Edition by Allen Donway.
:Usually, a '''function''' contains '''programming code''' and most likely placed near the top of a python file, and before the main processing section.
: When a Python program is run, the '''function's code is read into the corresponding process memory''', waiting to be executed when the function is called. Until a Function is specifically called, its code will sit still in the memeory.
:When creating programs that define and use functions, '''a large programming task can be broken-down into smaller elements''' (or '''modules'''). This is why creating programs that use functions is referred to as '''"modular programming"'''.
== Four type of Functions ==
:There are four major type of functions based on the arguments they will take and the value (or stuff) they will return:
:# function that does not take any function argument or return a value
:# function that does not take any function argument but do return a value
:# function that takes function argument(s) but does not return a value
:# function that takes function argument((s) and also return value(s)
== PART 1 - How User-Defined Functions are Declared and Run ==
:Functions may be designed:
* '''not to accept arguments or return a value''',
* to '''not accept arguments but return a value''',
* to '''accept arguments and not return a value''',
* or to '''both accept arguments and return a value'''.
In this investigation, we will focus on creating functions that either do NOT return a value, or return a value.
'''Functions and Strings'''
: You will now learn how to define and run functions that will return '''string data''' when a function is called.
:'''Perform the Following Steps:'''
:#Create a new python file for testing code in this section.
:#Whenever you want to create a function, you must start with the keyword "'''def'''". The '''def''' keyword is used to start the definition of the function, it does not run the code you write. Functions, just like '''if''' statements, must have all code under them indented.<source lang="python">
def hello():
print('Hello World')
print('Inside a Function')
</source>
:#Executing your file you should have noticed that nothing happened. Well actually, something did happen... the function called '''hello(''') has been defined and stored in internal memory in order for it to run when called by its function name. Now that our function was created, we can use it over and over.
:#To execute the code inside the function, run the function name with "'''()'''" '''brackets''' at the end of the function name.<br>Try running the '''hello()''' function by name three times like this:<source lang="python">
hello()
hello()
hello()
</source>You should notice that the function just does the same thing over-and-over no matter how many times your call the function by name. By the way, that is OK. On the other hand, you may want to create and use a function to do something, like perform error checking or some other task that returns a value to the '''main''' program for further processing. For example, a '''true''' or '''false''' value if the error checking function that was called was detected no errors or detected an error. But let's stick to some simple examples first, before tackling more complex use of functions.<br><br>
:#Let's create a function that '''returns''' some data after the function is called. This function does not print out any text: instead; it creates new variables and at the end returns the value of one of the variables.<source lang="python">
def return_text_value():
name = 'Terry'
greeting = 'Good Morning ' + name
return greeting
</source>
:# Call the function like this:<source lang="python">
return_text_value()
</source>One major difference between a function '''returning a value''' and simply '''printing a value''' is that '''returned''' values can be caught and stored in variables used in the program (that called the function) for later use. Once the returned value has been stored, it can be printed, manipulated, compared in IF statements, etc. Below will cover how to store a returned value.<br><br>
:#Notice that this syntax looks just the call to the input() function which you've used in the last lab:<source lang="python">
text = return_text_value()
</source>
:#Now the returned text from the function has been stored in the variable "'''text'''". It can be used like any string value now.<source lang="python">
print(text)
</source>
'''Functions and Numbers (Integers)'''
: You will now learn how to define and run functions that will return '''integer data''' when a function is called. In this section, you will define a function that will be returning integer values instead of text. There is not a big difference, but when returning number values, care needs to be taken if you try combining it with a string!
:'''Perform the Following steps:'''
:#Define the return_number_value() function:<source lang="python">
def return_number_value():
num1 = 10
num2 = 5
num3 = num1 + num2
return num3
</source>
:#And call it:<source lang="python">
number = return_number_value()
print(number)
print(number + 5)
print(return_number_value() + 10)
</source> What do you notice?<br><br>
:#Now, display both strings and numbers:<source lang="python">
number = return_number_value()
print('my number is ' + number)
</source> What do you notice? You should notice a warning message. This occurs because the returning value is a '''number''' and NOT a '''string'''! Combining numbers and strings in a statement (such as '''print()''') can cause errors. The error message should appear similar to the one displayed below: <source>
Traceback (most recent call last):
File "test.py", line 2, in <module>
print('my number is ' + number)
TypeError: cannot concatenate 'str' and 'int' objects
</source>
:#If a number needs to be combined with a string, use the '''str()''' predefined function that was discussed in a previous lab in order to convert the returned number into a string:<source lang="python">
number = return_number_value()
print('my number is ', number)
print('my number is ' + str(number))
print('my number is ' + str(return_number_value()))
</source>
== PART 2 - Creating a Python Script with Functions and Importing Functions ==
'''Creating a Python Script'''
:Now it's time to create a Python script that uses two functions. One function returns a string value to greet the user, where the other function returns the result of adding two values (stored in variables within the function).
:'''Perform the following Instructions:'''
:#Create a new script '''~/ops435/lab3/lab3a.py'''
:::'''Input / Output Requirements'''
:::*The script should have a '''Shebang line'''
:::*Below the '''Shebang line''', add an '''empty line''' followed by a comment stating: '''# return_text_value() function'''
:::*Add an '''empty line''' followed by the '''return_text_value()''' function '''definition''' that you previously entered in the shell.
:::*Add another '''empty line''' followed by a comment stating: '''# return_number_value() function'''
:::*Add another '''empty line''' following by the '''return_number_value()''' function '''definition''' that you previously entered in the shell.
:::*Add a '''couple of empty lines''', following by a comment stating: '''# Main Program'''
:::*Add another '''couple of empty lines''', followed by the statements displayed below:<source lang="python">
if __name__ == '__main__':
print('python code')
text = return_text_value()
print(text)
number = return_number_value()
print(str(number))
</source>
:Running your program you should have seen three lines being displayed: the text "python code", a greeting, and a result of a math calculation. The '''if''' statement in the code above is a special '''if''' statement needed to make sure that your "main" code only runs when you want it to. More on that later.
'''Importing Functions From other Python Scripts'''
In order to use functions from other scripts, you use the '''import''' statement.<br><br>
:# Let's see what happens if we forget to import functions from your lab3a.py script prior to calling a function. Create a new python file and try to call the return_text_value() function:<source lang="python">
text = lab3a.return_text_value()
</source>You should notice an error indicating '''"name 'lab3a' is not defined"'''. This error occurs since you failed to instruct python to '''import''' or "load existing defined functions from your lab3a.py script" to '''internal memory'''.<br><br>
:# Modify your program like this:<source lang="python">
import lab3a
text = lab3a.return_text_value()
print(text)
lab3a.return_number_value()
</source> You should notice that all of the function calls should now work.
:# Download the checking script and check your work. Enter the following commands from the bash shell.<source lang="bash">
cd ~/ops435/lab3/
pwd #confirm that you are in the right directory
ls CheckLab3.py || wget https://raw.githubusercontent.com/Seneca-CDOT/ops435/master/LabCheckScripts/CheckLab3.py
python3 ./CheckLab3.py -f -v lab3a
</source>
:# Before proceeding, make certain that you identify any and all errors in lab3a.py. When the checking script tells you everything is OK before proceeding to the next step.
<br><br>
= INVESTIGATION 2: CREATING FUNCTIONS WITH ARGUMENTS AND RETURN VALUES =
== PART 1 - Providing Functions With Arguments ==
:Functions can receive '''arguments''' - data to be used for processing. In this section, you will learn how to define functions that accept arguments and learn how to call functions with arguments (such as mathematical operations or testing conditions, which is useful for error-checking).
'''Passing up Single and Multiple Arguments to a Function'''
:'''Perform the Following Steps:'''
:#Create a new Python file for testing.
:#When passing arguments to functions, you put data such as '''strings''', '''numbers''', or '''variable names''' within brackets immediately following the function name.<br><br>'''NOTE:''' If a function accepts arguments, then those arguments must be declared (using variable names) when the function is declared. Those declared variable names are then used within the function for processing. Also, when you call a function with arguments, the number of arguments passed up to the function must match the number of arguments that were specified in the function declaration.<br><br>
:#Define a function called '''square()''':<source lang="python">
def square(number):
return number ** 2
</source>'''FYI:'''You may have learned that you multiple a number by itself in order to "square" the number. In python, the '''**''' operator will raise the operand on the left to the power of the operand on the right.<br><br>When calling functions with multiple arguments, the arguments are separated by '''commas'''. See what happens if you provide strings, strings without using quotes, or numbers with decimals in the following examples.
