Python enum meta making typing module crash

Question

I've been breaking my head on this and I can't seem to find a solution to the problem. I use an enum to manage my access in a flask server. Short story I need the enum to return a default value if a non-existent enum value is queried. First I created a meta class for the enum:

class AuthAccessMeta(enum.EnumMeta):
def __getattr__(self, item):
    try:
        return super().__getattr__(item)
    except Exception as _:
        if self == AuthAccess and item not in ['_subs_tree']:
            Loggers.SYS.warn('Access {} doesn\'t exist, substituting with MISSING.'.format(item))
            return AuthAccess.MISSING
@unique
class AuthAccess(str, AutoName, metaclass=AuthAccessMeta):
    ...

You can see I exclude the _subs_tree attribute since neither EnumMeta or Enum has it. Only place I found this method is in the typing module. Then I type an argument with AuthAcess elsewhere and it gives me this weird error:

C:\Users\[USER]\AppData\Local\Programs\Python\Python36\python.exe -m src.main
[SYS][INFO][11:18:54]: Instance 76cb0042196d4a75b3794ce0b9c1590c is running on project 'local/project1'
Traceback (most recent call last):
  File "C:\Users\[USER]\AppData\Local\Programs\Python\Python36\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "C:\Users\[USER]\AppData\Local\Programs\Python\Python36\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Users\[USER]\Documents\code\sollumcloudplatform\src\main.py", line 19, in <module>
    from src.procedures import create_app
  File "C:\Users\[USER]\Documents\code\sollumcloudplatform\src\procedures.py", line 191, in <module>
    def satisfy_role(role: {}, access_need: Tuple[List[AuthAccess]]) -> bool:
  File "C:\Users\[USER]\AppData\Local\Programs\Python\Python36\lib\typing.py", line 626, in inner
    return func(*args, **kwds)
  File "C:\Users\[USER]\AppData\Local\Programs\Python\Python36\lib\typing.py", line 1062, in __getitem__
    orig_bases=self.__orig_bases__)
  File "C:\Users\[USER]\AppData\Local\Programs\Python\Python36\lib\typing.py", line 965, in __new__
    self.__tree_hash__ = hash(self._subs_tree()) if origin else hash((self.__name__,))
  File "C:\Users\[USER]\AppData\Local\Programs\Python\Python36\lib\typing.py", line 1007, in _subs_tree
    tree_args = _subs_tree(self, tvars, args)
  File "C:\Users\[USER]\AppData\Local\Programs\Python\Python36\lib\typing.py", line 548, in _subs_tree
    tree_args.append(_replace_arg(arg, tvars, args))
  File "C:\Users\[USER]\AppData\Local\Programs\Python\Python36\lib\typing.py", line 517, in _replace_arg
    return arg._subs_tree(tvars, args)
TypeError: 'NoneType' object is not callable

I've tried returning the method from the typing module but Python tells me it doesn't exists either. Am I using the meta class wrong ? Should I just remove the typing on the argument ?

Ethan Furman · Accepted Answer · 2020-05-20 16:10:26Z

8

Returning a default value can be done with the right version of enum.

The problem you are having now, I suspect, is because in your except branch you do not return a value, nor raise an exception, if the if fails -- so None is returned instead.

class AuthAccessMeta(enum.EnumMeta):
def __getattr__(self, item):
    try:
        return super().__getattr__(item)
    except Exception as _:
        if self == AuthAccess and item not in ['_subs_tree']:
            Loggers.SYS.warn('Access {} doesn\'t exist, substituting with MISSING.'.format(item))
            return AuthAccess.MISSING
        # need something here, like simply reraising the exception
        raise

answered May 20, 2020 at 16:10

Ethan Furman

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2 Comments

BinarSkugga Over a year ago

Thank you ! To my understanding, unless you raise an exception it tries to find the implementation inside the argument class. Raising here tells typing to use its own default one. Very insightful !

Ethan Furman Over a year ago

@BinarSkugga: Sadly, I don't know enough about the typing module to be insightful, but standard Python is that everything returns something, and if you don't specify what that is, then the something is None.

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