Python: What does the slash mean in the output of help(range)?

What does the / mean in Python 3.4's help output for range before the closing parenthesis?

    >>> help(range)
    Help on class range in module builtins:

    class range(object)
     |  range(stop) -> range object
     |  range(start, stop[, step]) -> range object
     |  Return a virtual sequence of numbers from start to stop by step.
     |  Methods defined here:
     |  __contains__(self, key, /)
     |      Return key in self.
     |  __eq__(self, value, /)
     |      Return self==value.


It signifies the end of the positional only parameters, parameters you cannot use as keyword parameters. Such parameters can 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.

The syntax has also been defined for possible future inclusion in Python, see PEP 457 - Syntax For Positional-Only Parameters. At the moment the PEP acts as a reservation on the syntax, there are no actual plans to implement it as such.