`stubgen --inspect-mode` crashes with `AttributeError on PEP 604 union syntax (X | Y)` *(mypy 2.1.0)*

[Mr-Ruben]

Environment

• mypy version: 2.1.0 (compiled: yes)
• Python: 3.11
• OS: Debian/Linux
• Install method: pip, user install (~/.local/lib/python3.11/site-packages)

Steps to Reproduce

1. Create a module using PEP 604 union syntax:

# test_union_pep604.py
def greet(name: str) -> str:
    return f"Hello, {name}"

def process(value: int | str) -> float | None:
    if isinstance(value, int):
        return float(value)
    return None

class Container:
    def __init__(self, data: list[str] | dict[str, int]) -> None:
        self.data = data

    def get(self, key: str) -> int | None:
        if isinstance(self.data, dict):
            return self.data.get(key)
        return None

2. Run:

stubgen --inspect-mode -o /tmp/stubs_test test_union_pep604.py

Expected Behavior

stubgen generates a .pyi stub file preserving the X | Y annotations, the same way it succeeds on the equivalent Union[X, Y] syntax:

stubgen --inspect-mode -o /tmp/stubs_test test_union_old.py
# → Generated /tmp/stubs_test/test_union_old.pyi  (exit 0)

Actual Behavior

stubgen crashes with an unhandled AttributeError and exits with code 1:

Traceback (most recent call last):
  File "/home/<user>/.local/bin/stubgen", line 6, in <module>
    sys.exit(main())
  File "mypy/stubgen.py", line 2057, in main
  File "mypy/stubgen.py", line 1860, in generate_stubs
  File "mypy/stubgen.py", line 1818, in generate_stub_for_py_module
  File "/home/<user>/.local/lib/python3.11/site-packages/mypy/stubgenc.py", line 452, in generate_module
    self.generate_function_stub(name, obj, output=functions)
  File "/home/<user>/.local/lib/python3.11/site-packages/mypy/stubgenc.py", line 631, in generate_function_stub
    default_sig = self.get_default_function_sig(obj, ctx)
  File "/home/<user>/.local/lib/python3.11/site-packages/mypy/stubgenc.py", line 344, in get_default_function_sig
    add_args(args, get_pos_default)
  File "/home/<user>/.local/lib/python3.11/site-packages/mypy/stubgenc.py", line 336, in add_args
    arglist.append(ArgSig(arg, get_annotation(arg), default=False))
  File "/home/<user>/.local/lib/python3.11/site-packages/mypy/stubgenc.py", line 311, in get_annotation
    return self.get_type_fullname(argtype)
  File "/home/<user>/.local/lib/python3.11/site-packages/mypy/stubgenc.py", line 775, in get_type_fullname
    typename = getattr(typ, "__qualname__", typ.__name__)
AttributeError: 'types.UnionType' object has no attribute '__name__'

Root Cause

--inspect-mode inspects live runtime objects rather than parsing source. When Python evaluates a X | Y annotation, the resulting object is an instance of types.UnionType (introduced in Python 3.10, PEP 604). stubgenc.py's get_type_fullname assumes any annotation object falls back to having a __name__, which types.UnionType does not define — hence the crash.

Proposed Fix

File: mypy/stubgenc.py

1. Import UnionType (near line 25):

- from types import FunctionType, ModuleType
+ from types import FunctionType, ModuleType, UnionType

2. Special-case it in get_type_fullname (near line 775):

  if typ is Any:
      return "Any"
+ if isinstance(typ, UnionType):
+     return " | ".join(self.get_type_fullname(a) for a in typ.__args__)
  typename = getattr(typ, "__qualname__", typ.__name__)

This recursively resolves each member of the union and rejoins them with |, matching the original annotation's syntax. types.UnionType is available in all Python versions mypy 2.1.0 targets (3.10+), so no version gating is needed.

Verification

With the patch applied, test_union_pep604.py generates a stub with unions preserved:

$ stubgen --inspect-mode -o /tmp/stubs_test test_union_pep604.py
Processed 1 modules
Generated /tmp/stubs_test/test_union_pep604.pyi

$ cat /tmp/stubs_test/test_union_pep604.pyi
def greet(name: str) -> str: ...
def process(value: int | str) -> float | None: ...

class Container:
    def __init__(self, data: list | dict) -> None: ...
    def get(self, key: str) -> int | None: ...

test_union_old.py (using typing.Union) no longer crashes either, but it does not preserve the union — it falls back to Incomplete for those annotations, which is a pre-existing, separate limitation of runtime inspection unrelated to this patch:

Existing Issue

test_union_old.py

"""Module using classic Union[X, Y] syntax — should work with --inspect-mode."""
from typing import Union, Optional


def greet(name: str) -> str:
    return f"Hello, {name}"


def process(value: Union[int, str]) -> Optional[float]:
    if isinstance(value, int):
        return float(value)
    return None


class Container:
    def __init__(self, data: Union[list[str], dict[str, int]]) -> None:
        self.data = data

    def get(self, key: str) -> Optional[int]:
        if isinstance(self.data, dict):
            return self.data.get(key)
        return None

produces

$ stubgen --inspect-mode -o /tmp/stubs_test test_union_old.py
Processed 1 modules
Generated /tmp/stubs_test/test_union_old.pyi

$ cat /tmp/stubs_test/test_union_old.pyi
from _typeshed import Incomplete

def greet(name: str) -> str: ...
def process(value) -> Incomplete: ...

class Container:
    def __init__(self, data) -> None: ...
    def get(self, key: str) -> Incomplete: ...

So: the patch fixes the crash for PEP 604 syntax and, as a bonus, correctly resolves those unions in the stub. It does not fix typing.Union resolution — that gap already existed before this patch and is out of scope here.

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