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diff --git a/Doc/faq/programming.rst b/Doc/faq/programming.rst index 7bc00ff..3161418 100644 --- a/Doc/faq/programming.rst +++ b/Doc/faq/programming.rst @@ -767,6 +767,41 @@ Yes. Usually this is done by nesting :keyword:`lambda` within Don't try this at home, kids! +.. _faq-positional-only-arguments: + +What does the slash(/) in the parameter list of a function mean? +---------------------------------------------------------------- + +A slash in the argument list of a function denotes that the parameters prior to +it are positional-only. Positional-only parameters are the ones without an +externally-usable name. Upon calling a function that accepts positional-only +parameters, arguments are mapped to parameters based solely on their position. +For example, :func:`pow` is a function that accepts positional-only parameters. +Its documentation looks like this:: + + >>> help(pow) + Help on built-in function pow in module builtins: + + pow(x, y, z=None, /) + Equivalent to x**y (with two arguments) or x**y % z (with three arguments) + + Some types, such as ints, are able to use a more efficient algorithm when + invoked using the three argument form. + +The slash at the end of the parameter list means that all three parameters are +positional-only. Thus, calling :func:`pow` with keyword aguments would lead to +an error:: + + >>> pow(x=3, y=4) + Traceback (most recent call last): + File "<stdin>", line 1, in <module> + TypeError: pow() takes no keyword arguments + +Note that as of this writing this is only documentational and no valid syntax +in Python, although there is :pep:`570`, which proposes a syntax for +position-only parameters in Python. + + Numbers and strings =================== |