[New-bugs-announce] [issue46571] Strange `@typing.no_type_check` behavior for class variables

[Nikita Sobolev]

New submission from Nikita Sobolev <mail at sobolevn.me>:

I was working on improving coverage and test quality of `typing.py`, when I saw that `@no_type_check` is quite strange.

Let's dive into this!

## Everything is correct

We will start with a basic example, that illustrates that everything works fine:

```
from typing import no_type_check, get_type_hints

class A:
    x: int = 1

class B:
    y: str = 'a'

print(get_type_hints(A))  # ok: {'x': <class 'int'>}
print(get_type_hints(B))  # ok: {'y': <class 'str'>}
```

Ok, let's move on.

## Adding `@no_type_check`

Now, adding `@no_type_check` to `B` will make the result of `get_type_hints()` call on it - an empty `dict`:

```
from typing import no_type_check, get_type_hints

class A:
    x: int = 1

@no_type_check
class B:
    y: str = 'a'

print(get_type_hints(A))  # ok: {'x': <class 'int'>}
print(get_type_hints(B))  # ok: {}
```

This is still ok.

## Broken?

And now we can add some class-level constant to `B`, like `delegate` to show how it breaks `A`:

```
from typing import no_type_check, get_type_hints

class A:
    x: int = 1

@no_type_check
class B:
    y: str = 'a'
    delegate = A  # adding this line will make `A` to have `__no_type_check__` as well

print(get_type_hints(A))  # {}, wait, what?
print(get_type_hints(B))  # {}, ok
```

Why is that important?

It introduces an unfortunate side-effect that can make some totally unrelated (!) class completely ignore `get_type_hints()` and break things like `pydantic`, `beartype`, `attrs, etc that rely on type hints.

By adding a class-level assignment to a class that has `@no_type_check` or other `no_type_check_decorator`.

Why does this happen?

It happens because `no_type_check` has this logic:

```
    if isinstance(arg, type):
        arg_attrs = arg.__dict__.copy()
        for attr, val in arg.__dict__.items():
            if val in arg.__bases__ + (arg,):
                arg_attrs.pop(attr)
        for obj in arg_attrs.values():
            if isinstance(obj, types.FunctionType):
                obj.__no_type_check__ = True
            if isinstance(obj, type):
                no_type_check(obj)
```

Source: https://github.com/python/cpython/blob/8b1b27f1939cc4060531d198fdb09242f247ca7c/Lib/typing.py#L1952-L1975

As you can see above, we traverse all `__dict__` values of the given `class` and for some reason recurse into all nested types. 

I think that the original goal was to handle cases like:

```
@no_type_check
class Outer:
    class Inner: ...
```

And now it also affects regular assignments.

So, what can we do?

0. Nothing, it works correctly (I disagree)
1. Do not cover nested classes at all with `@no_type_check`, only cover methods
2. Only cover types that are **defined** in this class, like my `Inner` class example
3. Something else?

I think that `(2)` is more inline with the currect implementation, so my vote is for it.

I would like to implement this when we will have the final agreement :)

----------
components: Library (Lib)
messages: 412073
nosy: AlexWaygood, Jelle Zijlstra, gvanrossum, kj, sobolevn
priority: normal
severity: normal
status: open
title: Strange `@typing.no_type_check` behavior for class variables
versions: Python 3.10, Python 3.11, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue46571>
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