`curses.complexstr` concatenation returns `NotImplemented` instead of raising

[fedonman]

curses.complexstr returns Py_NotImplemented from its sq_concat slot for a non-complexstr operand, and PyNumber_Add() returns an sq_concat result verbatim, so the singleton reaches Python code instead of a TypeError.

import curses

s = curses.complexstr('ab')
print('s + "x" ->', repr(s + 'x'))
s += 'x'
print('after s += "x":', repr(s))

s + "x" -> NotImplemented
after s += "x": NotImplemented

Expected: TypeError from both. The augmented assignment is the same slot, so it silently rebinds the name to NotImplemented rather than failing.

complexstr is new in 3.16, so no released version is affected.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-156963: Raise TypeError when concatenating complexstr with another type #156964