:#Test your '''square()''' function:<source lang="python">
square(5)
square(10)
square(12)
square(square(2))
square('2')
</source>Notice that nothing is printed, you need to print the values the functions return to see what they are.
:#The last function call should produce an '''error message'''. This is caused by sending a '''string''' instead of a number that is processed by the function. We could use the int() function to convert any value passed in as a string by mistake to an integer number.<br><br>
:#Declare the function '''sum_numbers()''':<source lang="python">
def sum_numbers(number1, number2):
return int(number1) + int(number2)
</source>
:#Call that function to see what happens:<source lang="python">
sum_numbers(5, 10)
sum_numbers(50, 100)
</source>
:#You can also do what looks like calling a function within another function, but it's actually just calling sum_numbers() first, then calling square() with the return from sum_numbers as an argument:<source lang="python">
square(sum_numbers(5, 5))
</source>'''NOTE:''' Running functions with multiple arguments is the same. When call a function as an argument of another function, the '''inner-most function will run first''', and the return the value from that will be used as the argument for the '''outer function'''. In the example below, '''sum_numbers(5, 5)''' will return '''10''', thus providing that result to be square with that value '''square(10)'''.<br><br>
'''Practice Creating a Function that Accepts Arguments and Returns a Value'''
:It is time to practice creating a shell script that uses a function that accepts arguments, and returns a value.
:'''Perform the Following Instructions:'''
:#Create a new script '''~/ops435/lab3/lab3b.py'''. Refer to the '''Python Script template''' and the '''Additional Requirements''' sections when creating your Python script. Refer to Sample Run and Sample Imports displayed below for exact prompt and output
:::'''Python Script Template'''
:::<source lang="python">#!/usr/bin/env python3
def sum_numbers(number1, number2):
# Make this function add number1 and number2 and return the value
def subtract_numbers(number1, number2):
# Make this function subtract number1 and number2 and return the value
# Remember to make sure the function accepts 2 arguments
def multiply_numbers(number1, number2):
# Make this function multiply number1 and number2 and return the value
# Remember to make sure the function accepts 2 arguments
if __name__ == '__main__':
print(sum_numbers(10, 5))
print(subtract_numbers(10, 5))
print(multiply_numbers(10, 5))
</source>
::'''Additional Requirements'''
::*All functions should accept two arguments
::*All functions should return an integer
::*The script should contain no errors
::'''Sample Run:'''<source lang="python">
./lab3b.py
15
5
50
</source>
::'''Other examples:'''<source lang="python">
import lab3b
lab3b.sum_numbers(10, 5)
# Will return 15
lab3b.sum_numbers(25, 25)
# Will return 50
lab3b.subtract_numbers(10, 5)
# Will return 5
lab3b.subtract_numbers(5, 10)
# Will return -5
lab3b.multiply_numbers(10, 5)
# Will return 50
lab3b.multiply_numbers(10, 2)
# Will return 20
</source>
::2. Download the checking script and check your work. Enter the following commands from the bash shell.<source lang="bash">
cd ~/ops435/lab3/
pwd #confirm that you are in the right directory
ls CheckLab3.py || wget https://raw.githubusercontent.com/Seneca-CDOT/ops435/master/LabCheckScripts/CheckLab3.py
python3 ./CheckLab3.py -f -v lab3b
</source>
::3. Before proceeding, make certain that you identify any and all errors in lab3b.py. When the checking script tells you everything is OK - proceed to the next step.
'''Passing up Multiple Arguments and Using Conditional Statements'''
:You will now create a more complex function that will not only pass arguments, but also include logic to control the flow of the function, and affect how your Python script will be run. You will create a function that uses an '''if/elif/else''' statement.
:'''Perform the Following Steps:'''
:#Use a temporary Python file to define the following function:<source lang="python">
def describe_temperature(temp):
if temp > 30:
return 'hot'
elif temp < 0:
return 'cold'
elif temp == 20:
return 'perfect'
return 'ok'
</source>The final '''return "ok"''' will only take place if a previous return has not taken place before it. Once return has been used in a function, the function immediately exits and returns the value.
:#Call describe_temperature like this to confirm the results:<source>
print(describe_temperature(50))
# Will return 'hot'
print(describe_temperature(20))
# Will return 'perfect'
print(describe_temperature(-50))
# Will return 'cold'
print(describe_temperature(25))
# Will return 'ok'
print(describe_temperature(10))
# Will return 'ok'
</source>
'''Create a Python Script Receiving Multiple Arguments'''
:'''Perform the Following Instructions:'''
:#Create the '''~/ops435/lab3/lab3c.py''' script. The purpose of the script is to have a single function that can perform addition, subtraction, or multiplication on a pair of numbers. But the function will allow us to choose exatly what operation we are performing on it when we call it. If the operate function does NOT understand the operator given, it should return an error message (e.g. calling the function to 'divide' two numbers).
:#Use this template to get started:<source lang="python">
#!/usr/bin/env python3
def operate(number1, number2, operator):
# Place logic in this function
if __name__ == '__main__':
print(operate(10, 5, 'add'))
print(operate(10, 5, 'subtract'))
print(operate(10, 5, 'multiply'))
print(operate(10, 5, 'divide'))
</source>
:::*The operate() function should use '''conditional''' statements<br> '''FYI:''' Remember that you MUST consistently '''indent ALL code''' for within each section (or test).
:::*The operate() function should accept '''three arguments'''.
:::*The operate() function should '''return''' the result.
:::*The operate() function should '''return''' an error message if the operation is unknown<br> '''FYI:''' Use single quotes or double-quotes to pass a string value.
:::*The script should contain show the exact output as the sample.
:::*The script should contain no errors.
:::*As an extra exercise, try to write your function with only one return statement.
:::'''Sample Run 1:'''<source>
./lab3c.py
15
5
50
Error: function operator can be "add", "subtract", or "multiply"
</source>
:::'''Sample Run 2 (using import from another Python file):'''<source>
import lab3c
lab3c.operate(10, 20, 'add')
# Will return 30
lab3c.operate(2, 3, 'add')
# Will return 5
lab3c.operate(100, 5, 'subtract')
# Will return 95
lab3c.operate(10, 20, 'subtract')
# Will return -10
lab3c.operate(5, 5, 'multiply')
# Will return 25
lab3c.operate(10, 100, 'multiply')
# Will return 1000
lab3c.operate(100, 5, 'divide')
# Will return Error: function operator can be "add", "subtract", or "multiply"
lab3c.operate(100, 5, 'power')
# Will return Error: function operator can be "add", "subtract", or "multiply"
</source>
:::3. Download the checking script and check your work. Enter the following commands from the bash shell.<source lang="bash">
cd ~/ops435/lab3/
pwd #confirm that you are in the right directory
ls CheckLab3.py || wget https://raw.githubusercontent.com/Seneca-CDOT/ops435/master/LabCheckScripts/CheckLab3.py
python3 ./CheckLab3.py -f -v lab3c
</source>
:::4. Before proceeding, make certain that you identify any and all errors in lab3c.py. When the checking script tells you everything is OK - proceed to the next step.
<br><br>
== PART 2 - Running System Commands with Subprocess ==
:The remainder of this investigation will allow you to run operating system commands via your Python script. Although there are different ways in which to issue operating system commands, you will learn two of them.
'''Perform the Following Steps:'''
:#Create a new python file for testing.
:#Import the '''''os''''' module in your python file.
:#You can issue operating system commands by using the '''system()''' function. Try it:<source lang="python">
os.system('ls')
os.system('whoami')
os.system('ifconfig')
</source>Notice that the output from the programs is printed in your script. Consider that may not always be what you want.<br><br>
:#Try this also:<source lang="python">
os.system('ipconfig')
</source>You should notice an error message: ''''ipconfig: command not found''''. That error occurs since that command was an MS Windows command, and our current platform is Linux.<br><br>It is not always a good idea to run system commands in Python, this makes your Python code less portable and makes it require a specific operating system or a system that has those commands available. You should think about that when you decide whether you should or should not use a system command to accomplish some task or stick to pure Python code only.<br><br>As you may recall from lab2, you issued '''import sys''' to import special variables from the system. You can import a subprocess in order to run common non OS specific commands securely.<br><br>
:#Import the subprocess module in your python file.
