What is the meaning of a forward slash "/" in a Python method signature, as shown by help(foo)? [duplicate]

Question

In the signature returned interactively by help(foo), what is the meaning of a /?

In [37]: help(object.__eq__)  Help on wrapper_descriptor:  __eq__(self, value, /)     Return self==value.  In [55]: help(object.__init__)  Help on wrapper_descriptor:  __init__(self, /, *args, **kwargs)     Initialize self.  See help(type(self)) for accurate signature. 

I thought it might be related to keyword-only arguments, but it's not. When I create my own function with keyword-only arguments, positional and keyword-only arguments are separated by * (as expected), not by /. What does the / mean?

Answer 1

As explained here, the / as an argument marks the end of arguments that are positional only (see here), i.e. arguments you can't use as keyword parameters. In the case of __eq__(self, value, /) the slash is at the end, which means that all arguments are marked as positional only while in the case of your __init__ only self, i.e. nothing, is positional only.

Edit: This was previously only used for built-in functions but since Python 3.8, you can use this in your own functions. The natural companion of / is * which allows to mark the beginning of keyword-only arguments. Example using both: 

# a, b are positional-only # c, d are positional or keyword # e, f are keyword-only def f(a, b, /, c, d, *, e, f):     print(a, b, c, d, e, f)  # valid call f(10, 20, 30, d=40, e=50, f=60)  # invalid calls: f(10, b=20, c=30, d=40, e=50, f=60)   # b cannot be a keyword argument f(10, 20, 30, 40, 50, f=60)           # e must be a keyword argument