BUG: irradiance.dirint raises KeyError with scalar inputs on pandas >= 2.0

[Omesh37]

Bug description

pvlib.irradiance.dirint raises KeyError: 'cannot use a single bool to index into setitem' when called with scalar solar_zenith (or other
scalar inputs) on pandas >= 2.0.

To reproduce

import pvlib
import pandas as pd

pvlib.irradiance.dirint(
    ghi=0,
    solar_zenith=90,
    times=pd.DatetimeIndex(['2023-06-21'], tz='UTC')
)

Traceback

KeyError: 'cannot use a single bool to index into setitem'
File "pvlib/irradiance.py", in _dirint_bins
zenith_bin[(zenith >= 0) & (zenith < 25)] = 1

Root cause

In _dirint_bins, when scalar values are passed, boolean expressions like
(zenith >= 0) & (zenith < 25) evaluate to a plain Python bool (e.g.
False). In pandas >= 2.0, series[False] = value raises KeyError
because pandas now treats scalar bools as label lookups, not boolean masks.

Expected behavior

Returns NaN or 0.0 cleanly without crashing.

Fix

Convert all scalar inputs to pd.Series at the top of _dirint_bins:

kt_prime = pd.Series(kt_prime, index=times, dtype=float)
zenith = pd.Series(zenith, index=times, dtype=float)
w = pd.Series(w, index=times, dtype=float)
delta_kt_prime = pd.Series(delta_kt_prime, index=times, dtype=float)

Environment

• pvlib: 0.15.2.dev2+g200de89ca
• pandas: 2.3.3
• numpy: 2.3.5
• Python: 3.13.9
• OS: Windows

I would like to submit a PR for this fix.

[cwhanse]

@Omesh37 thanks for the report, and we'd welcome help addressing it.

I think the fix goes beyond #2752 and I'm not sure that casting the inputs to Series internal to dirint is the way to go.

For starters, the docstring for dirint says the type of several inputs (the key one is solar_zenith) is array-like. Some time ago we had a long debate #765 about what array-like meant, which in pvlib means np.array or pd.Series.

If we want to extend dirint to accept float then the type should be numeric. This is the first decision.

If we do want float input, then I think the fix is to make irradiance._dirint_bins work with float, which may involve casting internal to that function.

[Omesh37]

Thanks @cwhanse for the feedback.

• Based on discussion in use array_like instead of numeric/array-like #765, what I think is that dirint should accept numeric inputs (float/int scalars in addition to array-like), since it's a natural use case to call it with a single value.
• My proposed updated approach:

1. Change the type annotation for ghi, solar_zenith, pressure, and temp_dew in the dirint docstring from array-like to numeric
2. Keep the fix inside _dirint_bins — cast scalar inputs to pd.Series using the times index so boolean masking always produces a boolean Series (not a scalar bool)
   Does that direction sound right to you? Happy to update the PR accordingly :)

[cwhanse]

@Omesh37 yes, that's what I was thinking. However, please keep in mind that a pvlib function should (not must) return the same type as its inputs. In this case, if the input to dirint is float, then dirint should return dni as float. We try to do that without introducing a lot of if isinstance() kind of statement.