pandas-dev/pandas · error · ValueError

cannot add indices of unequal length

Error message

cannot add indices of unequal length

What it means

Raised by _sub_datetime_arraylike (and reused textually for sibling ops) when len(self) != len(other). DatetimeArray elementwise subtraction requires equal-length arrays; pandas does not broadcast datetimelike arrays against each other in this path.

Solutions

1. Reindex or align the two arrays to equal length first (e.g. .reindex_like, Index alignment, or pd.Series arithmetic with proper index).
2. Filter both operands with the same mask so lengths match.
3. If broadcasting a scalar is intended, pass a Timestamp rather than a length-1 DatetimeArray.

Example fix

// before
res = dta_a - dta_b  # ValueError if len(dta_a) != len(dta_b)

// after
common = dta_a.get_indexer(dta_b)  # or reindex as appropriate
res = dta_a - dta_a.reindex_like(dta_b)

import pandas as pd

def sub_aligned(a, b):
    if len(a) != len(b):
        a = pd.Series(a)
        b = pd.Series(b).reindex(a.index) if hasattr(b, 'index') else pd.Series(b, index=a.index)
    return a - b

try:
    res = a - b
except ValueError as e:
    if 'unequal length' in str(e):
        res = pd.Series(a) - pd.Series(b)  # rely on index alignment
    else:
        raise

When it happens

Trigger: DatetimeArray.__sub__ where the two arrays have different lengths; df[date_col_a] - df[date_col_b] with misaligned indexes that did not get reindexed; subtracting DatetimeIndex objects of different lengths.

Common situations: Misaligned DataFrame indexes after a filter/groupby that are not reindexed; concatenating arrays from different sources without length checks; subtracting a slice against the full array.

Related errors

• abs(axis) must be less than ndim
• cannot add Period to a
• Cannot add and
• cannot add and
• Cannot modify read-only array

AI-assisted analysis of pandas-dev/pandas@3b7651241d (2026-08-11). Data as JSON: /api/errors/e754e02723bb95fb. Report an issue: GitHub.

Appendix: source

Thrown at pandas/core/arrays/datetimelike.py:1112

        self = cast("DatetimeArray", self)
        # subtract a datetime from myself, yielding an ndarray[timedelta64[ns]]

        if isna(other):
            # i.e. np.datetime64("NaT")
            return self - NaT

        ts = Timestamp(other)

        self, ts = self._ensure_matching_resos(ts)
        return self._sub_datetimelike(ts)

    @final
    def _sub_datetime_arraylike(self, other: DatetimeArray) -> TimedeltaArray:
        if self.dtype.kind != "M":
            raise TypeError(f"cannot subtract a datelike from a {type(self).__name__}")

        if len(self) != len(other):
            raise ValueError("cannot add indices of unequal length")

        self = cast("DatetimeArray", self)

        self, other = self._ensure_matching_resos(other)
        return self._sub_datetimelike(other)

    @final
    def _sub_datetimelike(self, other: Timestamp | DatetimeArray) -> TimedeltaArray:
        self = cast("DatetimeArray", self)

        from pandas.core.arrays import TimedeltaArray

        try:
            self._assert_tzawareness_compat(other)
        except TypeError as err:
            new_message = str(err).replace("compare", "subtract")
            raise type(err)(new_message) from err

View on GitHub (pinned to 3b7651241d)