6.4: Parameters

Learning Objectives

By the end of this section you should be able to

• Identify a function's arguments and parameters.
• Describe how mutability affects how a function can modify arguments.

Arguments and parameters

What if a programmer wants to write a function that prints the contents of a list? Good practice is to pass values directly to a function rather than relying on global variables. A function argument is a value passed as input during a function call. A function parameter is a variable representing the input in the function definition. Note: The terms "argument" and "parameter" are sometimes used interchangeably in conversation and documentation.

Checkpoint: Global and local variables in a program with a function

Concepts in Practice: Arguments and parameters

Consider the following:

1	def print_welcome(name):
2	print(f"Welcome {name}!")
3
4	username = int(input("Enter new username: "))
5	print_welcome(username)

Concepts in Practice \(\PageIndex{1}\): Arguments and parameters

Which is an argument?

1. name
2. username
3. name and username

Answer

b. username is the argument passed to print_welcome(). name is print_welcome()'s parameter that is assigned with username's value.

Concepts in Practice \(\PageIndex{2}\): Arguments and parameters

Which is a parameter?

1. name
2. username
3. name and username

Answer

a. name is the only parameter defined with print_welcome(). username is the argument passed on line 5's function call.

Concepts in Practice \(\PageIndex{3}\): Arguments and parameters

What is the scope of name?

1. print_welcome() only
2. whole program

Answer

a. name is a parameter that exists only within a call to print_welcome().

Concepts in Practice \(\PageIndex{4}\): Arguments and parameters

What would happen if username was changed to name on lines 4 and 5?

1. same output
2. error/wrong output

Answer

a. The program would run correctly. Arguments and parameters can have the same name, and a local variable called name would still be created in print_welcome(). Scope can be confusing with variables of the same name.

Multiple arguments and parameters

Functions can have multiple parameters. Ex: A function uses two parameters, length and width, to compute the square footage of a room. Function calls must use the correct order and number of arguments to avoid undesired behavior and errors (unless using optional or keyword arguments as discussed later).

Example 6.1: Using multiple arguments in a function call

    def print_div(op_1, op_2):
      """ Prints division operation """
      print(f"{op_1}/{op_2} = {op_1/op_2}")

    num_1 = 6
    num_2 = 3
    print_div(num_1, num_2) # Prints "6/3 = 2.0"
    print_div(num_2, num_1) # Prints "3/6 = 0.5"
    print_div(num_1) # Error: Missing argument: op_2

Concepts in Practice: Multiple arguments and parameters

Consider the following:

    def calc_distance(x1, y1, x2, y2):
      dist = math.sqrt((x2-x1)**2 + (y2-y1)**2)
      print(dist)

    p1_x =int(input("Enter point 1's x: "))
    p1_y =int(input("Enter point 1's y: "))
    p2_x =int(input("Enter point 2's x: "))
    p2_y =int(input("Enter point 2's y: "))
    calc_distance(p1_x, p1_y, p2_x, p2_y)

Concepts in Practice \(\PageIndex{5}\): Multiple arguments and parameters

Which is an argument?

1. p1_x
2. x1

Answer

a. p1_x is the first argument used in line 9's function call. x1 is defined as the first parameter of calc_distance().

Concepts in Practice \(\PageIndex{6}\): Multiple arguments and parameters

Which is a parameter?

1. p1_y
2. y1

Answer

b. y1 is defined as calc_distance()'s second parameter. p1_y is the second argument on line 9's function call.

Concepts in Practice \(\PageIndex{7}\): Multiple arguments and parameters

What would be the value of x2 for the function call, calc_distance(2, 4, 3, 6)?

1. 2
2. 4
3. 3
4. 6
5. Error

Answer

c. 3 is a literal passed as the third argument, which corresponds to x2, the third parameter.

