CS152
Lab Exercise 3: Modular Design and Lists
The goal of this week's lab/project pair is to learn how to organize our Python programs in such a way that the functions inside of one Python file can be used by other Python programs. The general term for this is code reuse and it is a central idea in software engineering. After all, what’s the benefit of spending a lot of time working on a program if it can only be used once and only to solve one specific problem. But in order to achieve efficient code reuse, we will need to learn how to structure our programs in a particular way. This organizational model is commonly referred to as modular design and means that we will build more complex programs by reusing modules (collections of functions) also known as Python programs.
This lab will prepare you for Project 3 by introducing more details on lists, uses of the main function, more file processing techniques, and command-line arguments.
So, let’s get started!
Lab Tasks (L1-L7)
L1. Set up your workspace
Make a folder and name it Lab03. Open VS Code and then open this folder from the File menu. Create a new file, list_practice.py.
L2. Working with lists
Lists are an important data structure in Python. A list is an ordered sequence of values. In Python the values of a list can be of any type, including other lists, and the list does not have to all be of the same type (unlike in other programming languages--we’re looking at you Java!). But for this assignment, we'll mostly be dealing with lists of numbers.
Put the following assignment into your list_practice.py file. It assigns to the variable numbers a new list with five values:
numbers = [5, 3, 6, 1, 2]
In Python a list is indicated by square brackets, and the elements of the list are separated by commas.
We can make an empty list like this:
my_empty_list = []
To access one element of a list, we use index notation with square brackets like this:
first_number = numbers[0]
Note: In Python we start counting indices at zero.
Add two more lines to your file to print out the 5 and the 1 (the first element in the list and the fourth element in the list--be sure to check how you count!).
To add items to the end of a list, use the append(item to append to end of list) method.
To call a method on a list we combine the name of the list, the “dot” operator, and the method name as follows:
numbers.append(7)
print( numbers )
You should now see a 7 at the end of the list.
Just like with accessing list elements, we can replace elements in a list using bracket notation. For example, the following changes the first element of the list to a 4 and then prints the
updated list:
numbers[0] = 4
Modify the value of two other locations in the list in your Python file, then print out the list and make sure it did what you think it should have.
L3. Building lists
So far, all of our programs have processed data while we read it from a file (Ok, technically we created a list using the split method, but we didn’t keep that list around for later processing). For this project, we want to read the file and build a data list that stores the incoming data in a list and then manipulate the list. The concept is straightforward: start with an empty list and as your program reads through the file append each value to the list.
A. Download the file hurricanes .csv from Google Classroom assignment and save it in your Lab03 folder.
B. Create a new Python file called analyze.py. Add a file header (module string) at the top that contains your name, today’s date, and the section of CS 152 you are in. Then copy and paste the following pseudocode. Convert each line of pseudocode into actual Python code. Each comment will correspond to one line of code.
def main ():
# assign to fp the result of opening the file hurricanes.csv for reading
# assign to line the first line of the data file
# assign to headers the result of splitting the line using commas
# print headers
# assign to a list variable named data an empty list
# for each line in the file
# assign to items the result of splitting the line using commas
# append the second item to data (be careful, which index is that?)
# close the data file
# print data
if __name__ == "__main__":
main()
Program Output:
[ 'Year ', 'Number ', 'Damage\n ']
[15, 5, 6, 8, 3, 12, 7, 10, 2, 6, 4, 7, 10, 8]
L4. Work with command-line arguments
One way to get information from the user into a Python program is to use the input(“prompt string”) technique. But there is a second way to do this--command-line arguments. Information can be entered on the command line when the program is started and Python stores that information in a special variable called sys.argv where argv stands for the list of arguments read from the command line
For example, it would be nice to modify our analyze.py program so that we could use it with any data file and any column. The following steps will show you how to get access to the command-line information.
A. Import the sys module
Create a new file com.py. Add a file header (module string) at the top that contains your name, today’s date, and the section of CS 152 you are in. Add the following statement below your header information:
import sys
B. Find out what the sys package can do
Put the following line of code in the com.py file:
print(sys.argv)
Save your file and run the com.py file. What do you see?
Rerun the last command line, but add additional data after python3 com.py. For example, try:
python3 com.py hello world 1 2 3
Now, what do you see?
The sys package gives you the ability to see what the user has typed on the command line.
Each individual string (separated by spaces) from the command line is an entry in the argv list.
C. Access individual strings from the command line
Add the following three lines to your com.py file:
print("Running program", sys.argv[0])
print("I'm going to open the file", sys.argv[1])
print("I'm going to extract column", int(sys.argv[2]))
Run the program using the following command.
python3 com.py hurricanes.csv 1
Do you begin to see how the command-line arguments can control both the file and the column of data to extract without the need to edit the Python file each time?
L5. Get user input from the command-line
Edit your analyze.py file so that the csv filename and column index are given on the command line.
A. Add the import sys statement to the top of the file.
B. Change the definition of main() so that it has two parameters: filename, column_id. C. Change the hardcoded value, hurricanes.csv, with the filename parameter.
D. On the line that appends the data to your list, replace the index with the column_id parameter.
E. When the main function is called from the conditional block at the bottom of the file, use sys.argv[1] and int(sys.argv[2]) as arguments to be passed into the main function:
main(sys.argv[1], int(sys.argv[2])
And add two parameters to the definition of the main function:
def main(filename, column_id):
Run your program using:
python3 analyze.py hurricanes.csv 1
The result should be identical to the prior case. Try changing the 1 to a 2 and see what happens.
What happens if youtube forget the extra data on the command line?
L6. A Library of Useful Functions
In the next task we're going to create a file (module) that holds a library of useful functions. In Python, a library, or package, or module, is a file that contains functions. When you import the module into another Python file, you can use those functions. We've already done this by importing the sys package into our program. Now we're going to create our own module and then import it into other Python files.
Create a new file called stats.py. Add a file header (module string) at the top that contains your name, today’s date, and the section of CS 152 you are in.
A. Write a function to compute the sum of a list of numbers
Create a new function called sum(numbers) that takes one argument. You can assume that the argument will be a list of numbers. This function should add together all of the values in the list and return the sum. The algorithm is as follows:
# Create a variable to hold the sum
# Initialize the variable to 0.0 (explicitly make it a floating point number).
# Define a for loop to iterate over the list passed in as the function parameter. # On each iteration, add the current number to the variable holding the sum. # Once the loop completes, return the sum.
B. Write a test function
To test your function, make a second function called test at the bottom of your stats.py file. The function does not require any arguments. As the first instruction in the test function, assign to a variable the list [1, 2, 3, 4]. For the second instruction assign to a second variable the result of calling sum with the list as the argument. For the third instruction, print out the variable
holding the result.
Put the following at the end of your stats.py file (just below your test function) to call the test function when stats.py is run directly (i.e., not imported). The test function will not run when
stats.py is imported into another Python file.
if __name__ == "__main__":
test()
Run your program and make sure you get the value 10.0 as an output.
L7. Using a Library
The last step in the lab is to import your stats library into another file and use a function in that module. Reopen your analyze.py file. At the top, after the import sys, and add the following line of code:
import stats
To call a function in your stats library, all you have to do is put stats. in front of the name of the function you want to use (e.g., stats.sum(numbers) )
Edit your main function so it uses the stats.sum(numbers) function to compute the sum of the values stored in the data list and prints out the sum.
If you run your analyze.py program using the hurricanes.csv file and the index of the second column, you should get a sum of 103 as the answer.
When you are done with the lab exercises, begin the project.