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Property annotated as `FunctionType` triggers false unreachable error on `isinstance` check

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@daleselaji-dev 已經在處理了。

開始於 2026年8月24日。

  • #21885 來自 @daleselaji-dev —— 未關閉

評估

難度
4/5
預估耗時
3-5 天
新手友好度
50/100
Issue 類型
缺陷
描述清晰度
基本清楚
活躍度
活躍
技術堆疊
python
領域
compilers

研究方向

從 issue 中的重現案例開始，追蹤 mypy 對 isinstance 斷言的可達性分析，著重於其對 FunctionType 和 MethodType 的處理。為使用 property 註解的情況新增回歸測試，並驗證 warn_unreachable 不會產生任何錯誤或警告。

由索引模型根據 Issue 內容生成。

描述

bug topic-descriptors topic-reachability

Bug Report

Mypy incorrectly reports an unreachable error when assert isinstance(..., FunctionType) on a @property annotated as returning FunctionType, even though the constructor enforces the invariant.
This is a false positive: no MethodType is involved, and the assertion is trivially true.

Also, the option that enables this error is warn_unreachable. Is this supposed to be a warning instead of an error?

As an aside, while asserting that a defined function (def func(): ...) is an instance of FunctionType is True, if a parameter of a function/method defines func: FunctionType, mypy will raise a arg-type error at the call site (error: Argument 1 to "FuncWrap" has incompatible type "Callable[[], int]"; expected "FunctionType" [arg-type]). This also seems like a false positive, as raw function definitions are implicitly of type FunctionType. Not what this bug is about, but something I ran into designing my wrapper.

To Reproduce

from types import FunctionType

class FuncWrap:
    def __init__(self, func: object) -> None:
        if not isinstance(func, FunctionType):
            raise TypeError()
        self._func = func

    @property
    def __func__(self) -> FunctionType:
        return self._func

def good() -> int:
    return 1

wrapped = FuncWrap(good)

assert isinstance(wrapped.__func__, FunctionType)

If you replace the assert with assert wrapped.__func__ is good, mypy reports a new comparison-overlap error: error: Non-overlapping identity check (left operand type: "MethodType", right operand type: "FunctionType") [comparison-overlap].

gist
playground

Expected Behavior

No errors/warnings.

Actual Behavior

main.py:18: error: Subclass of "MethodType" and "FunctionType" cannot exist: "MethodType" is final  [unreachable]
main.py:18: error: Subclass of "MethodType" and "FunctionType" cannot exist: "FunctionType" is final  [unreachable]
Found 2 errors in 1 file (checked 1 source file)

Your Environment

  • Mypy version used: 2.3.1, 1.20.x, 1.10.x
  • Mypy command-line flags: N/A
  • Mypy configuration options from mypy.ini (and other config files): 
    [tool.mypy]
  cache_dir = "$MYPY_CONFIG_FILE_DIR/.mypy_cache"
  check_untyped_defs = true
  color_output = true
  disallow_any_generics = true
  disallow_incomplete_defs = true
  disallow_untyped_decorators = true
  disallow_untyped_defs = true
  exclude = []
  explicit_package_bases = false
  files = [ "src/", "tests/" ]
  implicit_optional = false
  implicit_reexport = false
  incremental = true
  local_partial_types = true
  mypy_path = "$MYPY_CONFIG_FILE_DIR/src"
  namespace_packages = false
  native_parser = true
  num_workers = 2
  plugins = []
  python_version = "3.11"
  show_error_codes = true
  show_error_context = true
  strict = true
  strict_equality = true
  strict_optional = true
  warn_no_return = true
  warn_redundant_casts = true
  warn_return_any = true
  warn_unreachable = true
  warn_unused_configs = true
  warn_unused_ignores = true
  • Python version used: 3.[11-14]
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