fix: make dask_property picklable

[ikrommyd]

🤖 AI text below 🤖

A class using @dask_property cannot be pickled by value:

import cloudpickle
import dask_awkward as dak

class Thing:
    @dak.dask_property
    def foo(self):
        return 1

    @foo.dask
    def foo(self, array):
        return 2

cloudpickle.dumps(Thing)
# TypeError: cannot pickle '_DaskProperty' object

property keeps fget/fset/fdel in C-level slots and provides no reduction of its own, so CPython refuses outright — pickle.dumps(property(f)) raises the same error. _DaskProperty inherits that refusal even though it does have an instance __dict__.

This matters because cloudpickle only pickles a class by reference when it is reachable as getattr(module, cls.__qualname__). Anything else — a behavior mixin written in __main__ or in a notebook cell, or one whose generated *Record/*Array classes have been shadowed in their module — gets pickled by value, which walks the class dict and hits the property. The result is that such behaviors cannot be sent to a distributed worker at all.

_DaskProperty now defines __reduce__, round-tripping fget/fset/fdel/__doc__/_dask_get.

_DaskMethod needs no change: it is a plain Python class whose whole state lives in the instance __dict__, so the default reduction already handles it and cloudpickle serializes the contained function and closure by value. It gets a test to keep it that way.

Found while debugging the coffea side of this, where the same copy of _DaskProperty broke FCCSchema under a distributed client: scikit-hep/coffea#1603.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.03%. Comparing base (3d90b72) to head (6c962dd).
⚠️ Report is 33 commits behind head on main.
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #629      +/-   ##
==========================================
- Coverage   90.05%   90.03%   -0.03%     
==========================================
  Files          24       24              
  Lines        3610     3601       -9     
==========================================
- Hits         3251     3242       -9     
  Misses        359      359              

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[martindurant]

OK. Looks fine. Wonder how you came onto this? Wonder whether the classes that have the property should be changed to have their own reduce() - but there's no downside to this either.

[ikrommyd]

OK. Looks fine. Wonder how you came onto this? Wonder whether the classes that have the property should be changed to have their own reduce() - but there's no downside to this either.

Claude found it while working on scikit-hep/coffea#1603. In coffea to make dask-awkward optional dependency we just vendored dask method and dask property. So I thought I'd just upstream the patch here too.