collections.UserString.replace() rejects the count keyword #161970

Bug report

Since 3.13, str.replace() accepts count as a keyword argument (gh-106487), and 3.15 did the same for bytes.replace() (gh-147856). collections.UserString.replace() names that parameter maxsplit instead, so the keyword form that works on str raises TypeError on a UserString. It looks like the name was copied from split() a long time ago.

from collections import UserString

print('aaa'.replace('a', 'b', count=1))
print(UserString('aaa').replace('a', 'b', count=1))

Actual:

baa
Traceback (most recent call last):
  ...
TypeError: UserString.replace() got an unexpected keyword argument 'count'

Expected: baa printed twice. (Conversely, UserString('aaa').replace('a', 'b', maxsplit=1) works although str.replace has no such parameter.)

CPython versions tested on:

3.15, CPython main branch

Operating systems tested on:

Linux

Found while building a Lean 4 model of CPython (https://broyojo.com/python-formalization/). AI tools (Claude) helped investigate the findings and prepare this report. The examples were rerun on CPython 3.15.2 and main, and the report received technical review. Record: corelibs-userstring-replace-count-keyword

Linked PRs

• gh-161970: Rename UserString.replace() parameter maxsplit to count #161971