TYPE_CHECKING and init=False

• cattrs version: 23.2.3
• Python version: 3.9.18
• Operating System: macOS

Description

Hi, I think I'm facing a similar situation as described in #160 but with a few subtleties that potentially motivate refining the internal type resolution of cattrs.

What I Did

Consider the following two files:

# inner.py
# --------

from attrs import define


@define
class Inner:
    pass

# outer.py
# --------

from typing import TYPE_CHECKING, Optional

import cattrs
from attrs import define, field

if TYPE_CHECKING:
    from inner import Inner


@define
class Outer:
    inner: Optional[Inner] = field(init=False, default=None)


outer = Outer()
cattrs.unstructure(outer)

Running the latter gives NameError: name 'Inner' is not defined because the inner class is not available in outer.py.

While one could fix this by informing cattrs about the class before attempting to unstructure, I don't think it's a convenient solution in my case because I don't even see a reason why it should be necessary in the first place: the corresponding attribute is init=False and with the default settings where such fields are ignored, cattrs shouldn't even need to bother about the field at all ... assuming of course that I do not overlook some other reason.

The motivation for my example: in my case, Inner relies on some heavy external dependencies (such as torch) and many execution paths of my code don't actually require loading it, so I instead use lazy-loading for improved startup time. That said, I wouldn't want to load it just to satisfy cattrs to perform an unstructuring operation that actually does not require the class due to init=False.

For now, I can simply remove that type annotation, which solves the problem, but this is of course suboptimal. Any thoughts would be highly appreciated.

Start by running the two-file reproduction in inner.py and outer.py, especially the cattrs.unstructure(outer) call. Read how cattrs resolves field annotations and handles init=False fields under its default settings. Done means the example no longer raises NameError without importing Inner, while the init=False field remains omitted from the unstructured result.