Defining a recursive type hint in Python?

Question

Let's say I have a function that accepts a Garthok, an Iterable[Garthok], an Iterable[Iterable[Garthok]], etc.

def narfle_the_garthoks(arg):
  if isinstance(arg, Iterable):
    for value in arg:
       narfle_the_garthoks(arg)
  else:
    arg.narfle()

Is there any way to specify a type hint for arg that indicates that it accepts any level of Iterables of Garthoks? I suspect not, but thought I'd check if I'm missing something.

As a workaround, I'm just specifying a few levels deep, and then ending with Iterable[Any].

Union[Garthok,
    Iterable[Union[Garthok,
        Iterable[Union[Garthok, 
            Iterable[Union[Garthok, Iterable[Any]]]]]]]]

For anyone looking for recursive generic types, see this issue (spoiler: still open as of now). — bfontaine
– bfontaine, Commented Jun 24 at 15:51

InSync · Accepted Answer · 2024-07-30 18:35:17Z

115

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

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

Mypy supports recursive types by default since v0.990, and Pyright/Pylance since v2020.9.4.

Some types of forward references are handled by PEP 563. You can use them starting from Python 3.7 by doing from __future__ import annotations – Konstantin

As of Python 3.12, __future__.annotations/stringifying is not necessary if the type is defined using a type statement:

type Garthoks = Garthok | Iterable[Garthoks]]

edited Jul 30, 2024 at 18:35

InSync

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answered Dec 19, 2018 at 5:28

gilch

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9 Comments

JesusFreke Over a year ago

Ah, gotcha. Thanks! It doesn't seem to be supported by IDEA/PyCharm either.

gilch Over a year ago

@JesusFreke There are a few other Python type checkers that I've heard of. Besides mypy and pycharm, there's also Facebook's Pyre and Google's pytype

gilch Over a year ago

@DanielH classes are types. You can also use TypeVars and subclass Generic for that [] syntax.

Konstantin Over a year ago

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

David C. Over a year ago

Unfortunately, it isn't possible to write NestedList = list[str | "NestedList"] instead of NestedList = list[Union[str, "NestedList"]] since it raises TypeError: unsupported operand type(s) for |: 'type' and 'str'.

|

Adnan MARSO · Accepted Answer · 2022-07-24 13:28:04Z

-2

MyPy comes with this limitation, it does not support cyclic reference yet, but I found a way to work it round using TypeVars like so:

from typing import TypeVar, TypeAlias, Iterable, Union

T = TypeVar('T')
_Garthoks: TypeAlias = Union[T, Iterable[T]]
Garthoks: TypeAlias = _Garthoks[_Garthoks[_Garthoks[_Garthoks]]]
# you can nest it as deep as you need...

Currently, I find it to be the best solution util MyPy support this feature.

I hope it solves your problem.

answered Jul 24, 2022 at 13:28

Adnan MARSO

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