:#There are many features available as part of the subprocess module, we are interested in "'''Popen'''". This method subprocess.Popen() can be used to run system commands as a child process to the Python script. The code below output will create a new child process, in Python we can control this through the new Python object we just created, "'''p'''". "'''p'''" now has a collection of methods(functions that are apart of a object) available.<br><br>
:#To demonstrate, issue the following:<source lang="python">
p = subprocess.Popen(['date'], stdout=subprocess.PIPE, stdin=subprocess.PIPE, stderr=subprocess.PIPE, shell=True)
</source>This function call and the following step is full of details we haven't yet talked about which is why it may look a little scary. By the time we're finished with the course - you will be able to look at code like this and not be intimidated. If you're curious and want to look ahead - you can find the definition for the [https://docs.python.org/3/library/subprocess.html#subprocess.Popen Popen function in the Python reference manual].
:#This next step is going to communicate with the process and get the retrieve it's output (stdout).<source>
output = p.communicate()
print(output)
print(output[0])
# The above stdout is stored in bytes
# Convert stdout to a string and strip off the newline characters
stdout = output[0].decode('utf-8').strip()
print(stdout)
</source>
:# Sometimes you will be able to use purely python code to get your job done, but often you will need to call existing system commands. It's important to learn how to call them and how to interact with those external processes.
'''Practice Running System Commands From Python'''
:'''Perform the Following Instructions:'''
<ol>
<li>Create the "'''~/ops435/lab3/lab3d.py'''" script. The purpose of this script is to create a Python function that can return the linux system's root directory free space.
:*The script should '''import the correct module'''
:*The script should use the linux command: '''<nowiki>df -h | grep '/$' | awk '{print $4}'</nowiki>'''
:*The script should contain the function called: '''free_space()'''
:*The function '''free_space()''' should return a string which is in '''utf-8''' and has '''newline characters strip'''
:*'''Note:''' your output may be completely different, the free/available disk space on every computers root directory may be different.
:'''Sample Run 1:''' <source>
./lab3d.py
9.6G
</source>
:'''Sample Run 2 (using import from another Python file):'''<source>
import lab3d
lab3d.free_space()
# Will return 9.6G
</source></li>
<li>Download the checking script and check your work. Enter the following commands from the bash shell.<source lang="bash">
cd ~/ops435/lab3/
pwd #confirm that you are in the right directory
ls CheckLab3.py || wget https://raw.githubusercontent.com/Seneca-CDOT/ops435/master/LabCheckScripts/CheckLab3.py
python3 ./CheckLab3.py -f -v lab3d
</source></li>
<li>Before proceeding, make certain that you identify any and all errors in lab3d.py. When the checking script tells you everything is OK - proceed to the next step.</li>
</ol>
= INVESTIGATION 3: USING LISTS =
:'''Lists''' are one of the most powerful '''data-types''' in Python. A list is a series of '''comma separated values found between square brackets'''. Values in a list can be anything: '''strings''', '''integers''', '''objects''', even '''other lists'''. In this section, you will introduce lists and how to use them effectively, you will further user lists in later labs. It is important to realise that although lists may appear very similar to arrays in other languages, they are different in a number of aspects including the fact that they don't have a fixed size.
== PART 1 - Navigating Items in Lists ==
:'''Perform the Following Steps'''
:#Create a new Python file for testing things in this section.
:#Create a few lists with different values: list1 contains only '''integers''', list2 contains only '''strings''', list3 contains a combination of both '''integers and strings'''
:#<source lang="python">
list1 = [ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]
list2 = [ 'uli101', 'ops235', 'ops335', 'ops435', 'ops535', 'ops635' ]
list3 = [ 'uli101', 1, 'ops235', 2, 'ops335', 3, 'ops435', 4, 'ops535', 5, 'ops635', 6 ]
</source>The best way to access individual '''elements''' in a list is using the list '''index'''.<br>The index is a number starting from 0 to ('''number_of_items - 1'''), the list index starts counting at '''0'''.<br><br>
:#Inspect specified elements in your lists:<source lang="python">
print(list1[0]) # First element in list1
print(list2[1]) # Second element in list2
print(list3[-1]) # Last element in list3
</source>
:#You can also retrieve ranges of items from a list (these are called slices): <source lang="python">
print(list1[0:5]) # Starting with index 0 and stopping before index 5
print(list2[2:4]) # Starting with index 2 and stopping before index 4
print(list3[3:]) # Starting with index 3 and going to the end
</source>
'''Practice Using Functions and Using the List Index'''
:'''Perform the Following Instructions'''
:#Create a Python script called: '''~/ops435/lab3/lab3e.py'''<br>The purpose of this script is to have a number of functions that output a different data storage in various elements of a list. Each function will return either a single item from the list OR will create a new list and return the entire new list.<br><br>
:#The template function names and the special if statement:<source lang="python">
#!/usr/bin/env python3
# Create the list called "my_list" here, not within any function defined below.
# That makes it a global variable. We'll talk about that in another lab.
def give_list():
# Does not accept any arguments
# Returns all of the global variable my_list unchanged
def give_first_item():
# Does not accept any arguments
# Returns a single string that is the first item in the global my_list
def give_first_and_last_item():
# Does not accept any arguments
# Returns a list that includes the first and last items in the global my_list
def give_second_and_third_item():
# Does not accept any arguments
# Returns a list that includes the second and third items in the global my_list
if __name__ == '__main__': # This section also referred to as a "main code"
print(give_list())
print(give_first_item())
print(give_first_and_last_item())
print(give_second_and_third_item())
</source>
:::'''Additional Requirements'''
:::*The script should declare a list called '''my_list''' created BEFORE any function definition
:::*The list called '''my_list''' should have the values: '''100''', '''200''', '''300''', and ''''six hundred''''
:::*The script should '''implement''' the empty functions - i.e. you have to fill in the bodies for these functions
:::'''Sample Run 1:'''<source>
./ lab3e.py
[100, 200, 300, 'six hundred']
100
[100, 'six hundred']
[200, 300]
</source>
:::'''Sample Run 2 (with import from another script):'''<source>
import lab3e
lab3e.give_list()
# Will print [100, 200, 300, 'six hundred']
lab3e.give_first_item()
# Will print 100
lab3e.give_first_and_last_item()
# Will print [100, 'six hundred']
lab3e.give_second_and_third_item()
# Will print [200, 300]
</source>
:::3. Download the checking script and check your work. Enter the following commands from the bash shell.<source>
cd ~/ops435/lab3/
pwd #confirm that you are in the right directory
ls CheckLab3.py || wget https://raw.githubusercontent.com/Seneca-CDOT/ops435/master/LabCheckScripts/CheckLab3.py
python3 ./CheckLab3.py -f -v lab3e
</source>
:::4. Before proceeding, make certain that you identify any and all errors in lab3e.py. When the checking script tells you everything is OK - proceed to the next step.
== PART 2 - Manipulating Items in Lists ==
:There are a number of ways to obtain information about lists as well as change the data that is contained within a list. In this section, you will learn how to manipulate lists.
:'''Perform the Following Steps:'''
:#Let's perform a simple change to a list element. Try the following code:<source lang="python">
courses = [ 'uli101', 'ops235', 'ops335', 'ops435', 'ops535', 'ops635' ]
print(courses[0])
courses[0] = 'eac150'
print(courses[0])
print(courses)
</source>
:#Below are some examples of using built-in functions to '''manipulate''' lists. Take your time to see how each function can be a useful tool for making changes to existing lists:<source lang="python">
courses.append('ops235') # Add a new item to the end of the list
print(courses)
courses.insert(0, 'hwd101') # Add a new item to the specified index location
print(courses)
courses.remove('ops335') # Remove first occurrence of value
print(courses)
sorted_courses = courses.copy() # Create a copy of the courses list
sorted_courses.sort() # Sort the new list
print(courses)
print(sorted_courses)
</source>
:#In addition to using functions to manipulate lists, there are functions that are useful to provide '''information''' regarding the list such as number of elements in a list, the smallest value and largest value in a list:<source lang="python">
list_of_numbers = [ 1, 5, 2, 6, 8, 5, 10, 2 ]
length_of_list = len(list_of_numbers) # Returns the length of the list
smallest_in_list = min(list_of_numbers) # Returns the smallest value in the list
largest_in_list = max(list_of_numbers) # Returns the largest value in the list
# Notice how the long line below is wrapped to fit on one screen:
print("List length is " + str(length_of_list) +
", smallest element in the list is " + str(smallest_in_list) +
", largest element in the list is " + str(largest_in_list))
</source>
== PART 3 - Iterating Over Lists ==
:This last section demonstrates an extremely useful for lists: the ability to quickly '''loop through every value in the list'''. '''For loops''' have a set number of times they loop. The '''for''' loop will execute all indented code for each item (element) in the list.
