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 when len(self) != len(other). DatetimeArray subtraction is elementwise (no broadcasting between two arrays), so unequal lengths are rejected with ValueError. Despite the word 'add' in the message, this guards a subtraction.

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 71959b8cb9)

Solutions

1. Align both arrays to a common index first: idx_a, idx_b = idx_a.align(idx_b) (alignment broadcasts NaN/NaT as needed).
2. Reindex one operand to the other's index: idx_b = idx_b.reindex(idx_a.index).
3. Slice the longer operand so lengths match, or broadcast a scalar instead of an array.
4. Confirm len(a) == len(b) with an assertion before the arithmetic.

Example fix

// before
out = datetime_idx_a - datetime_idx_b  # ValueError when lengths differ
// after
a, b = datetime_idx_a.align(datetime_idx_b)
out = a - b

if len(a) != len(b):
    a, b = a.align(b)
assert len(a) == len(b)
out = a - b

try:
    out = a - b
except ValueError as e:
    if 'unequal length' in str(e):
        a, b = a.align(b)
        out = a - b
    else:
        raise

When it happens

Trigger: Subtracting two DatetimeIndex/DatetimeArray objects of different lengths: idx_a - idx_b where len(idx_a) != len(idx_b). Reached via __sub__ line 1412 then _sub_datetime_arraylike line 1111.

Common situations: Joining or aligning series whose indexes disagree; slicing one series but not the other before a datetime subtraction; misconfigured fixtures in tests.

Related errors

• left and right must have the same length
• Lengths must match to compare
• length mismatch: {len(self)} vs. {len(other)}
• operands have mismatched length {len(self)} and {len(other)}
• cannot broadcast result

AI-assisted analysis of pandas-dev/pandas@71959b8cb9 (2026-08-07). Data as JSON: /api/errors/e754e02723bb95fb. Report an issue: GitHub.