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

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?

Asked By: gerrit

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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

Answered By: runDOSrun

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

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