:'''Perform the Following Steps'''
::The following '''for''' loop will store the value of each element from list_of_numbers within a variable named '''item''' and run code indented below the loop for each item.<br><br>
:#Run this from a temporary Python file:<source lang="python">
list_of_numbers = [ 1, 5, 2, 6, 8, 5, 10, 2 ]
for item in list_of_numbers:
print(item)
</source>
:#As you can see: instead of writing eight function calls for each element of the list, we can call the function in a loop. And we won't have to rewrite code if the length of the list changes.<br><br>
:#Run the following code:<source lang="python">
list_of_numbers = [ 1, 5, 2, 6, 8, 5, 10, 2 ]
def square(num):
return num * num
for value in list_of_numbers:
print(square(value))
</source>The code above only prints the squares and does not save them for future use. The next example uses a function that loops through list, squares the values, and also saves the squares in a new list.<br><br>
:#Run the following code:<source lang="python">
list_of_numbers = [ 1, 5, 2, 6, 8, 5, 10, 2 ]
# Squares each item in a list of numbers, returns new list with squared numbers
def square_list(number_list):
new_list = []
for number in number_list:
new_list.append(number * number)
return new_list
new_list_of_numbers = square_list(list_of_numbers)
print(list_of_numbers)
print(new_list_of_numbers)
</source>The above is just one example of a quick use of for loops mixed with lists. But be careful when passing lists into functions. When you give a function a list as an argument, it is the actual list reference and NOT a copy. This means a function can change the list without making a new list. While you do have to be careful, this can also be useful. A function can modify any given list ''without'' have to return it.<br><br>
:#To demonstrate, run the following code:<source lang="python">
list_of_numbers = [ 1, 5, 2, 6, 8, 5, 10, 2 ]
def delete_numbers(numbers):
numbers.remove(5)
numbers.remove(6)
numbers.remove(8)
numbers.remove(5)
delete_numbers(list_of_numbers)
print(list_of_numbers)
</source>
'''Practice Functions, Lists, Loops'''
:'''Perform the Following Instructions:'''
:#Create the '''~/ops435/lab3/lab3f.py''' script. The purpose of this script is to use functions to modify items inside a list. <source lang="python">
#!/usr/bin/env python3
# Place my_list below this comment (before the function definitions)
def add_item_to_list(ordered_list):
# Appends new item to end of list with the value (last item + 1)
def remove_items_from_list(ordered_list, items_to_remove):
# Removes all values, found in items_to_remove list, from my_list
# Main code
if __name__ == '__main__':
print(my_list)
add_item_to_list(my_list)
add_item_to_list(my_list)
add_item_to_list(my_list)
print(my_list)
remove_items_from_list(my_list, [1,5,6])
print(my_list)
</source>
'''Additional Requirements'''
:::*The missing list should have the values: '''1, 2, 3, 4, 5'''
:::*The program should have a function called '''add_item_to_list(ordered_list)'''<dd><dl>This function takes a single argument which is a list name itself. It will then look at the value of the last existing item in the list, it will then append a new value that is one unit bigger (i.e. '''+1''' and modifying that same list without returning any value).</dl></dd>
:::*The script should have a function called '''remove_items_from_list(ordered_list, items_to_remove)'''<dd><dl>This function takes two arguments: a list, and a list of numbers to remove from the list. This function will then check if those items exist within that list, and if they exist, then they will be removed. This function will modify the list without returning any value.</dl></dd>
:::'''Sample Run 1:'''<source>
run lab3f.py
[1, 2, 3, 4, 5]
[1, 2, 3, 4, 5, 6, 7, 8]
[2, 3, 4, 7, 8]
</source>
:::'''Sample Run 2 (with import):'''<source>
from lab3f import * print(my_list)
# Will print [1, 2, 3, 4, 5]
add_item_to_list(my_list)
add_item_to_list(my_list)
add_item_to_list(my_list)
print(my_list)
# Will print [1, 2, 3, 4, 5, 6, 7, 8]
remove_items_from_list(my_list, [1,5,6])
print(my_list)
# Will print [2, 3, 4, 7, 8]
</source>
:::2. Download the checking script and check your work. Enter the following commands from the bash shell.<source>
cd ~/ops435/lab3/
pwd #confirm that you are in the right directory
ls CheckLab3.py || wget https://raw.githubusercontent.com/Seneca-CDOT/ops435/master/LabCheckScripts/CheckLab3.py
python3 ./CheckLab3.py -f -v lab3f
</source>
:::3. Before proceeding, make certain that you identify any and all errors in lab3f.py. When the checking script tells you everything is OK - proceed to the next step.
= LAB 3 SIGN OFF (SHOW INSTRUCTOR) =
[[Image:lab1_signoff.png|thumb|right|450px|Students should be prepared with '''all required commands (system information) displayed in a terminal (or multiple terminals) prior to calling the instructor for signoff'''.]]
:'''Have Ready to Show Your Instructor:'''
::<span style="color:green;font-size:1.5em;">✓</span> Output of: <code>./CheckLab3.py -f -v</code>
::<span style="color:green;font-size:1.5em;">✓</span> Output of: <code>cat lab3a.py lab3b.py lab3c.py lab3d.py lab3e.py lab3f.py</code>
:'''Be able to answer any questions about the lab to show that you understood it!'''
<br><br>
= LAB REVIEW =
:# What is the purpose of using functions in a Python script?
:# Write Python code to define a function called '''greetings()''' that when called will greet the user by name and on the next line display the current date
:# Why is it useful for functions to accept '''arguments''' passed-up upon function execution?
:# What is the purpose of the '''import''' command? What can be the consequence if the import command is not used prior to running a function by name?
:# Write Python code to define a function called '''join()''' that excepts two arguments which will be be stored as the variables called '''word1''' and '''word2''' respectively during the execution of the function.
:# What is the command to return a value from a function?
:# What is the purpose of the '''system()''' function?
:# What is the purpose of a '''list'''?
:# Assume that the following list has been defined: '''mylist = [ 'apple', 1, 'grape', 2, 'banana', 3, ]'''<br>Based on that, what will the following contain?<source lang="python">mylist[0]
mylist[3]
mylist[-1]
mylist[0:1]</source>
:# Assume that the following list has been defined: '''combined_list = [ [7, 5], ['x', 'y'], [ 5, 'f' ] ]'''<br>Based on that, what will the following contain?<source lang="python">
combined_list[0]
combined_list[1]
combined_list[1][0]
combined_list[2][0:2]</source>
:# Briefly explain the purpose of each of the following functions (methods) that can be used with lists: '''append''', '''insert''', '''remove''', '''sort''', '''copy'''.</li>
:# Write the '''functions''' that perform the following operations on a list:<ol type="a"><li>Returns the length of the list</li><li>Returns the smallest value in the list</li><li>Returns the largest value in the list</li></ol>
:# Write a Python script to display all of the elements within a simple list.
[[Category:OPS435-Python]]
= <font color='red'>Still under Construction - Please do not use</font> =
= LAB OBJECTIVES =
:In this lab, you will learn how to identify and create different types of '''functions''', study the properties of '''list objects''' and learn how to manipulate them using their built-in methods, and finally we will explore how '''loop''' can be used to reduce code repetition.
