Functions in Python

A function is defined with def name(parameters): and returns a value with return; without a return statement it returns None. Arguments can be positional or keyword (f(b=1, a=5)), and a positional argument may not follow a keyword argument. Parameters can have defaults (b=2). *args collects extra positional arguments into a tuple and **kwargs collects extra keyword arguments into a dict; f(*t) unpacks a tuple into separate arguments.

Returning several values, as in return a, b, actually returns one tuple. Variables created by assignment inside a function are local, so they do not change a global of the same name unless declared with global. Functions can call themselves; recursion needs a base case, for example fact(n) = 1 if n <= 1 else n * fact(n - 1).

A documentation string placed first in the body becomes the function's __doc__.

Exam trap: default values are evaluated once, when def runs. A mutable default such as bag=[] is shared by every call that omits it, so values accumulate. Use None and create the list inside.