Cannot implement __len__ classmethod (0.21+)

[timkpaine]

Bug Description

This used to work in pyo3<0.21:

    #[classmethod]
    fn __len__(cls: &Bound<'_, PyType>) -> PyResult<usize> {
        Ok(0)
    }

error[E0425]: cannot find value `_slf_ref` in this scope
  --> lib1-py/src/lib.rs:15:1
   |
15 | #[pymethods]
   | ^^^^^^^^^^^^ not found in this scope

Steps to Reproduce

Include the above block in any #[pymethods] block

Backtrace

error[E0425]: cannot find value `_slf_ref` in this scope
  --> lib1-py/src/lib.rs:15:1
   |
15 | #[pymethods]
   | ^^^^^^^^^^^^ not found in this scope

Your operating system and version

macOS 15.0.1

Your Python version (python --version)

Python 3.11.10

Your Rust version (rustc --version)

rustc 1.79.0 (129f3b996 2024-06-10)

Your PyO3 version

0.22.5

How did you install python? Did you use a virtualenv?

brew install [email protected], pip install pyo3

Additional Info

No response

[LilyFoote]

Defining __len__ as a @classmethod seems very unusual to me. What's your use case?

[timkpaine]

Not unusual at all for certain types:

from enum import Enum, auto

class MyEnum(Enum):
    A = auto()
    B = auto()
    
print(len(MyEnum))  # > 2

My use case is the same, creating a class in rust representing an enum (where len is the enumeration of static members, very similar to the pure python example above).

[LilyFoote]

In the Enum case, __len__ is defined on the EnumType metaclass, not the Enum class.

https://github.com/python/cpython/blob/4b55d53316f607fdd83831e92f585288842c8988/Lib/enum.py#L803-L807

I think you'll need #906 to be implemented to achieve this in PyO3.

[timkpaine]

I think you'll need #906 to be implemented to achieve this in PyO3.

Or downgrade to 0.19, which is the more annoying bit as this used to work fine

[davidhewitt]

Agreed this is a legitimate bug.

From a quick test, it seems like Python allows most (all?) the dunder methods to be @classmethod or even @staticmethod, so this is simply just a case which we should add tests for and support 👍

[Icxolu]

I attempted a fix in #4654.