Normal arguments vs. keyword arguments

Question

How are \"keyword arguments\" different from regular arguments? Can\'t all arguments be passed as name=value instead of using positional syntax?

Answer 1

There is one last language feature where the distinction is important. Consider the following function:

def foo(*positional, **keywords):
    print "Positional:", positional
    print "Keywords:", keywords

The *positional argument will store all of the positional arguments passed to foo(), with no limit to how many you can provide.

>>> foo('one', 'two', 'three')
Positional: ('one', 'two', 'three')
Keywords: {}

The **keywords argument will store any keyword arguments:

>>> foo(a='one', b='two', c='three')
Positional: ()
Keywords: {'a': 'one', 'c': 'three', 'b': 'two'}

And of course, you can use both at the same time:

>>> foo('one','two',c='three',d='four')
Positional: ('one', 'two')
Keywords: {'c': 'three', 'd': 'four'}

These features are rarely used, but occasionally they are very useful, and it's important to know which arguments are positional or keywords.

Answer 2

I'm surprised no one has mentioned the fact that you can mix positional and keyword arguments to do sneaky things like this using *args and **kwargs (from this site):

def test_var_kwargs(farg, **kwargs):
    print "formal arg:", farg
    for key in kwargs:
        print "another keyword arg: %s: %s" % (key, kwargs[key])

This allows you to use arbitrary keyword arguments that may have keys you don't want to define upfront.

Answer 3

Using keyword arguments is the same thing as normal arguments except order doesn't matter. For example the two functions calls below are the same:

def foo(bar, baz):
    pass

foo(1, 2)
foo(baz=2, bar=1)

Answer 4

Using Python 3 you can have both required and non-required keyword arguments:

---

Optional: (default value defined for param 'b')

def func1(a, *, b=42):
    ...
func1(value_for_a) # b is optional and will default to 42

---

Required (no default value defined for param 'b'):

def func2(a, *, b):
    ... 
func2(value_for_a, b=21) # b is set to 21 by the function call
func2(value_for_a) # ERROR: missing 1 required keyword-only argument: 'b'`

This can help in cases where you have many similar arguments next to each other especially if they are of the same type, in that case I prefer using named arguments or I create a custom class if arguments belong together.