Functions

Introduction

a function is introduced with def and a freely selectable function name

def functionname(parameters):
    statement

a function returns one or more return statements if the return is left empty "None" is returned the same if there is no return statement

def helloWorld(name):
    return "Hello " + name + " !"

print(helloWorld("John"))
# Hello John !

f = "John"
f = helloWorld(f)
print(f)
# Hello John !

Default Value

you also have the option to assign a default value to a parameter

def helloWorld(name="World"):
    return "Hello " + name + " !"

helloWorld("John")
# 'Hello John !'

helloWorld()
# 'Hello World !'

def add(value1=4, value2=2):
    return value1 + value2

add(4)
#6

def add(value1=4, value2=2):
    return value1 + value2

add(4, 4)
#8

However, there is a problem, if we try to use only the second parameter without defining the first one since the arguments are passed in order.

If only one value is passed it is automatically the first one, so in our case value1

Key-Word-Parameters

we can easily solve the problem described in the previous chapter and at the same time improve the readability of the program

def add(value1=4, value2=2):
    return value1 + value2

add(value1=5)
#7

def add(value1=4, value2=2):
    return value1 + value2

add(value2=12)
#16

def add(value1=4, value2=2):
    return value1 + value2

add(value2=12, value1=33)
#45

Docstring

docstrings are used for documentation they are stored in the __doc__ attribute and do not influence the execution of the function

they are defined directly under the def line of a function

def heyWorld(name="World"):
    """returns Hey to the parameter name"""
    return "Hey " + name + " !"

heyWorld("John")
# 'Hey John !'

docstrings can also be placed at the beginning of a Python file

"""
Voluptate sit proident dolore reprehenderit
enim sunt pariatur irure occaecat anim tempor.
"""


def heyWorld(name="World"):
    """returns Hey to the parameter name"""
    return "Hey " + name + " !"

heyWorld("John")
# 'Hey John !'

Multiple Return Values

a function can only output one object but also lists or dictionaries for example

def fib_interv(x):
    if x < 0:
        return -1
    old, new = 0,1
    while True:
        if new < x:
            old, new = new, old+new
        else:
            return (old, new)

for i in (0, 1, 2, 3, 4, 5, 7, 8, 9, 12, 13, 14):
    print(i, fib_interv(i))

# 0 (0, 1)
# 1 (0, 1)
# 2 (1, 2)
# 3 (2, 3)
# 4 (3, 5)
# 5 (3, 5)
# 7 (5, 8)
# 8 (5, 8)
# 9 (8, 13)
# 12 (8, 13)
# 13 (8, 13)
# 14 (13, 21)

Local & Global Variables

variables are, by default, local in a functional but you can also read global variables within a function

def f():
    print(gh)

gh = "Test123"
f()
# Test123

def f():
    gh = "no Test"
    print(gh)

gh = "Test123"
f()
# no Test
print(gh)
# Test123

the last example shows very easily that the variable gh in f is local

def f():
    print(gh)
    gh = "no Test"
    print(gh)

gh = "Test123"
f()
# UnboundLocalError:
print(gh)

the variable gh is now ambiguous in f()

if you want to write to a variable within a function, you have to define it explicitly as global

def f():
    global gh
    print(gh)
    gh = "no Test"
    print(gh)

gh = "Test123"
f()
# Test123
# no Test

print(gh)
# no Test

gh
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# NameError: name 'gh' is not defined

def f():
    global gh
    gh = 123

gh
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# NameError: name 'gh' is not defined

f()

gh
# 123

Command-Line Arguments

when starting a Python program you can transfer so-called start arguments

these are separated from each other by a space

they are available inside the Python script in a list (sys.argv) in addition to the parameters the file name is stored in the list at the first position (sys.argv[0])

import sys

for Argument in sys.argv:
    print(Argument)

Variadic Functions

variadic functions allow flexibility in defining functions where the number of parameters is not fixed at definition

for this, we use "*"

def print_args(*args):
    for argument in args:
        print(argument)

print_args(123, "Lorem bli bla blu", "more bli bla")
# 123
# Lorem bli bla blu
# more bli bla

def print_args(number, *text):
    print(number, text)

print_args(123, "Lorem bli bla blu", "more bli bla")
# 123 ('Lorem bli bla blu', 'more bli bla')

* in function calls

when using a * in the function call, an argument is unpacked this can be useful when you want to separate the contents of a list or tuple

def print_args(number1, number2, number3):
    print(number1, number2, number3)

list1 = [3, 6, 9]
print_args(*list1)
# 3 6 9

if you want to pass lists you have to unpack them before you pass them otherwise, this list is only summarized in a tuple

def print_args(number, *text):
    print(number, text)

list1 = [3, 4, 2, 6, 9]
print_args(123, list1)
# 123 ([3, 4, 2, 6, 9],)

def print_args(number, *text):
    print(number, text)

list1 = [3, 4, 2, 6, 9]
print_args(123, *list1)
# 123 (3, 4, 2, 6, 9)

Any Keyword Parameter

With ** we can generate any number of key-word parameters

which are then stored in dictionaries

def gh(**args):
    print(args)

gh()
# {}

gh(de="German", en="English")
# {'de': 'German', 'en': 'English'}

as in the last article, we can also use ** in the function call

def f(a, b, c, h):
    print(a, b, c, h)

d = {'a':'aaaaaaa', 'b':'bbbbbbb', 'c':'ccccc', 'h':'hhhhh'}
f(**d)
# aaaaaaa bbbbbbb ccccc hhhhh