If you assign them in the call of the function you can pre-empt which parameter you are passing in.

def foo( a, b=None, c=None):
    print("{},{},{}".format(a,b,c))

>>> foo(4) 
4,None,None
>>> foo(4,c=5)
4,None,5

There's nothing wrong with just listing one parameter per line.

def many_argument_function(
        required1,
        required2,
        optional1=1,
        optional2=2,
        optional3=None,
        optional4="Test",
        optional5="g",
        optional6=2,
        optional7=(0, 255, 0),
        optional8=0.5
    ):

    """
    This function does some stuff.
    """

    # Do stuff

PEP 8 endorses this style of indentation, and although its own example puts multiple parameters on a single line, I find it much easier to scan a large list of parameters if each is on its own line.

By default, the first argument passed to the function will be assigned to the first parameter. If you want to assign the first argument to the second (or n:th) parameter, you must give it as keyword argument. See, for example

In [19]: def myfunc(x='X', y=5):
    ...:     print(x,y)
    ...:
    ...:

# No arguments -> Using default parameters
In [20]: myfunc()
X 5

# Only one positional argument -> Assigned to the first parameter, which is x
In [21]: myfunc(100)
100 5

# One keyword argument -> Assigned by name to parameter y
In [22]: myfunc(y=100)
X 100

The type of the arguments do not matter, but the order you used in the function definition.

Notes on terminology

• By parameter, I mean the variable in the function definition
• By argument, I mean the actual value passed to the function.

I would recommend you read into the argparse standard library. It should have more than you need to start parsing command line arguments!

from argparse import ArgumentParser

parser = ArgumentParser()
parser.add_argument('-i', '--input', required=True, type=str, help="...")
...
args = parser.parse_args()

print(args.input)