In #6640, it was pointed out that groupby() doesn't work on non-numerics. The issue: hash() is used to group, hash() gives different responses for different Python processes. The solution was to set a hash seed.

Unfortunately, Distributed has the same issue (dask/distributed#4141) and it's harder to solve. E.g. distributed-worker without a nanny doesn't have a good way to set a consistent seed.

Given that:

1. Until now groupby was broken for non-numerics and no one noticed. So probably not a huge use case.
2. It's hard to solve this with hash().

A better approach might be a different hash algorithm. For example, https://deepdiff.readthedocs.io/en/latest/deephash.html.

deephash seems promising:

from deepdiff import DeepHash
from pprint import pprint

class CustomObj:

    def __init__(self, x):
        self.x = x

    def __repr__(self):
        return f"CustomObj({self.x})"


objects = [
    125,
    "lalala",
    (12, 17),
    CustomObj(17),
    CustomObj(17),
    CustomObj(25),
]

for o in objects:
    print(repr(o), "has hash", DeepHash(o)[o])

Results in:

$ python example.py 
125 has hash 08823c686054787db6befb006d6846453fd5a3cbdaa8d1c4d2f0052a227ef204
'lalala' has hash 2307d4a08f50b743ec806101fe5fc4664e0b83c6c948647436f932ab38daadd3
(12, 17) has hash c7129efa5c7d19b0efabda722ae4cd8d022540b31e8160f9c181d163ce4f1bf6
CustomObj(17) has hash 477631cfc0315644152933579c23ed0c9f27743d632613e86eb7e271f6fc86e4
CustomObj(17) has hash 477631cfc0315644152933579c23ed0c9f27743d632613e86eb7e271f6fc86e4
CustomObj(25) has hash 8fa8427d493399d6aac3166331d78e9e2a2ba608c2b3c92a131ef0cf8882be14

Downsides:

1. This hashing approach, serializing the object to a string and then hashing the string, is probably much more expensive than hash().
2. Custom __hash__ won't work.

That being said, bag has used hash() this way for 4 years, maybe more, it's been broken with Distributed the whole time I assume, so probably that second downside is less relevant. Not sure about the performance aspect, though.