bpo-38237: Fix "versionchanged" for pow named arguments

[mdickinson]

The ability to use named arguments in "pow" was introduced in Python 3.8, not Python 3.9.

https://bugs.python.org/issue38237

[ammaraskar]

Thanks Mark! Looks like the backport to 3.8 was added after my original PR so this got missed. Should https://github.com/python/cpython/blob/master/Doc/whatsnew/3.8.rst be updated as well?

Probably an entry near the:

For integers, the three-argument form of the :func:pow function now permits the exponent to be negative in the case where the base is relatively prime to the modulus. It then computes a modular inverse to the base when the

and

One use case for this notation is that it allows pure Python functions to fully emulate behaviors of existing C coded functions. For example, the built-in :func:pow function does not accept keyword arguments:

def pow(x, y, z=None, /):
   "Emulate the built in pow() function"
  r = x ** y
   return r if z is None else r%z

updated to divmod to match the original PR?

[mdickinson]

Should https://github.com/python/cpython/blob/master/Doc/whatsnew/3.8.rst be updated as well?

It should, but it looks as though there isn't good agreement on the issue about how. So we may as well get this uncontroversial fix in, and then make a follow-up PR once we've decided what to do about the what's new document.

[aeros]

LGTM; I locally confirmed that kwarg support for pow() was indeed added in 3.8.

Python 3.8.2 (default, Feb 26 2020, 22:21:03) 
[GCC 9.2.1 20200130] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> pow(base=2, exp=10)
1024

Python 3.7.7+ (heads/3.7:4e3a7f9205, Mar 19 2020, 00:23:06) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> pow(base=2, exp=10)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: pow() takes no keyword arguments

[miss-islington]

Thanks @mdickinson for the PR 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[bedevere-bot]

GH-19079 is a backport of this pull request to the 3.8 branch.

[mdickinson]

@ammaraskar @aeros Thanks. I'll aim to find some time this weekend to look at the "what's new" part of the issue (though if someone else gets there first, even better!).