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A function is a piece of code that accomplishes a specific task. It usually takes in argument(s) as input, does some processing, and returns a result. Once a function is written, it can be called over and over again.


Functions take advantage of abstraction: we don't care how the function carries out its task, we just care that it gives us a correct result.

In addition, functions allow us to reuse code instead of rewriting it to work with different values.

Pure function

A pure function produces no effects other than returning a value. Its return value depends only on the inputs. Given the same arguments, it always returns the same value.


  • add always returns the sum of the arguments and has no side effects.
  • square

Non-pure function

A non-pure function or impure function produces a side effect (e.g., mutation, printing to screen). Its return value may depend on external state.



The print function displays a value on the screen and returns None.

Functions that modify a variable in another frame

The following get_a() function modifies the variable a in the global frame:

a = 0
def get_a():
    global a
    a += 1
    return a - 1

The following tick function modifies the variable n in the countdown frame:

def countdown(n):
    def tick():
        nonlocal n
        n -= 1
        return n
    return tick

Functions that mutate an object in another frame

The following add_square function mutates lst in the global frame:

lst = []
def append_square(n):