[BUG] Pymod operation is missing null values

[galipremsagar]

Describe the bug
When there is 0 in denominator of pymod operation, we seem to be returning garbage value instead of null value.

Steps/Code to reproduce bug

Type "help", "copyright", "credits" or "license" for more information.
>>> 1 % 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero
>>> import pandas as pd
>>> x = pd.Series([1, 2, 3, 4, 0])
>>> x  % 2
0    1
1    0
2    1
3    0
4    0
dtype: int64
>>> 2 % x
0    0.0
1    0.0
2    2.0
3    2.0
4    NaN
dtype: float64

>>> import cudf
>>> x = cudf.from_pandas(x)
>>> x % 2
0    1
1    0
2    1
3    0
4    0
dtype: int64
>>> 2 % x
0             0
1             0
2             2
3             2
4    4294967295   #<- This could either be `nan` or `null`
dtype: int64

Expected behavior
Return either nan or null inplace of garbage values

Environment overview (please complete the following information)

• Environment location: Docker
• Method of cuDF install: from source (0.15)

Additional context
This kind of looks similar to #5722 but not sure if they share the same code-flows, so Just putting it out there.

[kkraus14]

I guess it makes sense for a modulo zero operation to return null since it's undefined behavior but this is somewhere we would differ from Pandas.

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[kkraus14]

cc @brandon-b-miller for more binaryop null things

[vyasr]

Binary operations in libcudf are designed so that nulls always propagate. As a result, although we have created a specialized mod operator PYMOD in libcudf for compatibility with Python's rules for handling negatives, we can't trivially exploit that for null handling since the operator doesn't return a nullable value.

However, in this instance there is some precedent for solving this at the libcudf level. Specifically, all the Null* operators have special handling to bypass null propagation in the wrapper for ops. While I would normally say that we should solve this issue by post-processing the result in Python (similar to how #11441 solves #7389), the existing infrastructure for solving this problem makes me less wary of introducing this logic into C++.

@jrhemstad I'd be curious what you think about this. The tl;dr is that in pandas x % 0 returns nulls, and we're trying to figure out how best to do that. Options are (1) using the same wrapping logic that we currently use in libcudf for the Null* operators, which I find slightly icky (but only slightly since we're already doing this, it's just a little less obvious why you'd do this for an operator that isn't strictly about specialized null handling) or (2) just post-processing the result in Python based on whether the input values contained zeros.

CC @brandon-b-miller

[jrhemstad]

although we have created a specialized mod operator PYMOD in libcudf for compatibility with Python's rules for handling negatives

"PYMOD" is a misnomer. It wasn't intended to be 100% compliant with Python's %. It was intended to just be the mathematical modulo operation vs C++'s % is the remainder operation.

See: https://stackoverflow.com/questions/13683563/whats-the-difference-between-mod-and-remainder

I wanted to rename MOD to REMAINDER and PYMOD would be MODULO.

While I would normally say that we should solve this issue by post-processing the result in Python

This is the correct answer. It's no different than #7389 (comment) imo.

[jrhemstad]

Found the original conversation: #1985 (review)

What's worse is calling an operation PyMod. It doesn't tell a user of the C++ interface what it actually means. In general, we should use descriptions that stand on their own and do not resort to knowledge of another language or library.

I still agree with my past self :)

[vyasr]

I'm fine with that explanation, although to @harrism's point we'd definitely need to document how we define remainder vs modulo since it's at best it's a nonstandard but common distinction. So perhaps we should do that rename while also addressing this issue here :)

I think resolving this issue then requires two things:

1. Rename PYMOD to REMAINDER (along with suitable documentation of the meaning)
2. Add the same kind of logic as Return zero when floor dividing an integer data by zero #11441 except for modulo.

@brandon-b-miller would be up for handling this? I'm hoping it would be a near carbon copy of #11441 except with a different fill value. I can take on 1 myself if you don't want to.

[brandon-b-miller]

happy to handle making the change 👍