Python 3.11 StrEnum error when member is named "title"

[samypr100]

Bug Report

Currently when running mypy==0.991 on a Python 3.11 file that's using StrEnum, it errors unexpectedly on an specific named member on my code. In this case, the member only needed to be named title to get an Incompatible types in assignment error.

I realize python 3.11 support gh-12840 is still ongoing, but I thought I should report it to help others who might encounter this.

It might also be worth noting that it happens in older Python versions (tested on 3.10) when using strenum backport.

To Reproduce

from enum import StrEnum


class TheBug0(StrEnum):
    lorem = "foo"
    ipsum = "ipsum"
    title = "bug0"  # Bug because left side is named "title"?


class TheBug1(StrEnum):
    lorem = "foo"
    title = "bug1"  # Bug because left side is named "title"?
    ipsum = "bar"


class TheBug2(StrEnum):
    title = "bug2"  # Bug because left side is named "title"?
    lorem = "foo"
    ipsum = "bar"


class NoBug(StrEnum):
    lorem = "foo"
    ipsum = "bar"

Playground Gist URL: https://mypy-play.net/?mypy=latest&python=3.11&gist=910f75dbbd0c2ca484dc018a1983b654

Expected Behavior

No Errors

Actual Behavior

main.py:7: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], str]")  [assignment]
main.py:12: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], str]")  [assignment]
main.py:17: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], str]")  [assignment]
Found 3 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.991
• Mypy command-line flags: mypy main.py
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.11.1

[ngnpope]

Interesting... This is because enum.StrEnum is a subclass of str, so the title member ends up shadowing str.title().

In fact, the same can be observed when trying to use member names that shadow other methods of str:

from enum import StrEnum


class BuggyEnum(StrEnum):
    capitalize = "capitalize"
    casefold = "casefold"
    center = "center"
    count = "count"
    encode = "encode"
    endswith = "endswith"
    expandtabs = "expandtabs"
    find = "find"
    format = "format"
    format_map = "format_map"
    index = "index"
    isalnum = "isalnum"
    isalpha = "isalpha"
    isascii = "isascii"
    isdecimal = "isdecimal"
    isdigit = "isdigit"
    isidentifier = "isidentifier"
    islower = "islower"
    isnumeric = "isnumeric"
    isprintable = "isprintable"
    isspace = "isspace"
    istitle = "istitle"
    isupper = "isupper"
    join = "join"
    ljust = "ljust"
    lower = "lower"
    lstrip = "lstrip"
    maketrans = "maketrans"
    partition = "partition"
    removeprefix = "removeprefix"
    removesuffix = "removesuffix"
    replace = "replace"
    rfind = "rfind"
    rindex = "rindex"
    rjust = "rjust"
    rpartition = "rpartition"
    rsplit = "rsplit"
    rstrip = "rstrip"
    split = "split"
    splitlines = "splitlines"
    startswith = "startswith"
    strip = "strip"
    swapcase = "swapcase"
    title = "title"
    translate = "translate"
    upper = "upper"
    zfill = "zfill"

This chucks out the following:

main.py:5: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], str]")  [assignment]
main.py:6: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], str]")  [assignment]
main.py:7: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, SupportsIndex, str], str]")  [assignment]
main.py:8: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, str, Optional[SupportsIndex], Optional[SupportsIndex]], int]")  [assignment]
main.py:9: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, str, str], bytes]")  [assignment]
main.py:10: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, Union[str, Tuple[str, ...]], Optional[SupportsIndex], Optional[SupportsIndex]], bool]")  [assignment]
main.py:11: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, SupportsIndex], str]")  [assignment]
main.py:12: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, str, Optional[SupportsIndex], Optional[SupportsIndex]], int]")  [assignment]
main.py:13: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, VarArg(object), KwArg(object)], str]")  [assignment]
main.py:14: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, _FormatMapMapping], str]")  [assignment]
main.py:15: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, str, Optional[SupportsIndex], Optional[SupportsIndex]], int]")  [assignment]
main.py:16: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], bool]")  [assignment]
main.py:17: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], bool]")  [assignment]
main.py:18: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], bool]")  [assignment]
main.py:19: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], bool]")  [assignment]
main.py:20: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], bool]")  [assignment]
main.py:21: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], bool]")  [assignment]
main.py:22: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], bool]")  [assignment]
main.py:23: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], bool]")  [assignment]
main.py:24: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], bool]")  [assignment]
main.py:25: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], bool]")  [assignment]
main.py:26: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], bool]")  [assignment]
main.py:27: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], bool]")  [assignment]
main.py:28: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, Iterable[str]], str]")  [assignment]
main.py:29: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, SupportsIndex, str], str]")  [assignment]
main.py:30: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], str]")  [assignment]
main.py:31: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, Optional[str]], str]")  [assignment]
main.py:32: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as overloaded function)  [assignment]
main.py:33: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, str], Tuple[str, str, str]]")  [assignment]
main.py:34: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, str], str]")  [assignment]
main.py:35: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, str], str]")  [assignment]
main.py:36: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, str, str, SupportsIndex], str]")  [assignment]
main.py:37: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, str, Optional[SupportsIndex], Optional[SupportsIndex]], int]")  [assignment]
main.py:38: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, str, Optional[SupportsIndex], Optional[SupportsIndex]], int]")  [assignment]
main.py:39: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, SupportsIndex, str], str]")  [assignment]
main.py:40: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, str], Tuple[str, str, str]]")  [assignment]
main.py:41: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, Optional[str], SupportsIndex], List[str]]")  [assignment]
main.py:42: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, Optional[str]], str]")  [assignment]
main.py:43: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, Optional[str], SupportsIndex], List[str]]")  [assignment]
main.py:44: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, bool], List[str]]")  [assignment]
main.py:45: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, Union[str, Tuple[str, ...]], Optional[SupportsIndex], Optional[SupportsIndex]], bool]")  [assignment]
main.py:46: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, Optional[str]], str]")  [assignment]
main.py:47: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], str]")  [assignment]
main.py:48: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], str]")  [assignment]
main.py:49: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, _TranslateTable], str]")  [assignment]
main.py:50: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str], str]")  [assignment]
main.py:51: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, SupportsIndex], str]")  [assignment]
Found 47 errors in 1 file (checked 1 source file)

https://mypy-play.net/?mypy=latest&python=3.11&gist=d21b0840b2caefcb83142ca1f0d0b7ca

[JelleZijlstra]

This error is correct. You have an incompatible override, which creates unsound code.

Consider:

def title_string(s: str) -> str:
    return s.title()

s(TheBug0.title)  # runtime error

[ngnpope]

Probably something that should be mentioned under the limitations in the documentation, if anyone cares enough to write a patch... 🤷🏻

[samypr100]

Note, this behavior should now fixed upstream due to python/cpython#103600