Positional and Keyword arguments¶

Function arguments: Introduction¶

Python offers three main ways to control function arguments, these are:

positional only
positional or keyword
keyword only

Here is a short overview of what we will cover here:

def example(pos_only /, pos_or_kwd, *, kwd_only)
    # pos_only is positional only, order matters and cannot be passed via pos_only=1
    # pos_or_kwd like traditional args (without /, *) can be specified by order (pos) or keyword explicitly
    # kwd_only must be specified as kwd_only=10
    ...

example(1, 2, kwd_only=3)  # Valid
example(1, pos_or_kwd=2, kwd_only=3)  # Valid
example(1,2,3) # Invalid (kwd_only must be keyword only)
example(pos_only=1, 2, kwd_only=3)  # Invalid (pos_only cannot be specified via keyword)

Positional Only Arguments¶

Arguments specified before a / in a function signature, indicate the argument should only be passed as a positional argument (e.g without a keyword and order matters):

def one(a, b, c, /):
    ...

In the above example, a,b,c must be passed explicitly without a keyword argument.

Positional OR Keyword Arguments¶

By default, when either / or * is omitted from a function signature, arguments are considered positional_or_keyword args, which means passing them positionally in order is completely valid and passing them via keyword explicitly is also valid:

def one(a,b,c):
    ...

one(1,2,3)  # Valid
one(a=10, b=20, c=30)  # Valid

Keyword only Arguments¶

Arguments which follow a trailing * are considered keyword only arguments. These must be explicitly passed via a keyword= call:

def one(a, b, /, * c):
    ...

one(10, 20, c=100)  # Valid, c= must be explicit
one(10, 20, 30)  # Invalid, TypeError: one takes two positional arguments by 3 were given