Functions

Built-in, module and user-defined functions: parameters and arguments, default and keyword arguments, return values, the flow of execution, and local and global scope.

Key ideas

def only defines a function; its body runs each time the function is called, and the call then returns to the line after it.
Arguments are matched by position, keyword arguments by name; a positional argument cannot follow a keyword argument, and each parameter gets exactly one value.
A default value is used only when that argument is not passed; parameters with defaults come after those without.
return sends a value back and ends the function; with no return a function gives None; return a, b gives a tuple.
A name given a value inside a function is local and hides a global of the same name; global x lets the function change the x outside.
A list passed to a function is the same object, so changes made to it stay; giving a parameter a new value (n = n + 5) does not change the caller’s variable.

Remember

Kinds of function	built-in: len(), int(), max(); from a module: math.sqrt(); user-defined: written with def
Defaults	def area(l, b=2): return l * b print(area(5), area(5, 3)) # 10 15
Keyword arguments	f(1, c=3, b=2) is fine; f(a=1, 2) is a SyntaxError
Two values back	def mm(a, b): return a + b, a * b, so mm(2, 3) gives (5, 6)
No return	r = f() for a function that only prints: r is None
Local and global	x = 9 inside a function makes a new local x; after global x the function changes the outer x

Watch out

Thinking a function runs where it is defined: it runs only when it is called.
Putting a keyword argument before a positional one, or giving one parameter two values.
Using a function that only prints as if it gave a value: it gives None.
Expecting a change to a number parameter to change the caller’s variable.

Worked example

What does this print?
def update(L, n):
    for i in range(len(L)):
        if L[i] % 2 == 0:
            L[i] = L[i] + 5
    n = n + 5

nums = [27, 18, 17, 13]
m = 10
update(nums, m)
print(nums, m)

Working: the list is the same object inside the function, so changes to it stay; n is a new local name

Answer: [27, 23, 17, 13] 10