It signifies the end of the positional only parameters, parameters you cannot use as keyword parameters. Before Python 3.8, such parameters could only be specified in the C API.
It means the
key argument to
__contains__ can only be passed in by position (
range(5).__contains__(3)), not as a keyword argument (
range(5).__contains__(key=3)), something you can do with positional arguments in pure-python functions.
Also see the Argument Clinic documentation:
To mark all parameters as positional-only in Argument Clinic, add a
/on a line by itself after the last parameter, indented the same as the parameter lines.
and the (very recent addition to) the Python FAQ:
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.
The syntax is now part of the Python language specification, as of version 3.8, see PEP 570 – Python Positional-Only Parameters. Before PEP 570, the syntax was already reserved for possible future inclusion in Python, see PEP 457 – Syntax For Positional-Only Parameters.
Positional-only parameters can lead to cleaner and clearer APIs, make pure-Python implementations of otherwise C-only modules more consistent and easier to maintain, and because positional-only parameters require very little processing, they lead to faster Python code.