Changes

:In previous labs, you learned some programming tools in order to make your Python scripts '''more functional''' and allowed your Python script to run differently based on different data or situations. These tools included '''objects/variables''', '''condition statements''' and '''loops'''. The utilization of these basic tools not only apply to Python scripts, but basically all programming languages including interpreted (including '''Perl scripts''', '''Bash Shell scripts''', '''JavaScript''', etc ) and compiled languages (including '''C''', '''C++''', '''Java''', etc).

:*'''Looping through lists''' using '''functions'''. Looping (iteration) is the ability for your program to repeatedly run the same code over and over. In this way, you can run a loop that contains a list to better send data to functions for better, more efficient execution of your Python script.

:A very simple definition of using '''functions''' is to create and reuse '''smaller programs within a larger program'''. In programming languages such as '''C''', '''C++''' and '''Java''', commonly used functions are pre-packaged in '''libraries'''. This relates to dependency issues that were discussed when compiling C programming code in your OPS235 course: if a supporting library is missing, the program would not be able to run the called function.

:Functions may be designed :* '''not to accept arguments or return a value''', designed * to '''not accept arguments but not return a value''', designed * to '''accept arguments and not return a value''', * or designed to '''both accept arguments and return a value'''. In this investigation, will we will focus of on creating functions that either do NOT return a value, or return a value.

:#Launch your Centos VM, open a shell terminal (as a regular user) and start Create a new ipython3 sessionpython file for testing code in this section.:<source>ipython3</source>#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.<br><br>:# Enter the following code in your ipython3 shell:<source lang="python">

</source>Remember to press '''ENTER''' a second time to return to the ipython prompt. You may :#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. <br><br>:#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 by issuing the following in the ipython3 shelllike this:<source lang="python">

:# Call the function in your ipython3 shell by issuing the followinglike this:<source lang="python">

:#Enter Notice that this syntax looks just the following call to the input() function which you've used in the ipython3 shell to see returning a variable's value workslast lab:<source lang="python">

:#Issue Define the following in your ipython3 shellreturn_number_value() function:<source lang="python">

:#Now, issue the following in the ipython3 shell to And call the '''return_number_value()''' functionit:<source lang="python">

:#Now, issue the following which use the print() statement to display both strings and numbers:<source lang="python">

---------------------------------------------------------------------------TypeError Traceback (most recent call last):<ipython-input-24-d80d5924146a> File "test.py", line 2, in <module>()----> 1 print('my numbr number is ' + number) TypeError: Cancannot concatenate 'str't convert and 'int' object to str implicitlyobjects

:::2. Run Running your lab3a.py script in your ipython3 shell by issuing:<source>run lab3a.py</source>You 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.

In order to use functions from other scripts, you use the '''import''' commandstatement.<br><br>We can demonstrate the use of re-using functions from another script by simply '''issuing statements from the ipython shell to call a function from your lab3a.py Python script'''. But care MUST be taken to first use the import command to load in the function declarations from your Python script to your ipython shell environment first.<br><br> :'''Perform the following Instructions:'''

:# Let's see what happens if we forget to import functions from your lab3a.py script prior to calling a function. Issue Create a new python file and try to call the followingreturn_text_value() function:<source lang="python">

</source>You should notice an error indicating '''"name 'lab3a' is not defined"'''. This error occurs since you failed to instruct the ipython shell python to '''import''' or "load existing defined functions from your lab3a.py script" to '''internal memory'''.<br><br>:# Issue the following within the ipython shellModify your program like this:<source lang="python">

print(text)

</source> You should notice that all of the function calls should now work. '''NOTE:''' since you are in the ipython shell, the import command only will work as long as you are '''logged into that shell'''. You will need to use other methods (contained in other Python Scripts (or future ipython shell sessions) to make these function imports '''persistent''', but you will learn about that at a later time.<br><br>:# Exit the ipython3 shell, download Download the checking script and check your work. Enter the following commands from the bash shell.<source lang="bash">

:#Start the ipython3 shellCreate a new Python file for testing.:<source>ipython3</source>#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 rguments 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>:#Issue the following from the ipython shell to declare Define a function called '''square()''':<source lang="python">

:#Issue the following to test Test your '''square()''' function:<source lang="python">

</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>:#Issue the following to declare Declare the function '''sum_numbers()''':<source lang="python">

:#Issue the following from the ipython shell Call that function to see what happens:<source lang="python">

:#Issue the following to issue 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">

</source>'''NOTE:''' Running functions with multiple arguments is the same. When you put call a function as a an argument of another function, the '''inner-most function will run first''', and the return the value '''10''' from the 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>

run ./lab3b.py

::'''Sample Run Using importOther examples:'''<source lang="python">

# Will return 15

# Will return 50

# Will return 5

# Will return -5

# Will return 50

# Will return 20

::2. Exit the ipython3 shell, download Download the checking script and check your work. Enter the following commands from the bash shell.<source lang="bash">

::3. Before proceeding, make certain that you identify any and all errors in lab3b.py. When the checking script tells you everything is OK before proceeding - proceed to the next step.

