Functions Parameters and Arguments

Learning objective: By the end of this lesson, students will be able to declare a function with parameters and call a function using arguments.

Parameters

In Python, parameters are variables that act as placeholders for values passed into a function.

# text is a parameter
def print_banner(text):
    print("=======================")
    print(text)
    print("=======================")

The function above accepts one parameter, text. When the function is called, a value must be provided for this parameter.

Arguments

An argument is the actual value you pass into a function when you call it. Just like in JavaScript, arguments are used to provide input to a function.

# "hello, world" is an argument
print_banner("hello, world")

In this example, the value "hello, world" is passed to the function as the argument for text.

Unlike JavaScript, Python requires that you provide the correct number of arguments when calling a function. If you don’t, Python will raise an error.

print_banner()

# Error:
# TypeError: print_banner() missing 1 required positional argument: 'text'

Accepting a variable number of arguments

In JavaScript, we can use rest parameters to allow a function to accept any number of arguments:

const sum = (...nums) => {
  total = 0;

  nums.forEach((num) => {
    total += num;
  });

  return total;
};

console.log(sum(1, 5, 10));
// prints: 16

Python’s `*args` syntax

In Python, you can use the * (asterisk) before a parameter name to accept a variable number of positional arguments. This is commonly written as *args.

def sum(*args):
    print(type(args))
    # prints: <class 'tuple'>

    total = 0
    for arg in args:
        total += arg

    return total

print(sum(1, 5, 10))
# prints: 16

The args parameter becomes a tuple that contains all the values passed to the function. The name args is a common convention, but you can technically use any name after the *.

Mixing required and variable arguments

If your function has both required and variable arguments, the required ones must come first.

def sum(greeting, *args):
    print(greeting)
    # prints: Hello, friend

    total = 0
    for arg in args:
        total += arg

    return total

print(sum("Hello, friend", 1, 5, 10))
# prints:
# Hello, friend
# 16