API / BUG: How do we differentiate between -9223372036854775808 and iNaT?

[gfyoung]

From #3707 (at 028188):

from datetime import datetime
from pandas import DataFrame

import numpy as np

max_int = np.iinfo(np.int64).max
min_int = np.iinfo(np.int64).min

df = DataFrame([max_int, min_int], index=[datetime(2013, 1, 1), datetime(2013, 1, 1)])
assert df.resample("M").apply(np.sum)[0][0] == -1
...
AssertionError

The assertion error occurs because during the aggregation, pandas checks in cython_operation in core/groupby.py via _is_cython_func from core/base.py whether there are any "missing" integer values (assuming the data is integer) before and after the aggregation, which are defined as iNaT = -9223372036854775808. If there are any such values, we automatically cast the data to float.

This logic is quite prevalent in the codebase, but it does seem quite fraught with pitfalls. For example, what if the output of a computation got the value -9223372036854775808 ? Also, what if the user intended to use -9223372036854775808 as a legitimate data point?

Unlikely, sure. But reasonable, absolutely.

[jreback]

so we already do this for transforms. https://github.com/pandas-dev/pandas/blob/master/pandas/core/groupby.py#L2100. should be easy to extend generally.

[gfyoung]

I don't follow you here: https://github.com/pandas-dev/pandas/blob/master/pandas/core/groupby.py#L2102 seems to suggest otherwise.

[jreback]

@gfyoung the output type of datetime/timedelta ops must be integers (though I suppose and this seems like the case here), that we have a non-datetimelike that returns an integer as well. So may need to sort thru these checks to avoid false positives.

[jbrockmendel]

I think the example in the OP still fails, but only because of floating point error, not iNaT ambiguity. IIRC there was a discussion about supporting int64_t directly in libgroupby.group_add and i think that would solve that particular example.

[rhshadrach]

I now get the expected result on main; not sure if this needs tests or not.