:'''<u>Objectives</u>'''
:Write Python code in order to:
:*'''Create and use built-in and user created functions''' - using the '''def''' and '''import''' keywords
:*'''Obtaining properties and manipulating list object''' - using the built-in function: len(), min(), max(), and list object methods: copy(), sort(), append(), and remove()
:*'''Looping through lists'''. Looping (iteration) is the method of using loop to process items in a list object one at a time using the same code to reduce code repetition in your Python script. This will result in a better, more efficient and effective program execution.
<br><br>
= INVESTIGATION 1: CREATING THE SIMPLE FUNCTIONS =
:"In the context of computer programming, a '''function''' is a named sequence of statements that perform a computation" -- Think Python 2nd Edition by Allen Donway.
:Usually, a '''function''' contains '''programming code''' and most likely placed near the top of a python file, and before the main processing section.
: When a Python program is run, the '''function's code is read into the corresponding process memory''', waiting to be executed when the function is called. Until a Function is specifically called, its code will sit still in the memeory.
:When creating programs that define and use functions, '''a large programming task can be broken-down into smaller elements''' (or '''modules'''). This is why creating programs that use functions is referred to as '''"modular programming"'''.
== Four type of Functions ==
:There are four major type of functions based on the arguments they will take and the value (or stuff) they will return:
:# function that does not take any function argument or return a value
:# function that does not take any function argument but do return a value
:# function that takes function argument(s) but does not return a value
:# function that takes function argument((s) and also return value(s)
== PART 1 - How User-Defined Functions are Declared and Run ==
:Functions may be designed:
* '''not to accept arguments or return a value''',
* to '''not accept arguments but return a value''',
* to '''accept arguments and not return a value''',
* or to '''both accept arguments and return a value'''.
In this investigation, we will focus on creating functions that either do NOT return a value, or return a value.
'''Functions and Strings'''
: You will now learn how to define and run functions that will return '''string data''' when a function is called.
:'''Perform the Following Steps:'''
:#Create a new python file for testing code in this section.
:#Whenever you want to create a function, you must start with the keyword "'''def'''". The '''def''' keyword is used to start the definition of the function, it does not run the code you write. Functions, just like '''if''' statements, must have all code under them indented.<source lang="python">
def hello():
print('Hello World')
print('Inside a Function')
</source>
:#Executing your file you should have noticed that nothing happened. Well actually, something did happen... the function called '''hello(''') has been defined and stored in internal memory in order for it to run when called by its function name. Now that our function was created, we can use it over and over.
:#To execute the code inside the function, run the function name with "'''()'''" '''brackets''' at the end of the function name.<br>Try running the '''hello()''' function by name three times like this:<source lang="python">
hello()
hello()
hello()
</source>You should notice that the function just does the same thing over-and-over no matter how many times your call the function by name. By the way, that is OK. On the other hand, you may want to create and use a function to do something, like perform error checking or some other task that returns a value to the '''main''' program for further processing. For example, a '''true''' or '''false''' value if the error checking function that was called was detected no errors or detected an error. But let's stick to some simple examples first, before tackling more complex use of functions.<br><br>
:#Let's create a function that '''returns''' some data after the function is called. This function does not print out any text: instead; it creates new variables and at the end returns the value of one of the variables.<source lang="python">
def return_text_value():
name = 'Terry'
greeting = 'Good Morning ' + name
return greeting
</source>
:# Call the function like this:<source lang="python">
return_text_value()
</source>One major difference between a function '''returning a value''' and simply '''printing a value''' is that '''returned''' values can be caught and stored in variables used in the program (that called the function) for later use. Once the returned value has been stored, it can be printed, manipulated, compared in IF statements, etc. Below will cover how to store a returned value.<br><br>
:#Notice that this syntax looks just the call to the input() function which you've used in the last lab:<source lang="python">
text = return_text_value()
</source>
:#Now the returned text from the function has been stored in the variable "'''text'''". It can be used like any string value now.<source lang="python">
print(text)
</source>
'''Functions and Numbers (Integers)'''
: You will now learn how to define and run functions that will return '''integer data''' when a function is called. In this section, you will define a function that will be returning integer values instead of text. There is not a big difference, but when returning number values, care needs to be taken if you try combining it with a string!
:'''Perform the Following steps:'''
:#Define the return_number_value() function:<source lang="python">
def return_number_value():
num1 = 10
num2 = 5
num3 = num1 + num2
return num3
</source>
:#And call it:<source lang="python">
number = return_number_value()
print(number)
print(number + 5)
print(return_number_value() + 10)
</source> What do you notice?<br><br>
:#Now, display both strings and numbers:<source lang="python">
number = return_number_value()
print('my number is ' + number)
</source> What do you notice? You should notice a warning message. This occurs because the returning value is a '''number''' and NOT a '''string'''! Combining numbers and strings in a statement (such as '''print()''') can cause errors. The error message should appear similar to the one displayed below: <source>
Traceback (most recent call last):
File "test.py", line 2, in <module>
print('my number is ' + number)
TypeError: cannot concatenate 'str' and 'int' objects
</source>
:#If a number needs to be combined with a string, use the '''str()''' predefined function that was discussed in a previous lab in order to convert the returned number into a string:<source lang="python">
number = return_number_value()
print('my number is ', number)
print('my number is ' + str(number))
print('my number is ' + str(return_number_value()))
</source>
== PART 2 - Creating a Python Script with Functions and Importing Functions ==
'''Creating a Python Script'''
:Now it's time to create a Python script that uses two functions. One function returns a string value to greet the user, where the other function returns the result of adding two values (stored in variables within the function).
:'''Perform the following Instructions:'''
:#Create a new script '''~/ops435/lab3/lab3a.py'''
:::'''Input / Output Requirements'''
:::*The script should have a '''Shebang line'''
:::*Below the '''Shebang line''', add an '''empty line''' followed by a comment stating: '''# return_text_value() function'''
:::*Add an '''empty line''' followed by the '''return_text_value()''' function '''definition''' that you previously entered in the shell.
:::*Add another '''empty line''' followed by a comment stating: '''# return_number_value() function'''
:::*Add another '''empty line''' following by the '''return_number_value()''' function '''definition''' that you previously entered in the shell.
:::*Add a '''couple of empty lines''', following by a comment stating: '''# Main Program'''
:::*Add another '''couple of empty lines''', followed by the statements displayed below:<source lang="python">
if __name__ == '__main__':
print('python code')
text = return_text_value()
print(text)
number = return_number_value()
print(str(number))
</source>
:Running your program you should have seen three lines being displayed: the text "python code", a greeting, and a result of a math calculation. The '''if''' statement in the code above is a special '''if''' statement needed to make sure that your "main" code only runs when you want it to. More on that later.
'''Importing Functions From other Python Scripts'''
In order to use functions from other scripts, you use the '''import''' statement.<br><br>
:# Let's see what happens if we forget to import functions from your lab3a.py script prior to calling a function. Create a new python file and try to call the return_text_value() function:<source lang="python">
text = lab3a.return_text_value()
</source>You should notice an error indicating '''"name 'lab3a' is not defined"'''. This error occurs since you failed to instruct python to '''import''' or "load existing defined functions from your lab3a.py script" to '''internal memory'''.<br><br>
:# Modify your program like this:<source lang="python">
import lab3a
text = lab3a.return_text_value()
print(text)
lab3a.return_number_value()
</source> You should notice that all of the function calls should now work.
:# Download the checking script and check your work. Enter the following commands from the bash shell.<source lang="bash">
cd ~/ops435/lab3/
pwd #confirm that you are in the right directory
ls CheckLab3.py || wget https://raw.githubusercontent.com/Seneca-CDOT/ops435/master/LabCheckScripts/CheckLab3.py
python3 ./CheckLab3.py -f -v lab3a
</source>
:# Before proceeding, make certain that you identify any and all errors in lab3a.py. When the checking script tells you everything is OK before proceeding to the next step.
<br><br>
= INVESTIGATION 2: CREATING FUNCTIONS WITH ARGUMENTS AND RETURN VALUES =
== PART 1 - Providing Functions With Arguments ==
:Functions can receive '''arguments''' - data to be used for processing. In this section, you will learn how to define functions that accept arguments and learn how to call functions with arguments (such as mathematical operations or testing conditions, which is useful for error-checking).
'''Passing up Single and Multiple Arguments to a Function'''
:'''Perform the Following Steps:'''
:#Create a new Python file for testing.
