What's the correct way to check if an object is a typing.Generic?
I'm trying to write code that validates type hints, and in order to do so I have to find out what kind of object the annotation is. For example, consider this snippet that's supposed to tell the user what kind of value is expected:
import typing
typ = typing.Union[int, str]
if issubclass(typ, typing.Union):
print('value type should be one of', typ.__args__)
elif issubclass(typ, typing.Generic):
print('value type should be a structure of', typ.__args__[0])
else:
print('value type should be', typ)
This should print "value type should be one of (int, str)", but instead it throws an exception:
Traceback (most recent call last):
File "untitled.py", line 6, in <module>
if issubclass(typ, typing.Union):
File "C:\Python34\lib\site-packages\typing.py", line 829, in __subclasscheck__
raise TypeError("Unions cannot be used with issubclass().")
TypeError: Unions cannot be used with issubclass().
isinstance doesn't work either:
>>> isinstance(typ, typing.Union)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "C:\Python34\lib\site-packages\typing.py", line 826, in __instancecheck__
raise TypeError("Unions cannot be used with isinstance().")
TypeError: Unions cannot be used with isinstance().
What's the correct way to check if typ is a typing.Generic?
If possible, I would like to see a solution that's backed by documentation or a PEP or some other resource. A "solution" that "works" by accessing undocumented, internal attributes is easy to find. But more likely than not, it'll turn out to be an implementation detail and will change in future versions. I'm looking for "the right way" to do it.
You may be looking for __origin__:
# * __origin__ keeps a reference to a type that was subscripted, # e.g., Union[T, int].__origin__ == Union;`
import typing
typ = typing.Union[int, str]
if typ.__origin__ is typing.Union:
print('value type should be one of', typ.__args__)
elif typ.__origin__ is typing.Generic:
print('value type should be a structure of', typ.__args__[0])
else:
print('value type should be', typ)
>>>value type should be one of (<class 'int'>, <class 'str'>)
The best I could find to advocate the use of this undocumented attribute is this reassuring quote from Guido Van Rossum (2 years ago):
The best I can recommend is using
__origin__-- if we were to change this attribute there would still have to be some other way to access the same information, and it would be easy to grep your code for occurrences of__origin__. (I'd be less worried about changes to__origin__than to__extra__.) You may also look at the internal functions_gorg()and_geqv()(these names will not be part of any public API, obviously, but their implementations are very simple and conceptually useful).
This caveat in the documentation seem to indicate that nothing is set in marble yet:
New features might be added and API may change even between minor releases if deemed necessary by the core developers.
There is no official way to obtain this information. The typing module is still in heavy development, and has no public API to speak of. (In fact, it will probably never have one.)
All we can do is to look at the module's internals and find the least gross way to get the information we're after. And because the module is still being worked on, its internals will change. A lot.
In python 3.5 and 3.6, generics had an __origin__ attribute that held a reference to the original generic base class (i.e. List[int].__origin__ would've been List), but this was changed in 3.7. Now the easiest way to find out if something is a generic is probably to check its __parameters__ and __args__ attributes.
Here is a set of functions that can be used to detect generics:
import typing
if hasattr(typing, '_GenericAlias'):
# python 3.7
def _is_generic(cls):
if isinstance(cls, typing._GenericAlias):
return True
if isinstance(cls, typing._SpecialForm):
return cls not in {typing.Any}
return False
def _is_base_generic(cls):
if isinstance(cls, typing._GenericAlias):
if cls.__origin__ in {typing.Generic, typing._Protocol}:
return False
if isinstance(cls, typing._VariadicGenericAlias):
return True
return len(cls.__parameters__) > 0
if isinstance(cls, typing._SpecialForm):
return cls._name in {'ClassVar', 'Union', 'Optional'}
return False
else:
# python <3.7
if hasattr(typing, '_Union'):
# python 3.6
def _is_generic(cls):
if isinstance(cls, (typing.GenericMeta, typing._Union, typing._Optional, typing._ClassVar)):
return True
return False
def _is_base_generic(cls):
if isinstance(cls, (typing.GenericMeta, typing._Union)):
return cls.__args__ in {None, ()}
if isinstance(cls, typing._Optional):
return True
return False
else:
# python 3.5
def _is_generic(cls):
if isinstance(cls, (typing.GenericMeta, typing.UnionMeta, typing.OptionalMeta, typing.CallableMeta, typing.TupleMeta)):
return True
return False
def _is_base_generic(cls):
if isinstance(cls, typing.GenericMeta):
return all(isinstance(arg, typing.TypeVar) for arg in cls.__parameters__)
if isinstance(cls, typing.UnionMeta):
return cls.__union_params__ is None
if isinstance(cls, typing.TupleMeta):
return cls.__tuple_params__ is None
if isinstance(cls, typing.CallableMeta):
return cls.__args__ is None
if isinstance(cls, typing.OptionalMeta):
return True
return False
def is_generic(cls):
"""
Detects any kind of generic, for example `List` or `List[int]`. This includes "special" types like
Union and Tuple - anything that's subscriptable, basically.
"""
return _is_generic(cls)
def is_base_generic(cls):
"""
Detects generic base classes, for example `List` (but not `List[int]`)
"""
return _is_base_generic(cls)
def is_qualified_generic(cls):
"""
Detects generics with arguments, for example `List[int]` (but not `List`)
"""
return is_generic(cls) and not is_base_generic(cls)
All of these functions should work in all python versions <= 3.7 (including anything <3.5 that uses the typing module backport).