dask: `groupby` does not seem to work for non-numerics

What happened:

Following along tutorial bag, I played around with it for better understanding:

b = db.from_sequence(list(range(10)))
b.groupby(lambda x: 'even' if (x % 2) == 0 else 'odd').compute()

Result:

[('odd', [3, 7]), ('even', [0, 2, 4, 6, 8]), ('odd', [1, 5, 9])]

What you expected to happen:

I expected:

[('odd', [1, 3, 5, 7, 9]), ('even', [0, 2, 4, 6, 8])]

Minimal Complete Verifiable Example:

Shown above.

Anything else we need to know?:

I tried to think of what could be causing this difference, so that I can understand what to watch out for, but could not manage:

• Maybe it was because the return object needs to be the same (pointing to the same object, not just the same value).
  • But the string literal should be the same object. Just to be certain, I added variables even and odd, defined globally, and still got the strange result.
• Maybe it was due to partitioning.
  • First, this seems strange, since groupby API specifically states that it makes the expensive traversal across all partitions.
  • Second, this is non-ideal: a major advantage is to allow me, as the user, to needn’t worry about partition details.
  • Third, why does it work fine when the return value (the groupby key) is a number?

Environment:

• Dask version: ‘2.20.0’
• Python version: ‘3.8.5’
• Operating System: ‘Mac OS 10.15.6’
• Install method (conda, pip, source): conda, specifically using the environment.yml as specified in the tutorial: conda env create -f binder/environment.yml