Modifying arguments and mutability

In Python, a variable is a name that refers to an object stored in memory, aka an object reference, so Python uses a pass-by-object-reference system. If an argument is changed in a function, the changes are kept or lost depending on the object's mutability. A mutable object can be modified after creation. A function's changes to the object then appear outside the function. An immutable object cannot be modified after creation. So a function must make a local copy to modify, and the local copy's changes don't appear outside the function.

Programmers should be cautious of modifying function arguments as these side effects can make programs difficult to debug and maintain.

Example 6.2: Converting temperatures

What are the values of weekend_temps and type after convert_temps() finishes?

    def convert_temps(temps, unit):
      if unit == "F":
        for i in range(len(temps)):
          temps[i] = (temps[i]-32) * 5/9
        unit = "C"
      else:
        for i in range(len(temps)):
          temps[i] = (temps[i]*9/5) + 32
        unit = "F"

    # Weekend temperatures in Fahrenheit.
    wknd_temps = [49.0, 51.0, 44.0]
    deg_sign = u"\N{DEGREE SIGN}" # Unicode
    metric = "F"

    # Convert from Fahrenheit to Celsius.
    convert_temps(wknd_temps, metric)
    for temp in wknd_temps:
      print(f"{temp:.2f}{deg_sign}{metric}", end=" ")

The output is 9.44°F 10.56°F 6.67°F. type was changed to "C" in the function but didn't keep the change outside of the function. Why is the list argument change kept and not the string argument change? (Hint: A list is mutable. A string is immutable.)

Checkpoint: Exploring a faulty function

Concepts in Practice \(\PageIndex{8}\): Mutability and function arguments

In convert_temps(), wknd_temps and temps refer to ________ in memory.

1. the same object
2. different objects

Answer

a. A list is mutable, so the object referred to by wknd_temps and temps is modified without needing a local copy.

Concepts in Practice \(\PageIndex{9}\): Mutability and function arguments

After unit is assigned with "C", metric and unit refer to ________ in memory.

1. the same object
2. different objects

Answer

b. A string is immutable, so convert_temps() makes a new object for unit to refer to.

Concepts in Practice \(\PageIndex{10}\): Mutability and function arguments

deg_sign is a string whose value cannot change once created. deg_sign is ________.

1. immutable
2. mutable

Answer

a. An immutable object's value cannot be changed.

Concepts in Practice \(\PageIndex{11}\): Mutability and function arguments

On line 16, unit ________.

1. refers to an object with the value "C"
2. does not exist

Answer

b. unit no longer exists after convert_temps() finishes and returns to line 14.

Try It: Printing right triangle area

Write a function, print_area(), that takes in the base and height of a right triangle and prints the triangle's area. The area of a right triangle is 
\(\frac{bh}{2}\), where \(b\) is the base and \(h\) is the height.

Given input:

    3
    4

The output is:

    Triangle area: 6.0

Interactive Code

# Define print_area()

base = float(input())
height = float(input())
print_area(base, height)

Input

3
4

Answer

def print_area(b, h):
area = (b * h) / 2.0
print("Triangle area:", area)

base = float(input())
height = float(input())
print_area(base, height)

Try It: Curving scores

Write a function, print_scores(), that takes in a list of test scores and a number representing how many points to add. For each score, print the original score and the sum of the score and bonus. Make sure not to change the list.

Given function call:

    print_scores([67, 68, 72, 71, 69], 10)

The output is:

    67 would be updated to 77
    68 would be updated to 78
    72 would be updated to 82
    71 would be updated to 81
    69 would be updated to 79

Interactive Code

# Define print_scores()

print_scores([67, 68, 72, 71, 69], 10)
print() # Print newline
print_scores([67, 68, 72, 71, 69], 5)

 

Answer

def print_scores(scores, bonus):
for score in scores:
print(score, "would be updated to", score + bonus)

print_scores([67, 68, 72, 71, 69], 10)
print() # Print newline
print_scores([67, 68, 72, 71, 69], 5)