Function

A function is a named block of code that you can call whenever you need it. It accepts input values, called parameters or arguments, does some work and may return a value. In some languages the same idea is called a method or a procedure.

Why it matters

Functions let you write logic once and reuse it, break large problems into small understandable pieces and test each piece separately. A function that does one thing and has a clear name makes a program much easier to read and to debug.

Example

def discounted_price(price, percent):
    return round(price * (1 - percent / 100), 2)

print(discounted_price(2000, 15))   # 1700.0

price and percent are parameters; 2000 and 15 are the arguments passed in the call; return sends the result back.

Common mistakes

• Writing functions that do several unrelated things, which makes them hard to name and test.
• Changing global state inside a function so the same call gives different results at different times.
• Using a mutable default argument, such as def f(items=[]) in Python, which is shared between calls.

Related

• Variable
• Recursion
• Functional programming
• Unit test

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