Defining a recursive type hint in Python?

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You can specify recursive types in the typing language by using type aliases and forward reference strings,

Garthoks = Union[Garthok, Iterable['Garthoks']]

Note that recursive types are not yet supported by mypy. But it will likely be added eventually.

Update 2020/9/14: Microsoft announces support for recursive types in Pyright/Pylance.

Some types of forward references are handled by PEP0563. You can use them starting from Python 3.7 by doing from __future__ import annotations – Konstantin

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