When to use typing.TYPE_CHECKING (feddit.de)

submitted 1 year ago* (last edited 1 year ago) by wasabi@feddit.de to c/python@programming.dev

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After learning about TYPE_CHECKING i made it a habit to put all imports that were only needed for type checking into an if TYPE_CHECKING: guard. But now I am wondering if that is actually intended to be used like that. Checking whether an import is only needed at type checking time can get quite tedious and sometimes you run into situations were you introduced some code that made the import a requirement at runtime.

How do you use TYPE_CHECKING? Whenever it is possible or only when using it actually solves a circular import?

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[-] o11c@programming.dev 0 points 1 year ago

You should have part of your test harness perform a separate import of every module. If your module is idempotent (most good code is) you could do this in a single process by cleaning sys.modules I guess ... but it still won't be part of your pytest process.

Static analyzers can only detect some cases, so can't be fully trusted.

I've also found there are a lot of cases where performant Python code has to be implemented in a distinct way from what the type-checker sees. You can do this with aggressive type: ignore but I often find it cleaner to use separate if blocks.