Python Function Parameter Matching Model General Rule keyword only Parameter Details

Python3 has a more general collation for function parameters, the Python3 keyword-only parameter, which is a parameter that must be passed by keyword only and not populated by a position parameter.This rule is useful for dealing with more than one parameter for a person.

keyword-only

kword_only(1, 2, 3, c=4)
print('-' * 20)
kword_only(a=1, c=3)

Example results:

1 (2, 3) 4
--------------------
1 () 3

Parameters after *args need to be passed in the call using a keyword or an exception will be thrown.

def kword_only(a, *b, c):
  print(a, b, c)
kword_only(1, 2, 3)

Exceptional results:

kword_only(1, 2, 3)
TypeError: kword_only() missing 1 required keyword-only argument: 'c'

We can also use a * character in the parameter list to indicate that the function does not accept a parameter list of variable lengths, but requires that the parameters after * be passed using keyword parameters.

def kword_only(a, *, b, c):
  print(a, b, c)
kword_only(1, b=2, c=3)
print('-'*20)
kword_only(b=2, c=3, a=1)

Example results:

1 2 3
--------------------
1 2 3

We can see that a can be passed using location parameters, but b, c must be passed through keyword parameters or an exception will be thrown.

def kword_only(a, *, b, c):
  print(a, b, c)
kword_only(1, 2, 3)

Exceptional results:

kword_only(1, 2, 3)
TypeError: kword_only() takes 1 positional argument but 3 were given

Of course, we can use the default value for the keyword-only parameter so that the a in this example can still be passed by location or keyword parameters, while b and c are optional, and of course, keyword parameters are still used to pass if necessary.

def kword_only(a, *, b=2, c=3):
  print(a, b, c)
kword_only(1)
print('-'*20)
kword_only(a=1)
print('-'*20)
kword_only(b=22, c=33, a=11)

Example results:

1 2 3
--------------------
1 2 3
--------------------
11 22 33

summary