PEP 781: Make ``TYPE_CHECKING`` a built-in constant

PEPs
149

Let’s explore the two alternatives I suggested with aiohttp, which is a heavy package to import.

You can create an empty _typing_imports.py module, and a corresponding _typing_imports.pyi module, containing just:

import aiohttp as aiohttp

Elsewhere in your code, you can import your typing-only module:

from . import _typing_imports as _t   # Let's shorten it too

async def fetch_some_data(value: _t.aiohttp.ClientSession) -> None: ...

This solution is slightly more involved, but it also has some benefits. In addition to avoiding the typing import, thus satisfying the motivation of this PEP:

  1. It approaches typing imports systematically, meaning:
    1. It makes the fact that the module you imported can only be used in (delayed) annotations explicit (without resorting to aliasing aiohttp itself wherever it might be imported).
    2. It makes guarding typing-only aiohttp imports throughout your codebase unnecessary.
  2. It allows you to evaluate your aiohttp annotations so that they can be used at runtime, e.g. by defining a module-level __getattr__ in your _typing_imports.py:
    def __getattr__(name):
    if name == 'aiohttp':
        import aiohttp

        return aiohttp

    raise AttributeError(name)
  3. It can be done today.
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