:#When passing arguments to functions, you put data such as '''strings''', '''numbers''', or '''variable names''' within brackets immediately following the function name.<br><br>'''NOTE:''' If a function accepts arguments, then those arguments must be declared (using variable names) when the function is declared. Those declared variable names are then used within the function for processing. Also, when you call a function with arguments, the number of arguments passed up to the function must match the number of arguments that were specified in the function declaration.<br><br>
:#Define a function called '''square()''':<source lang="python">
def square(number):
return number ** 2
</source>'''FYI:'''You may have learned that you multiple a number by itself in order to "square" the number. In python, the '''**''' operator will raise the operand on the left to the power of the operand on the right.<br><br>When calling functions with multiple arguments, the arguments are separated by '''commas'''. See what happens if you provide strings, strings without using quotes, or numbers with decimals in the following examples.
:#Test your '''square()''' function:<source lang="python">
square(5)
square(10)
square(12)
square(square(2))
square('2')
</source>Notice that nothing is printed, you need to print the values the functions return to see what they are.
:#The last function call should produce an '''error message'''. This is caused by sending a '''string''' instead of a number that is processed by the function. We could use the int() function to convert any value passed in as a string by mistake to an integer number.<br><br>
:#Declare the function '''sum_numbers()''':<source lang="python">
def sum_numbers(number1, number2):
return int(number1) + int(number2)
</source>
:#Call that function to see what happens:<source lang="python">
sum_numbers(5, 10)
sum_numbers(50, 100)
</source>
:#You can also do what looks like calling a function within another function, but it's actually just calling sum_numbers() first, then calling square() with the return from sum_numbers as an argument:<source lang="python">
square(sum_numbers(5, 5))
</source>'''NOTE:''' Running functions with multiple arguments is the same. When call a function as an argument of another function, the '''inner-most function will run first''', and the return the value from that will be used as the argument for the '''outer function'''. In the example below, '''sum_numbers(5, 5)''' will return '''10''', thus providing that result to be square with that value '''square(10)'''.<br><br>
'''Practice Creating a Function that Accepts Arguments and Returns a Value'''
:It is time to practice creating a shell script that uses a function that accepts arguments, and returns a value.
:'''Perform the Following Instructions:'''
:#Create a new script '''~/ops435/lab3/lab3b.py'''. Refer to the '''Python Script template''' and the '''Additional Requirements''' sections when creating your Python script. Refer to Sample Run and Sample Imports displayed below for exact prompt and output
:::'''Python Script Template'''
:::<source lang="python">#!/usr/bin/env python3
def sum_numbers(number1, number2):
# Make this function add number1 and number2 and return the value
def subtract_numbers(number1, number2):
# Make this function subtract number1 and number2 and return the value
# Remember to make sure the function accepts 2 arguments
def multiply_numbers(number1, number2):
# Make this function multiply number1 and number2 and return the value
# Remember to make sure the function accepts 2 arguments
if __name__ == '__main__':
print(sum_numbers(10, 5))
print(subtract_numbers(10, 5))
print(multiply_numbers(10, 5))
</source>
::'''Additional Requirements'''
::*All functions should accept two arguments
::*All functions should return an integer
::*The script should contain no errors
::'''Sample Run:'''<source lang="python">
./lab3b.py
15
5
50
</source>
::'''Other examples:'''<source lang="python">
import lab3b
lab3b.sum_numbers(10, 5)
# Will return 15
lab3b.sum_numbers(25, 25)
# Will return 50
lab3b.subtract_numbers(10, 5)
# Will return 5
lab3b.subtract_numbers(5, 10)
# Will return -5
lab3b.multiply_numbers(10, 5)
# Will return 50
lab3b.multiply_numbers(10, 2)
# Will return 20
</source>
::2. Download the checking script and check your work. Enter the following commands from the bash shell.<source lang="bash">
cd ~/ops435/lab3/
pwd #confirm that you are in the right directory
ls CheckLab3.py || wget https://raw.githubusercontent.com/Seneca-CDOT/ops435/master/LabCheckScripts/CheckLab3.py
python3 ./CheckLab3.py -f -v lab3b
</source>
::3. Before proceeding, make certain that you identify any and all errors in lab3b.py. When the checking script tells you everything is OK - proceed to the next step.
'''Passing up Multiple Arguments and Using Conditional Statements'''
:You will now create a more complex function that will not only pass arguments, but also include logic to control the flow of the function, and affect how your Python script will be run. You will create a function that uses an '''if/elif/else''' statement.
:'''Perform the Following Steps:'''
:#Use a temporary Python file to define the following function:<source lang="python">
def describe_temperature(temp):
if temp > 30:
return 'hot'
elif temp < 0:
return 'cold'
elif temp == 20:
return 'perfect'
return 'ok'
</source>The final '''return "ok"''' will only take place if a previous return has not taken place before it. Once return has been used in a function, the function immediately exits and returns the value.
:#Call describe_temperature like this to confirm the results:<source>
print(describe_temperature(50))
# Will return 'hot'
print(describe_temperature(20))
# Will return 'perfect'
print(describe_temperature(-50))
# Will return 'cold'
print(describe_temperature(25))
# Will return 'ok'
print(describe_temperature(10))
# Will return 'ok'
</source>
'''Create a Python Script Receiving Multiple Arguments'''
:'''Perform the Following Instructions:'''
:#Create the '''~/ops435/lab3/lab3c.py''' script. The purpose of the script is to have a single function that can perform addition, subtraction, or multiplication on a pair of numbers. But the function will allow us to choose exatly what operation we are performing on it when we call it. If the operate function does NOT understand the operator given, it should return an error message (e.g. calling the function to 'divide' two numbers).
:#Use this template to get started:<source lang="python">
#!/usr/bin/env python3
def operate(number1, number2, operator):
# Place logic in this function
if __name__ == '__main__':
print(operate(10, 5, 'add'))
print(operate(10, 5, 'subtract'))
print(operate(10, 5, 'multiply'))
print(operate(10, 5, 'divide'))
</source>
:::*The operate() function should use '''conditional''' statements<br> '''FYI:''' Remember that you MUST consistently '''indent ALL code''' for within each section (or test).
:::*The operate() function should accept '''three arguments'''.
:::*The operate() function should '''return''' the result.
:::*The operate() function should '''return''' an error message if the operation is unknown<br> '''FYI:''' Use single quotes or double-quotes to pass a string value.
:::*The script should contain show the exact output as the sample.
:::*The script should contain no errors.
:::*As an extra exercise, try to write your function with only one return statement.
:::'''Sample Run 1:'''<source>
./lab3c.py
15
5
50
Error: function operator can be "add", "subtract", or "multiply"
</source>
:::'''Sample Run 2 (using import from another Python file):'''<source>
import lab3c
lab3c.operate(10, 20, 'add')
# Will return 30
lab3c.operate(2, 3, 'add')
# Will return 5
lab3c.operate(100, 5, 'subtract')
# Will return 95
lab3c.operate(10, 20, 'subtract')
# Will return -10
lab3c.operate(5, 5, 'multiply')
# Will return 25
lab3c.operate(10, 100, 'multiply')
# Will return 1000
lab3c.operate(100, 5, 'divide')
# Will return Error: function operator can be "add", "subtract", or "multiply"
lab3c.operate(100, 5, 'power')
# Will return Error: function operator can be "add", "subtract", or "multiply"
</source>
:::3. Download the checking script and check your work. Enter the following commands from the bash shell.<source lang="bash">
cd ~/ops435/lab3/
pwd #confirm that you are in the right directory
ls CheckLab3.py || wget https://raw.githubusercontent.com/Seneca-CDOT/ops435/master/LabCheckScripts/CheckLab3.py
python3 ./CheckLab3.py -f -v lab3c
</source>
:::4. Before proceeding, make certain that you identify any and all errors in lab3c.py. When the checking script tells you everything is OK - proceed to the next step.
<br><br>
== PART 2 - Running System Commands with Subprocess ==
:The remainder of this investigation will allow you to run operating system commands via your Python script. Although there are different ways in which to issue operating system commands, you will learn two of them.
'''Perform the Following Steps:'''
:#Create a new python file for testing.
:#Import the '''''os''''' module in your python file.
