Introduction

A function packages a useful operation under a name. Parameters supply input, and a return value carries the result back to the caller. Keep printing separate from calculation so you can reuse and test the function.

Syntax & example

def double(number):
    return number * 2

print(double(7))

Expected output

14

Common mistakes

  • Using a variable before assigning a value to it.
  • Mixing text and numbers without converting deliberately.
  • Skipping an empty input or a boundary value when testing.

Practice

Write a function that doubles its input.

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