How to pass optional arguments in a python function ?

Examples of how to pass optional arguments in a python function?

Basic python function

Let's start by creating a simple function in python:

def function_test(a,b):
    print(a,b)
    return None

myfunc(1,2)

gives

1 2

However if we try to pass a third variable

myfunc(1,2,7)

will generate an error message

too many values to unpack (expected 2)

def myfunc(a,b):
    print('function print', a,b)
    return a, b, a+b

output = myfunc(1,2)

print(output[0])
print(output[1])
print(output[2])

gives

1 2

1
2
3

Passing optional arguments with *args

def myfunc(a,b, *args):
    output = a + b
    for ar in args:
        print('Optional argument:', ar)
        output += ar
    return output

myfunc(1,2,7)

gives

Optional argument: 7

10

Another example

def myfunc(a,b, *args):
    output = a + b
    for ar in args:
        print('Optional argument:', ar)
        output += ar
    return output

myfunc(1,2,7,10,20)

gives

Optional argument: 7
Optional argument: 10
Optional argument: 20

40

Pass a list

def myfunc(a,b, *args):
    output = a + b
    for ar in args:
        print('Optional argument:', ar)
    return output

l = [7,10,20]

myfunc(1,2,l)

gives

Optional argument: [7, 10, 20]

Pass a dictionary

def myfunc(a,b, *args):
    output = a + b
    for ar in args:
        print('Optional argument:', ar)
    return output

d = {'bob':42, 'emma':47}

myfunc(1,2,d)

gives

Optional argument: {'bob': 42, 'emma': 47}

Passing optional arguments with **kwargs

Another possibility is to use **kwargs to name optional variables directly:

def myfunc(a,b, **kwargs):
    print('kwargs: ', kwargs)
    return None

myfunc(1,2,lang='fr')

gives

{'lang': 'fr'}

In the function to test if there is an optional argument like 'lang' we can use:

if 'lang' in kwargs:

to get the value of the 'lang' variable:

kwargs['lang']

Example

def myfunc(a,b, **kwargs):
    print(kwargs)
    if 'lang' in kwargs:
        print("found it")
        print( kwargs['lang'] )
    return a + b

myfunc(1,2,lang='fr', )

gives

{'lang': 'fr'}
found it
fr

Global variable

In python we can also define global variables

global c

c = 123

def myfunc(a,b):
    print(c)
    return a + b

myfunc(1,2)

gives

123

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