:#You can issue operating system commands by using the '''system()''' function. Try it:<source lang="python">
os.system('ls')
os.system('whoami')
os.system('ifconfig')
</source>Notice that the output from the programs is printed in your script. Consider that may not always be what you want.<br><br>
:#Try this also:<source lang="python">
os.system('ipconfig')
</source>You should notice an error message: ''''ipconfig: command not found''''. That error occurs since that command was an MS Windows command, and our current platform is Linux.<br><br>It is not always a good idea to run system commands in Python, this makes your Python code less portable and makes it require a specific operating system or a system that has those commands available. You should think about that when you decide whether you should or should not use a system command to accomplish some task or stick to pure Python code only.<br><br>As you may recall from lab2, you issued '''import sys''' to import special variables from the system. You can import a subprocess in order to run common non OS specific commands securely.<br><br>
:#Import the subprocess module in your python file.
:#There are many features available as part of the subprocess module, we are interested in "'''Popen'''". This method subprocess.Popen() can be used to run system commands as a child process to the Python script. The code below output will create a new child process, in Python we can control this through the new Python object we just created, "'''p'''". "'''p'''" now has a collection of methods(functions that are apart of a object) available.<br><br>
:#To demonstrate, issue the following:<source lang="python">
p = subprocess.Popen(['date'], stdout=subprocess.PIPE, stdin=subprocess.PIPE, stderr=subprocess.PIPE, shell=True)
</source>This function call and the following step is full of details we haven't yet talked about which is why it may look a little scary. By the time we're finished with the course - you will be able to look at code like this and not be intimidated. If you're curious and want to look ahead - you can find the definition for the [https://docs.python.org/3/library/subprocess.html#subprocess.Popen Popen function in the Python reference manual].
:#This next step is going to communicate with the process and get the retrieve it's output (stdout).<source>
output = p.communicate()
print(output)
print(output[0])
# The above stdout is stored in bytes
# Convert stdout to a string and strip off the newline characters
stdout = output[0].decode('utf-8').strip()
print(stdout)
</source>
:# Sometimes you will be able to use purely python code to get your job done, but often you will need to call existing system commands. It's important to learn how to call them and how to interact with those external processes.
'''Practice Running System Commands From Python'''
:'''Perform the Following Instructions:'''
<ol>
<li>Create the "'''~/ops435/lab3/lab3d.py'''" script. The purpose of this script is to create a Python function that can return the linux system's root directory free space.
:*The script should '''import the correct module'''
:*The script should use the linux command: '''<nowiki>df -h | grep '/$' | awk '{print $4}'</nowiki>'''
:*The script should contain the function called: '''free_space()'''
:*The function '''free_space()''' should return a string which is in '''utf-8''' and has '''newline characters strip'''
:*'''Note:''' your output may be completely different, the free/available disk space on every computers root directory may be different.
:'''Sample Run 1:''' <source>
./lab3d.py
9.6G
</source>
:'''Sample Run 2 (using import from another Python file):'''<source>
import lab3d
lab3d.free_space()
# Will return 9.6G
</source></li>
<li>Download the checking script and check your work. Enter the following commands from the bash shell.<source lang="bash">
cd ~/ops435/lab3/
pwd #confirm that you are in the right directory
ls CheckLab3.py || wget https://raw.githubusercontent.com/Seneca-CDOT/ops435/master/LabCheckScripts/CheckLab3.py
python3 ./CheckLab3.py -f -v lab3d
</source></li>
<li>Before proceeding, make certain that you identify any and all errors in lab3d.py. When the checking script tells you everything is OK - proceed to the next step.</li>
</ol>
= INVESTIGATION 3: USING LISTS =
:'''Lists''' are one of the most powerful '''data-types''' in Python. A list is a series of '''comma separated values found between square brackets'''. Values in a list can be anything: '''strings''', '''integers''', '''objects''', even '''other lists'''. In this section, you will introduce lists and how to use them effectively, you will further user lists in later labs. It is important to realise that although lists may appear very similar to arrays in other languages, they are different in a number of aspects including the fact that they don't have a fixed size.
== PART 1 - Navigating Items in Lists ==
:'''Perform the Following Steps'''
:#Create a new Python file for testing things in this section.
:#Create a few lists with different values: list1 contains only '''integers''', list2 contains only '''strings''', list3 contains a combination of both '''integers and strings'''
:#<source lang="python">
list1 = [ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]
list2 = [ 'uli101', 'ops235', 'ops335', 'ops435', 'ops535', 'ops635' ]
list3 = [ 'uli101', 1, 'ops235', 2, 'ops335', 3, 'ops435', 4, 'ops535', 5, 'ops635', 6 ]
</source>The best way to access individual '''elements''' in a list is using the list '''index'''.<br>The index is a number starting from 0 to ('''number_of_items - 1'''), the list index starts counting at '''0'''.<br><br>
:#Inspect specified elements in your lists:<source lang="python">
print(list1[0]) # First element in list1
print(list2[1]) # Second element in list2
print(list3[-1]) # Last element in list3
</source>
:#You can also retrieve ranges of items from a list (these are called slices): <source lang="python">
print(list1[0:5]) # Starting with index 0 and stopping before index 5
print(list2[2:4]) # Starting with index 2 and stopping before index 4
print(list3[3:]) # Starting with index 3 and going to the end
</source>
'''Practice Using Functions and Using the List Index'''
:'''Perform the Following Instructions'''
:#Create a Python script called: '''~/ops435/lab3/lab3e.py'''<br>The purpose of this script is to have a number of functions that output a different data storage in various elements of a list. Each function will return either a single item from the list OR will create a new list and return the entire new list.<br><br>
:#The template function names and the special if statement:<source lang="python">
#!/usr/bin/env python3
# Create the list called "my_list" here, not within any function defined below.
# That makes it a global variable. We'll talk about that in another lab.
def give_list():
# Does not accept any arguments
# Returns all of the global variable my_list unchanged
def give_first_item():
# Does not accept any arguments
# Returns a single string that is the first item in the global my_list
def give_first_and_last_item():
# Does not accept any arguments
# Returns a list that includes the first and last items in the global my_list
def give_second_and_third_item():
# Does not accept any arguments
# Returns a list that includes the second and third items in the global my_list
if __name__ == '__main__': # This section also referred to as a "main code"
print(give_list())
print(give_first_item())
print(give_first_and_last_item())
print(give_second_and_third_item())
</source>
:::'''Additional Requirements'''
:::*The script should declare a list called '''my_list''' created BEFORE any function definition
:::*The list called '''my_list''' should have the values: '''100''', '''200''', '''300''', and ''''six hundred''''
:::*The script should '''implement''' the empty functions - i.e. you have to fill in the bodies for these functions
:::'''Sample Run 1:'''<source>
./ lab3e.py
[100, 200, 300, 'six hundred']
100
[100, 'six hundred']
[200, 300]
</source>
:::'''Sample Run 2 (with import from another script):'''<source>
import lab3e
lab3e.give_list()
# Will print [100, 200, 300, 'six hundred']
lab3e.give_first_item()
# Will print 100
lab3e.give_first_and_last_item()
# Will print [100, 'six hundred']
lab3e.give_second_and_third_item()
# Will print [200, 300]
</source>
:::3. Download the checking script and check your work. Enter the following commands from the bash shell.<source>
cd ~/ops435/lab3/
pwd #confirm that you are in the right directory
ls CheckLab3.py || wget https://raw.githubusercontent.com/Seneca-CDOT/ops435/master/LabCheckScripts/CheckLab3.py
python3 ./CheckLab3.py -f -v lab3e
</source>
:::4. Before proceeding, make certain that you identify any and all errors in lab3e.py. When the checking script tells you everything is OK - proceed to the next step.
== PART 2 - Manipulating Items in Lists ==
:There are a number of ways to obtain information about lists as well as change the data that is contained within a list. In this section, you will learn how to manipulate lists.
