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Is there an elegant way to import type hints without circular imports?

lopz 4y ago
Something like this does the trick: if TYPE_CHECKING: import WhateverClass https://docs.python.org/3/library/typing.html#typing.TYPE_CHECKING https://docs.python.org/3/library/typing.html#typing.TYPE_CH...
- nomel 4y ago
  I’m a fan of having a single state, used for everything. Splitting the code up into two states, one for the linter and one for the execution, seems like a recipe for incorrectness and confusion. I would hate to refactor something like that.
  - closed 4y ago
    The issue is that sometimes a function can take a type that is an optional dependency, so you don't want to import it unless you are type checking. (And some types are defined in the typeshed so only exist to be imported during type checking; eg the type checker lib itself is a dependency in this case)
    - deleted 4y ago
      [deleted]
- int_19h 4y ago
  It's hardly elegant, though.
  - Spivak 4y ago
    Yeah if you’re like just put what is essentially if False: import blah unironically as good design and more than a necessary evil until a long-term solution emerges then we’ve jumped the shark.
deleted 4y ago
[deleted]
mdaniel 4y ago
Type hints can also be strings, which at least PyCharm resolves as if they were real type references from typing import List class Alpha: @staticmethod def doit(b: "Beta") -> List["Beta"]: return [b] class Beta: @staticmethod def doit(a: "Alpha") -> List["Alpha"]: return [a]
- squeaky-clean 4y ago
  This only works if they're in the same file, or Alpha imports Beta later in the file and vice versa. Once you split these into separate files (say for example adding a gamma.py file and Gamma class that uses Alpha and Beta) you get Unresolved reference 'Alpha' and Unresolved reference 'Beta', and typechecking calls to Alpha.doit and Beta.doit from Gamma does not work.
  - mdaniel 4y ago
    I thought it also accepted fully qualified type names but I just tried it and you're right. Relevant to this submission's title, if your packages are side-effect free, just importing the namespace then allows referencing the type names without touching the "actual" type import gamma def doit(bar: "gamma.Gamma"): ... In Java my answer to circular deps is the introduction of an interface that the concrete types can implement but then breaks the cycle

lopz 4y ago

Something like this does the trick: if TYPE_CHECKING: import WhateverClass https://docs.python.org/3/library/typing.html#typing.TYPE_CHECKING https://docs.python.org/3/library/typing.html#typing.TYPE_CH...

nomel 4y ago
I’m a fan of having a single state, used for everything. Splitting the code up into two states, one for the linter and one for the execution, seems like a recipe for incorrectness and confusion. I would hate to refactor something like that.
- closed 4y ago
  The issue is that sometimes a function can take a type that is an optional dependency, so you don't want to import it unless you are type checking. (And some types are defined in the typeshed so only exist to be imported during type checking; eg the type checker lib itself is a dependency in this case)
  - deleted 4y ago
    [deleted]
int_19h 4y ago
It's hardly elegant, though.
- Spivak 4y ago
  Yeah if you’re like just put what is essentially if False: import blah unironically as good design and more than a necessary evil until a long-term solution emerges then we’ve jumped the shark.

deleted 4y ago

[deleted]

mdaniel 4y ago

Type hints can also be strings, which at least PyCharm resolves as if they were real type references from typing import List class Alpha: @staticmethod def doit(b: "Beta") -> List["Beta"]: return [b] class Beta: @staticmethod def doit(a: "Alpha") -> List["Alpha"]: return [a]

squeaky-clean 4y ago
This only works if they're in the same file, or Alpha imports Beta later in the file and vice versa. Once you split these into separate files (say for example adding a gamma.py file and Gamma class that uses Alpha and Beta) you get Unresolved reference 'Alpha' and Unresolved reference 'Beta', and typechecking calls to Alpha.doit and Beta.doit from Gamma does not work.
- mdaniel 4y ago
  I thought it also accepted fully qualified type names but I just tried it and you're right. Relevant to this submission's title, if your packages are side-effect free, just importing the namespace then allows referencing the type names without touching the "actual" type import gamma def doit(bar: "gamma.Gamma"): ... In Java my answer to circular deps is the introduction of an interface that the concrete types can implement but then breaks the cycle