How can I create my own "parameterized" type in Python (like `Optional[T]`)?

Question

I want to create my own parameterized type in Python for use in type hinting:

class MaybeWrapped:
    # magic goes here

T = TypeVar('T')

assert MaybeWrapped[T] == Union[T, Tuple[T]]

Never mind the contrived example; how can I implement this? I looked at the source for Union and Optional, but it looks like some fairly low-level hackery that I'd like to avoid.

The only suggestion in the documentation comes from an example re-implementation of Mapping[KT,VT] that inherits from Generic. But that example is more about the __getitem__ method than about the class itself.

Answer 1

If you're just trying to create generic classes or functions, try taking a look at the documentation on mypy-lang.org about generic types -- it's fairly comprehensive, and more detailed then the standard library typing docs.

If you're trying to implement your specific example, it's worth pointing out that type aliases work with typevars -- you can simply do:

from typing import Union, TypeVar, Tuple

T = TypeVar('T')

MaybeWrapped = Union[T, Tuple[T]]

def foo(x: int) -> MaybeWrapped[str]:
    if x % 2 == 0:
        return "hi"
    else:
        return ("bye",)

# When running mypy, the output of this line is:
# test.py:13: error: Revealed type is 'Union[builtins.str, Tuple[builtins.str]]'
reveal_type(foo(3))

However, if you're trying to construct a generic type with genuinely new semantics, you're very likely out of luck. Your remaining options are to:

1. Construct some kind of custom class/metaclass thing that PEP 484-compliant type checkers can understand and use that.
2. Modify the type checker you're using somehow (mypy has an experimental "plugin" system, for example)
3. Petition to modify PEP 484 to include your new, custom type (you can do this by opening an issue in the typing module repo).