Function Parameters

Function Parameters#

The two birthday functions in the previous section were nearly identical. The only difference was the name. Parameters let us pass data into a function, eliminating duplication.

Defining a Parameter#

Add a parameter name inside the parentheses of the def line:

def happy_birthday(person):
    print("Happy Birthday to you!")
    print("Happy Birthday to you!")
    print(f"Happy Birthday, dear {person}.")
    print("Happy Birthday to you!")

The parameter person is a local variable that receives a value when the function is called.

Calling with an Argument#

When you call a function with a parameter, you supply an argument — the value to pass in:

>>> def happy_birthday(person):
...     print("Happy Birthday to you!")
...     print("Happy Birthday to you!")
...     print(f"Happy Birthday, dear {person}.")
...     print("Happy Birthday to you!")
...
>>> happy_birthday("Emily")
Happy Birthday to you!
Happy Birthday to you!
Happy Birthday, dear Emily.
Happy Birthday to you!

One function, but different results for different arguments. Edit the call to happy_birthday("Andre") and run it again to see.

How it Works#

When Python sees happy_birthday("Emily"), it:

  1. Creates a local variable person inside the function.

  2. Assigns it the value "Emily".

  3. Executes the body of the function.

  4. Discards person when the function finishes.

The next call happy_birthday("Andre") does the same thing with "Andre".

Parameters vs. Arguments#

The terms parameter and argument are often used interchangeably, but technically:

  • A parameter is the name listed in the def line: person.

  • An argument is the value supplied in the call: "Emily".