:'''Perform the Following Steps:'''
:#Let's perform a simple change to a list element. Try the following code:<source lang="python">
courses = [ 'uli101', 'ops235', 'ops335', 'ops435', 'ops535', 'ops635' ]
print(courses[0])
courses[0] = 'eac150'
print(courses[0])
print(courses)
</source>
:#Below are some examples of using built-in functions to '''manipulate''' lists. Take your time to see how each function can be a useful tool for making changes to existing lists:<source lang="python">
courses.append('ops235') # Add a new item to the end of the list
print(courses)
courses.insert(0, 'hwd101') # Add a new item to the specified index location
print(courses)
courses.remove('ops335') # Remove first occurrence of value
print(courses)
sorted_courses = courses.copy() # Create a copy of the courses list
sorted_courses.sort() # Sort the new list
print(courses)
print(sorted_courses)
</source>
:#In addition to using functions to manipulate lists, there are functions that are useful to provide '''information''' regarding the list such as number of elements in a list, the smallest value and largest value in a list:<source lang="python">
list_of_numbers = [ 1, 5, 2, 6, 8, 5, 10, 2 ]
length_of_list = len(list_of_numbers) # Returns the length of the list
smallest_in_list = min(list_of_numbers) # Returns the smallest value in the list
largest_in_list = max(list_of_numbers) # Returns the largest value in the list
# Notice how the long line below is wrapped to fit on one screen:
print("List length is " + str(length_of_list) +
", smallest element in the list is " + str(smallest_in_list) +
", largest element in the list is " + str(largest_in_list))
</source>
== PART 3 - Iterating Over Lists ==
:This last section demonstrates an extremely useful for lists: the ability to quickly '''loop through every value in the list'''. '''For loops''' have a set number of times they loop. The '''for''' loop will execute all indented code for each item (element) in the list.
:'''Perform the Following Steps'''
::The following '''for''' loop will store the value of each element from list_of_numbers within a variable named '''item''' and run code indented below the loop for each item.<br><br>
:#Run this from a temporary Python file:<source lang="python">
list_of_numbers = [ 1, 5, 2, 6, 8, 5, 10, 2 ]
for item in list_of_numbers:
print(item)
</source>
:#As you can see: instead of writing eight function calls for each element of the list, we can call the function in a loop. And we won't have to rewrite code if the length of the list changes.<br><br>
:#Run the following code:<source lang="python">
list_of_numbers = [ 1, 5, 2, 6, 8, 5, 10, 2 ]
def square(num):
return num * num
for value in list_of_numbers:
print(square(value))
</source>The code above only prints the squares and does not save them for future use. The next example uses a function that loops through list, squares the values, and also saves the squares in a new list.<br><br>
:#Run the following code:<source lang="python">
list_of_numbers = [ 1, 5, 2, 6, 8, 5, 10, 2 ]
# Squares each item in a list of numbers, returns new list with squared numbers
def square_list(number_list):
new_list = []
for number in number_list:
new_list.append(number * number)
return new_list
new_list_of_numbers = square_list(list_of_numbers)
print(list_of_numbers)
print(new_list_of_numbers)
</source>The above is just one example of a quick use of for loops mixed with lists. But be careful when passing lists into functions. When you give a function a list as an argument, it is the actual list reference and NOT a copy. This means a function can change the list without making a new list. While you do have to be careful, this can also be useful. A function can modify any given list ''without'' have to return it.<br><br>
:#To demonstrate, run the following code:<source lang="python">
list_of_numbers = [ 1, 5, 2, 6, 8, 5, 10, 2 ]
def delete_numbers(numbers):
numbers.remove(5)
numbers.remove(6)
numbers.remove(8)
numbers.remove(5)
delete_numbers(list_of_numbers)
print(list_of_numbers)
</source>
'''Practice Functions, Lists, Loops'''
:'''Perform the Following Instructions:'''
:#Create the '''~/ops435/lab3/lab3f.py''' script. The purpose of this script is to use functions to modify items inside a list. <source lang="python">
#!/usr/bin/env python3
# Place my_list below this comment (before the function definitions)
def add_item_to_list(ordered_list):
# Appends new item to end of list with the value (last item + 1)
def remove_items_from_list(ordered_list, items_to_remove):
# Removes all values, found in items_to_remove list, from my_list
# Main code
if __name__ == '__main__':
print(my_list)
add_item_to_list(my_list)
add_item_to_list(my_list)
add_item_to_list(my_list)
print(my_list)
remove_items_from_list(my_list, [1,5,6])
print(my_list)
</source>
'''Additional Requirements'''
:::*The missing list should have the values: '''1, 2, 3, 4, 5'''
:::*The program should have a function called '''add_item_to_list(ordered_list)'''<dd><dl>This function takes a single argument which is a list name itself. It will then look at the value of the last existing item in the list, it will then append a new value that is one unit bigger (i.e. '''+1''' and modifying that same list without returning any value).</dl></dd>
:::*The script should have a function called '''remove_items_from_list(ordered_list, items_to_remove)'''<dd><dl>This function takes two arguments: a list, and a list of numbers to remove from the list. This function will then check if those items exist within that list, and if they exist, then they will be removed. This function will modify the list without returning any value.</dl></dd>
:::'''Sample Run 1:'''<source>
run lab3f.py
[1, 2, 3, 4, 5]
[1, 2, 3, 4, 5, 6, 7, 8]
[2, 3, 4, 7, 8]
</source>
:::'''Sample Run 2 (with import):'''<source>
from lab3f import * print(my_list)
# Will print [1, 2, 3, 4, 5]
add_item_to_list(my_list)
add_item_to_list(my_list)
add_item_to_list(my_list)
print(my_list)
# Will print [1, 2, 3, 4, 5, 6, 7, 8]
remove_items_from_list(my_list, [1,5,6])
print(my_list)
# Will print [2, 3, 4, 7, 8]
</source>
:::2. Download the checking script and check your work. Enter the following commands from the bash shell.<source>
cd ~/ops435/lab3/
pwd #confirm that you are in the right directory
ls CheckLab3.py || wget https://raw.githubusercontent.com/Seneca-CDOT/ops435/master/LabCheckScripts/CheckLab3.py
python3 ./CheckLab3.py -f -v lab3f
</source>
:::3. Before proceeding, make certain that you identify any and all errors in lab3f.py. When the checking script tells you everything is OK - proceed to the next step.
= LAB 3 SIGN OFF (SHOW INSTRUCTOR) =
[[Image:lab1_signoff.png|thumb|right|450px|Students should be prepared with '''all required commands (system information) displayed in a terminal (or multiple terminals) prior to calling the instructor for signoff'''.]]
:'''Have Ready to Show Your Instructor:'''
::<span style="color:green;font-size:1.5em;">✓</span> Output of: <code>./CheckLab3.py -f -v</code>
::<span style="color:green;font-size:1.5em;">✓</span> Output of: <code>cat lab3a.py lab3b.py lab3c.py lab3d.py lab3e.py lab3f.py</code>
:'''Be able to answer any questions about the lab to show that you understood it!'''
<br><br>
= LAB REVIEW =
:# What is the purpose of using functions in a Python script?
:# Write Python code to define a function called '''greetings()''' that when called will greet the user by name and on the next line display the current date
:# Why is it useful for functions to accept '''arguments''' passed-up upon function execution?
:# What is the purpose of the '''import''' command? What can be the consequence if the import command is not used prior to running a function by name?
:# Write Python code to define a function called '''join()''' that excepts two arguments which will be be stored as the variables called '''word1''' and '''word2''' respectively during the execution of the function.
:# What is the command to return a value from a function?
:# What is the purpose of the '''system()''' function?
:# What is the purpose of a '''list'''?
:# Assume that the following list has been defined: '''mylist = [ 'apple', 1, 'grape', 2, 'banana', 3, ]'''<br>Based on that, what will the following contain?<source lang="python">mylist[0]
mylist[3]
mylist[-1]
mylist[0:1]</source>
:# Assume that the following list has been defined: '''combined_list = [ [7, 5], ['x', 'y'], [ 5, 'f' ] ]'''<br>Based on that, what will the following contain?<source lang="python">
combined_list[0]
combined_list[1]
combined_list[1][0]
combined_list[2][0:2]</source>
:# Briefly explain the purpose of each of the following functions (methods) that can be used with lists: '''append''', '''insert''', '''remove''', '''sort''', '''copy'''.</li>
:# Write the '''functions''' that perform the following operations on a list:<ol type="a"><li>Returns the length of the list</li><li>Returns the smallest value in the list</li><li>Returns the largest value in the list</li></ol>
:# Write a Python script to display all of the elements within a simple list.
[[Category:OPS435-Python]]