In Python, function parameters can be Positional-Only, Positional-or-Keyword, or Keyword-Only. Let’s consider all three cases in the example function f:

def f(pos1, pos2, /, pos_or_kwd1, pos_or_kwd2=None, *, kwd1, kwd2):
    """
    pos1, pos2 - are Positional-Only
    pos_or_kwd1, pos_or_kwd2 - are Positional-or-Keyword
    kwd1, kwd2 - are Keyword-Only
    """
    pass

The rules for defining these parameters are straightforward:

1. Everything before the / symbol is Positional-Only parameters.
2. Everything after the * symbol is Keyword-Only parameters.
3. What’s between / and * can be passed as both Positional and Keyword.
4. If there is no / symbol, there are no Positional-Only parameters.
5. If there is no * symbol, there are no Keyword-Only parameters.
6. If / and * are not present, arguments can be passed to a function by position or by keyword.

Here are some valid calls to this function:

f(1, 2, 'a', kwd1='c', kwd2='d')

f(1, 2, 'a', 'b', kwd1='c', kwd2='d')

f(1, 2, 'a', pos_or_kwd2='b', kwd1='c', kwd2='d')

f(1, 2, pos_or_kwd1='a', kwd1='c', kwd2='d')

Note that the first two arguments are Positional-Only, and we cannot pass them as pos1=1. Meanwhile, the last two arguments are Keyword-Only, and we cannot omit the use of the name when passing the argument value.

Here are some invalid calls to the function f that will raise an exception:

f(1, 2, 'a', 'b', 'c', kwd2='d')  # raises... 
# TypeError: f() takes from 3 to 4 positional arguments but 5 positional arguments (and 1 Keyword-Only argument) were given

f(pos1=1, pos2=2, pos_or_kwd1='a', kwd1='c', kwd2='d')  # raises... 
# TypeError: f() got some Positional-Only arguments passed as keyword arguments: 'pos1, pos2'

The logic around optional values for Positional-Only parameters remains the same as for Positional-or-Keyword parameters. For example:

def f(pos1, pos2=2, /, pos_or_kwd1='a', *, kwd1, kwd2):
    pass

Once a Positional-Only parameter is specified with a default, the following Positional-Only and Positional-or-Keyword parameters need to have defaults as well.

As guidance, use Positional-Only if names do not matter or have no meaning, and there are only a few arguments that will always be passed in the same order. Use Keyword-Only when names have meaning, and the function definition is more understandable by being explicit with names.

For more information, refer to PEP 3102 (Keyword-Only Arguments) and PEP 570 (Python Positional-Only Parameters).