AttributeError: __metadata__ on Python 3.13

[kamyarghajar]

There is compatibility problem which raises the following exception:

File "venv/lib/python3.13/site-packages/injector/__init__.py", line 1768, in build
    return self._build_class(self._target, **kwargs)
           ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^
  File "venv/lib/python3.13/site-packages/injector/__init__.py", line 1763, in _build_class
    return self._injector.create_object(cls, additional_kwargs=kwargs)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "venv/lib/python3.13/site-packages/injector/__init__.py", line 1177, in create_object
    self.call_with_injection(init, self_=instance, kwargs=additional_kwargs)
    ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "venv/lib/python3.13/site-packages/injector/__init__.py", line 1216, in call_with_injection
    bindings = get_bindings(callable)
  File "venv/lib/python3.13/site-packages/injector/__init__.py", line 1358, in get_bindings
    has_injectable_parameters = any(
        _is_specialization(v, Annotated) and _inject_marker in v.__metadata__
        for v in type_hints.values()
    )
  File "venv/lib/python3.13/site-packages/injector/__init__.py", line 1359, in <genexpr>
    _is_specialization(v, Annotated) and _inject_marker in v.__metadata__
                                                           ^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.11/Frameworks/Python.framework/Versions/3.13/lib/python3.13/typing.py", line 1378, in __getattr__
    raise AttributeError(attr)
AttributeError: __metadata__. Did you mean: '__getattr__'?

which might be fixed by replacing:

has_injectable_parameters = any(
    _is_specialization(v, Annotated) and _inject_marker in v.__metadata__
    for v in type_hints.values()
)

in __init__.py file with:

has_injectable_parameters = any(
    _is_specialization(v, Annotated) and _inject_marker in getattr(v, "__metadata__", ())
    for v in type_hints.values()
)

[kamyarghajar]

Pull request: #304

[kamyarghajar]

MRE

import sys
from dataclasses import dataclass
from typing import Literal
from injector import Injector

# Trigger: Python 3.13 + Literal + slots=True
# In Python 3.13, the internal representation of Literal can sometimes 
# satisfy _is_specialization(v, Annotated) but lack the __metadata__ attribute.
@dataclass(slots=True)
class ServiceConfig:
    environment: Literal["prod", "test"] = "test"

def reproduce():
    print(f"Python version: {sys.version}")
    injector = Injector()
    
    print("Attempting to create object (this triggers get_bindings)...")
    try:
        # This will trigger the AttributeError on Python 3.13 with injector <= 0.23.0
        injector.get(ServiceConfig)
        print("Success: No error raised.")
    except AttributeError as e:
        print(f"\nCRASH DETECTED: {e}")
        if "__metadata__" in str(e):
            print("Confirmed: This is the Python 3.13 __metadata__ regression.")
    except Exception as e:
        print(f"Unexpected error: {type(e).__name__}: {e}")

if __name__ == "__main__":
    reproduce()