python - How to access the type arguments of typing.Generic?

Question

The typing module provides a base class for generic type hints: The typing.Generic class.

Subclasses of Generic accept type arguments in square brackets, for example:

list_of_ints = typing.List[int]
str_to_bool_dict = typing.Dict[str, bool]

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My question is, how can I access these type arguments?

That is, given str_to_bool_dict as input, how can I get str and bool as output?

Basically I'm looking for a function such that

>>> magic_function(str_to_bool_dict)
(<class 'str'>, <class 'bool'>)

Answer 1

Python >= 3.8

As of Python3.8 there is typing.get_args:

print( get_args( List[int] ) ) # (<class 'int'>,)

PEP-560 also provides __orig_bases__[n], which allows us the arguments of the nth generic base:

from typing import TypeVar, Generic, get_args

T = TypeVar( "T" )

class Base( Generic[T] ):
    pass

class Derived( Base[int] ):
    pass

print( get_args( Derived.__orig_bases__[0] ) ) # (<class 'int'>,)

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Python >= 3.6

As of Python 3.6. there is a public __args__ and (__parameters__) field. For instance:

print( typing.List[int].__args__ )

This contains the generic parameters (i.e. int), whilst __parameters__ contains the generic itself (i.e. ~T).

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Python < 3.6

Use typing_inspect.getargs

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Some considerations

typing follows PEP8. Both PEP8 and typing are coauthored by Guido van Rossum. A double leading and trailing underscore is defined in as: "“magic” objects or attributes that live in user-controlled namespaces".

The dunders are also commented in-line; from the official repository for typing we can see:

• "__args__ is a tuple of all arguments used in subscripting, e.g., Dict[T, int].__args__ == (T, int)".

However, the authors also note:

• "The typing module has provisional status, so it is not covered by the high standards of backward compatibility (although we try to keep it as much as possible), this is especially true for (yet undocumented) dunder attributes like __union_params__. If you want to work with typing types in runtime context, then you may be interested in the typing_inspect project (part of which may end up in typing later)."

I general, whatever you do with typing will need to be kept up-to-date for the time being. If you need forward compatible changes, I'd recommend writing your own annotation classes.