Classes with `cached_property` can't be cloudpickled

[durumu]

I believe this is a bug.

Here is an example:

from functools import cached_property
import pickle
import cloudpickle


class MyExample:
    def __init__(self, foo):
        self.foo = foo

    @cached_property
    def bar(self) -> int:
        return self.foo * 3


example = MyExample(2)
pickle.dumps(example)      # works
cloudpickle.dumps(example) # crashes

This fails with TypeError: cannot pickle '_thread.RLock' object.

[pierreglaser]

Hii @durumu,

pickling cached objects such as function/methods/properties is not yet supported by cloudpickle (see #178).

A possible minimal fix (that would not attempt at pickling the cache contents when pickling the cached object, see #178 (comment)) should be relatively simple to implement, PR welcome.

[Hoeze]

This seemingly also does not work for the class definition itself:

from functools import cached_property
import cloudpickle


class MyExample:
    def __init__(self, foo):
        self.foo = foo

    @cached_property
    def bar(self) -> int:
        return self.foo * 3


cloudpickle.dumps(MyExample) # fails

[praateekmahajan]

Ran into a similar issue, inspired from this wrote similar functionality that uses copyreg to pickle cached_property objects

from functools import cached_property


# This function is used to extract the necessary information to pickle a cached_property object.
def pickle_cached_property(obj: cached_property):  # noqa: ANN202
    # The `func` attribute of a cached_property object is the original method
    # that was decorated with @cached_property.
    # The `__globals__` attribute of that function contains a reference to the
    # global namespace where the function was defined, which may include the owner class.

    for cls in obj.func.__globals__.values():
        # Check if the value is a class (owner of the cached_property).
        if isinstance(cls, type):
            for name, prop in cls.__dict__.items():
                # Check if the property matches the cached_property we're trying to pickle.
                if prop is obj:
                    return (unpickle_cached_property, (obj.func, cls, name))

    # If the loop completes without returning, it means the owner class wasn't found.
    err = f"Could not find the owner class for cached_property {obj}"
    raise TypeError(err)


# Define a function to reconstruct the cached_property object during unpickling
def unpickle_cached_property(func, owner, name):  # noqa: ANN202;
    # Create a new cached_property object with the original function
    prop = cached_property(func)
    # The `__set_name__` method of a cached_property is normally called automatically
    # when the property is assigned to a class attribute. However, since we are
    # reconstructing the object during unpickling, we need to call it manually.
    # This method sets up the internal state of the cached_property object
    # so that it can function correctly when accessed as a class attribute.
    prop.__set_name__(owner, name)
    return prop


# Register the pickling and unpickling functions for the cached_property class
copyreg.pickle(cached_property, pickle_cached_property, unpickle_cached_property)