Use def to create new functions

def add(x, y):
    print("x is {} and y is {}".format(x, y))
    return x + y  # Return values with a return statement

keyword parameters

# Calling functions with parameters
add(5, 6)  # => prints out "x is 5 and y is 6" and returns 11

# Another way to call functions is with keyword arguments
add(y=6, x=5)  # Keyword arguments can arrive in any order.

multiple return values

# Returning multiple values (with tuple assignments)
def swap(x, y):
    return y, x  # Return multiple values as a tuple without the parenthesis.
                 # (Note: parenthesis have been excluded but can be included)

x = 1
y = 2
x, y = swap(x, y)     # => x = 2, y = 1

variable arguments

# You can define functions that take a variable number of
# positional arguments
def varargs(*args):
    return args

varargs(1, 2, 3)  # => (1, 2, 3)

# You can define functions that take a variable number of
# keyword arguments, as well
def keyword_args(**kwargs):
    return kwargs

# Let's call it to see what happens
keyword_args(big="foot", loch="ness")  # => {"big": "foot", "loch": "ness"}


# You can do both at once, if you like
def all_the_args(*args, **kwargs):
    print(args)
    print(kwargs)
"""
all_the_args(1, 2, a=3, b=4) prints:
    (1, 2)
    {"a": 3, "b": 4}
"""

# When calling functions, you can do the opposite of args/kwargs!
# Use * to expand args (tuples) and use ** to expand kwargs (dictionaries).
args = (1, 2, 3, 4)
kwargs = {"a": 3, "b": 4}
all_the_args(*args)            # equivalent: all_the_args(1, 2, 3, 4)
all_the_args(**kwargs)         # equivalent: all_the_args(a=3, b=4)
all_the_args(*args, **kwargs)  # equivalent: all_the_args(1, 2, 3, 4, a=3, b=4)

first class functions and closures

# Python has first class functions
def create_adder(x):
    def adder(y):
        return x + y
    return adder

add_10 = create_adder(10)
add_10(3)   # => 13

# Closures in nested functions:
# We can use the nonlocal keyword to work with variables in nested scope which shouldn't be declared in the inner functions.
def create_avg():
    total = 0
    count = 0
    def avg(n):
        nonlocal total, count
        total += n
        count += 1
        return total/count
    return avg
avg = create_avg()
avg(3)  # => 3.0
avg(5)  # (3+5)/2 => 4.0
avg(7)  # (8+7)/3 => 5.0

anonymous functions

(lambda x: x > 2)(3)                  # => True
(lambda x, y: x ** 2 + y ** 2)(2, 1)  # => 5

doc comments

Adding a doc comment in python involves writing a string literal on the first line of your function.

def greet():
    """Display a simple greeting."""
    print("Hello")

greet()