How do you define and call a function in Python?

Functions are defined with the def keyword, a name, parentheses for parameters, a colon, and an indented body. They are first-class objects — you can assign them to variables, pass them as arguments, and return them from other functions.

def greet(name, greeting='Hello'):
    """Return a personalised greeting string."""
    return f'{greeting}, {name}!'

print(greet('Alice'))           # Hello, Alice!
print(greet('Bob', 'Hi'))       # Hi, Bob!
print(greet(greeting='Hey', name='Carol'))  # keyword args

The string literal immediately after the def line is the docstring — accessible via help() or function.__doc__. Always write docstrings for anything that will be reused.

Parameter types to know for interviews:

• Positional: matched left to right.
• Default: greeting='Hello' — must come after positional args.
• *args: captures any number of extra positional arguments as a tuple.
• **kwargs: captures any number of keyword arguments as a dictionary.
• Keyword-only: parameters after a bare * must be passed by name.

def summary(*args, separator=', '):
    return separator.join(str(a) for a in args)

print(summary(1, 2, 3))             # 1, 2, 3
print(summary(1, 2, 3, separator='-'))  # 1-2-3

A function without an explicit return statement returns None. Returning multiple values looks like separate values but Python actually returns a tuple: return x, y is return (x, y).

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