07 Checkpoint: Lists

Purpose

Reinforce your understanding that Python numbers are immutable and Python lists are mutable.

Problem Statement

Within a Python program, variables of type boolean, integer, float, and string are immutable which means after a value is assigned to one of those variables, the value cannot be changed. When a computer executes a line of code that appears to change a boolean, integer, float, or string, the computer doesn’t change the old value of the variable but instead creates a new value and assigns the new value to the variable. For example, on line 4 in example 1 the int function creates and returns an integer value that the computer assigns to the variable x. Line 5 appears to change the value of x by adding 5 to it, but what really happens is the computer creates a new integer value that is 5 larger than the old value and assigns the new value to the variable x.



# Example 1

def main():
    x = int(input("Please enter an integer: "))
    x += 5
    print(x)


    
Unlike booleans, integers, floats, and strings, within a Python
    program, lists are mutable which means the value of a
    list can be changed. When a computer executes a line of code that
    adds, updates, or removes an element from a list, the computer
    doesn’t create a new list but instead changes the list. For example,
    line 4 in
    example 2 creates a list with two elements.
    Line 5 changes the
    list by adding the element "silk" to the list.




# Example 2

def main():
    list1 = ["denim", "linen"]
    list1.append("silk")
    print(list1)

    


    

    
Assignment

    
During this checkpoint, you will examine and run a Python program
    that passes both a number and a list to function. Do the following:


    
    
        
  1. Download and save the
        modify_args.py Python file
        and then open it in VS Code.
    2. 

        
  2. Run the modify_args.py program on your
        computer. Examine the code and the output and answer the
        following three questions:
        
      
            
    1. In main, there are two local variables
            named x and lx. At line 15, both
            of these local variables are passed to the
            modify_args function. After the call to
            modify_args finished, which of
            main’s two local variables did the
            modify_args function change?
            
        
                
      1. x
        2. 
                
      2. lx
        3. 
                
      3. both of them
        4. 
                
      4. neither of them
        5. 
            
      2. 

            
    2. Why is the modify_args function NOT
            able to change the value of main’s local
            variable x?
            
        
                
      1. Because x is a number and numbers in
                Python are immutable
        2. 
                
      2. Because x is a list and lists in
                Python are mutable
        3. 
            
      3. 

            
    3. Why is the modify_args function able
            to change the value of main’s local variable
            lx?
            
        
                
      1. Because lx is a number and numbers
                in Python are immutable
        2. 
                
      2. Because lx is a list and lists in
                Python are mutable
        3. 
            
      4. 
        
    3. 
    

    


    

    
Sample Run


> python modify_args.py
main()
    Before calling modify_args(): x 5  lx [7, -2]
    modify_args(n, alist)
        Before changing n and alist: n 5  alist [7, -2]
        After changing n and alist:  n 6  alist [7, -2, 4]
    After calling modify_args():  x 5  lx [7, -2, 4]

    


    

    
Submission

    
When complete, report your progress in the associated I‑Learn
    quiz.