'''Passing up Multiple Arguments and Using Logic Conditional Statements'''

:You will now create a more complex function that will not only pass-up 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''' logic statement.

:#Start the ipython3 shell<source>ipython3</source>:#Define Use a temporary Python file to define the following function in your ipython shell:<source lang="python">

:#Issue the following functions (with arguments) 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'

:#Create the '''~/ops435/lab3/lab3c.py''' script. The purpose of the script is to make 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 the functionit. 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).

:::*The operate() function should use '''logicconditional''' statements<br> &nbsp; '''FYI:''' Remember that you MUST consistently '''indent ALL code''' for within each logic section (or test): otherwise, it may not allow the logic statement to work correctly. :::*The operate() function should accept '''three arguments'''.:::*The operate() function should '''return''' the result.

:::*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.

run ./lab3c.py

:::'''Sample Run 2 (with using importfrom another Python file):'''<source>

# Will return 30

# Will return 5

# Will return 95

# Will return -10

# Will return 25

# Will return 1000

# Will return Error: function operator can be "add", "subtract", or "multiply"

# Will return Error: function operator can be "add", "subtract", or "multiply"

:::3. Exit the ipython3 shell, download Download the checking script and check your work. Enter the following commands from the bash shell.<source lang="bash">

:::4. Before proceeding, make certain that you identify any and all errors in lab3c.py. When the checking script tells you everything is OK before proceeding - proceed to the next step.

: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 how to issue two of them within a Python script to run in a secure manner regardless of the type of operating system platform (eg Linux, Windows, MacOSX).

:#Start Create a new python file for testing.:#Import the ipython3 shell'''''os''''' module in your python file.:<source>ipython3</source> #You can issue operating system commands by using the '''system()''' function.<br><br>:#Issue the following in ipythonTry it:<source lang="python">os.system('ls')os.system('whoami')os.system('ifconfig')</source> You should notice Notice that the output from the Linux commands that you called programs is not printed to the console automatically, it's printed by ipython onlyin your script. Make sure Consider that may not always be what you understand the differencewant.<br><br>:#Issue the following in ipythonTry 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. Also, allowing python to execute commands on the operating system can be a '''security problem'''. For these reasons You should think about that when you decide whether you should only or should not use '''sub-process''' and '''a system commands''' as a last resort and 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>:#Issue Import the following subprocess module in ipython:<source lang="your python">import subprocess</source>file.:#To view the available modules and attributes to obtain non OS specific command-like information, issue the following:<source>dir(subprocess)</source>There are many available modules and attributes features available as part of the subprocessmodule, we are interested in "'''Popen'''". This method subprocess.Popen() can be used to run system commands as a child process to the Python script. This 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, view them with '''dir()'''.<br><br>

print(output)print(output[0])

print(stdout)

:#<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>run ./lab3d.py

:::'''Sample Run 2 (with using importfrom another Python file):'''<source>

'# Will return 9.6G'</source></li>:::3. Exit the ipython3 shell, download <li>Download the checking script and check your work. Enter the following commands from the bash shell.<source lang="bash">

</source></li>:::4. <li>Before proceeding, make certain that you identify any and all errors in lab3d.py. When the checking script tells you everything is OK before proceeding - proceed to the next step.</li></ol>

:'''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 arraysin other languages, they are different in a number of aspects including which functions are used to manipulate lists as opposed to which functions are used to manipulate arraysthe fact that they don't have a fixed size.

:#Start the ipython3 shellCreate a new Python file for testing things in this section.:<source>ipython3</source>You will now create #Create a few lists with different values: list1 contains only '''integers''', list2 contains only '''strings''', list3 contains a combination of both '''integers and strings'''.<br><br>:#Issue the following from the ipython shell:<source lang="python">

</source>List are constructed similar to arrays. The best way to get access individual '''elements''' from 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>:#Issue the following Inspect specified elements in the ipython shell to obtain stored datayour lists:<source lang="python">print(list1[0] ) # First element in list1print(list2[1] ) # Second element in list2print(list3[-1] ) # Last element in list3

:#Issue the following to You can also retrieve ranges (slices) of items from a list(these are called slices): <source lang="python">print(list1[0:5] ) # Starting with index 0 and stopping before index 5print(list2[2:4] ) # Starting with index 2 and stopping before index 4print(list3[3:] ) # Starting with index 3 and going to the end</source>Lists can also contain other lists. This means data can be contained in: lists of strings, lists of integers, or lists contains a combination of strings and integers.<br><br>:#Issue of the following to create a list that contains lists:<source lang="python">list4 = [ [1, 2, 3, 4], ['a', 'b', 'c', 'd'], [ 5, 6, 'e', 'f' ] ]</source>The list just created only has 3 index locations. Each index points the individual list stored as the list element. <source>list4[0]list4[1]list4[2]</source>:#To access a list inside another list, a second index is needed. Spend some time trying out the syntax and try and navigate to a specific spot in the list.<source lang="python">list4[0][0] # First element in first listlist4[0][-1] # Last element in first listlist4[2][0:2] # First two elements in third list</source>:#You can use different elements from existing lists to create new lists. To demonstrate, issue the following:<source lang="python">first_only_list = [ list1[0], list2[0], list3[0] ]first_only_list

# Create the list called "my_list" below this command (here, not within any function defined below).# That makes it a global variable. We'll talk about that in another lab.

