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.

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).

    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

    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)

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.

    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=" ")