# Returns all of the entire list global variable my_list unchanged

# Returns a single string that is the first item in the listglobal my_list

# Returns a list that includes the first and last items in the listglobal my_list

# Returns a list that includes the second and third items in the listglobal my_list

if __name__ == '__main__': # This section also referred to as a "boiler platemain code"

run ./ lab3e.py

:::'''Sample Run 2 (with importfrom another script):'''<source>

# Will print [100, 200, 300, 'six hundred']

# Will print 100

# Will print [100, 'six hundred']

# Will print [200, 300]

:::3. Exit the ipython3 shell, download Download the checking script and check your work. Enter the following commands from the bash shell.<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 before proceeding - proceed to the next step.

:#Start the ipython3 shell:<source>ipython3</source>:#Let's perform a simple change to a list element. Issue Try the following in the ipython shellcode:<source lang="python">

print(courses[0])

print(courses[0])print(courses)</source>It might be useful to change a list element-by-element, but there are other more efficient methods of changing a list (for example: using functions). You will now use the '''dir()''' and '''help()''' functions to see what functions and attributes are available for manipulating lists. The '''help()''' function will also give us tips on how to use those functions. For example, you can use issue help(list-name) in order to see what functions are available to use for that list specific list.<br><br>:#Issue the following:<source lang="python">dir(courses)help(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.<br><br>:#Issue the following:<source lang="python">help(courses.append)

print(courses)

print(courses)

print(courses)

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.<br><br> :#Issue the following:<source lang="python">

length_of_list = len(list_of_numbers) # Returns the length of the listsmallest_in_list = min(list_of_numbers) # Returns the smallest value in the listlargest_in_list = max(list_of_numbers) # Returns the largest value in the list</source>In addition to manipulating and obtaining characteristics of a list, it # Notice how the long line below is also useful wrapped to be able to '''perform searches''' for values within lists and obtain the location of values for elements contained within a list. The '''indexfit on one screen:print()''' function allows searching inside a list for a value, it will return the index number of the first occurence. <source lang="pythonList length is ">number = 10help+ str(list_of_numbers.indexlength_of_list)+ list_of_numbers.index(number) # Return index of the number searched for</source>One common annoyance that can occur when performing searches are '''error messages''' when performing a search for an ", smallest element that happens NOT to exist in the list. A good way to prevent those type of errors is to use an '''if''' statement to check to see if the value for an element is in a list, then the appropriate search can be performed for that existing element value.<br><br>:#Issue the following:<source lang="python">list_of_numbers = [ 1, 5, 2, 6, 8, 5, 10, 2 ]number = 7if number in list_of_numbers: # Returns True if value in list, returns False if item not in list number_index = list_of_numbers.index(number) print('index is: ' + str(number_index)smallest_in_list)+else: # If ", largest element in the statement list is False, the else will run print(" + str(numberlargest_in_list) + ' is not in list_of_numbers')

: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. Using loops with list allow for efficient processing of stored data.

::Let's take a moment to understand how the '''for''' loop works. This The following '''for''' loop will store the value of each list element from list_of_numbers within a variable named '''item''' and run code indented below the loop for each item.<br><br>:#Issue the following in the ipython shellRun this from a temporary Python file:<source lang="python">

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 completely change the list without making a new list. While you do have to be careful , this is can also be useful, a . A function can modify any given list, ''without '' have to return or store it.<br><br>

print(list_of_numbers)

# Appends new item to end of list which is with the value (last item + 1)

:::*The script 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>

from lab3f import * [1/1899]print(my_list)# Will print [1, 2, 3, 4, 5]

print(my_list)# Will print [1, 2, 3, 4, 5, 6, 7, 8]

print(my_list)# Will print [2, 3, 4, 7, 8]

:::2. Exit the ipython3 shell, download Download the checking script and check your work. Enter the following commands from the bash shell.<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 before proceeding - proceed to the next step.

:# Assume that the following command was issued at the ipython3 promptlist has been defined: '''mylist = [ 'apple', 1, 'grape', 2, 'banana', 3, ]'''<br>Based on the command issued abovethat, what are the results of will the following commandscontain?<blockquotesource lang="python">'''mylist[0]''' , '''mylist[3]''' , '''mylist[-1]''' , '''mylist[0:1]'''</blockquotesource>:# Assume that the following command was issued at the ipython3 promptlist has been defined: '''combined_list = [ [7, 5], ['x', 'y'], [ 5, 'f' ] ]'''<br>Based on the command issued abovethat, what are the results of will the following commandscontain?<blockquotesource lang="python">'''combined_list[0]''' , '''combined_list[1]''' , '''combined_list[1][0]''' , '''combined_list[2][0:2]'''</blockquotesource>