pandas-dev/pandas · error · ValueError

Could not infer freq from start/end

Error message

Could not infer freq from start/end

What it means

Defensive branch in _get_ordinal_range (marked pragma: no cover) that fires when freq is None and neither start nor end ended up as a Period object, so there is no frequency to inherit. In normal flow at least one bound becomes a Period via Period(start, freq); reaching this line means the inputs defeated that construction path. It is essentially an internal-invariant failure rather than an expected user error.

Source

Thrown at pandas/core/arrays/period.py:1544

        start = Period(start, freq)
    if end is not None:
        end = Period(end, freq)

    is_start_per = isinstance(start, Period)
    is_end_per = isinstance(end, Period)

    if is_start_per and is_end_per and start.freq != end.freq:
        raise ValueError("start and end must have same freq")
    if start is NaT or end is NaT:
        raise ValueError("start and end must not be NaT")

    if freq is None:
        if is_start_per:
            freq = start.freq
        elif is_end_per:
            freq = end.freq
        else:  # pragma: no cover
            raise ValueError("Could not infer freq from start/end")
        mult = freq.n

    if periods is not None:
        periods = periods * mult
        if start is None:
            data = np.arange(
                end.ordinal - periods + mult, end.ordinal + 1, mult, dtype=np.int64
            )
        else:
            data = np.arange(
                start.ordinal, start.ordinal + periods, mult, dtype=np.int64
            )
    else:
        data = np.arange(start.ordinal, end.ordinal + 1, mult, dtype=np.int64)

    return data, freq

View on GitHub (pinned to 71959b8cb9)

Solutions

1. Always pass an explicit freq= to period_range / _get_ordinal_range so freq inference is not required.
2. If calling internals directly, ensure at least one bound is a real Period before invoking.
3. Report as a pandas bug if hit through the public API, since the branch is marked no-cover.

Example fix

// before
pd.core.arrays.period._get_ordinal_range(start, end, periods, freq=None)
// after
pd.core.arrays.period._get_ordinal_range(start, end, periods, freq='D')

import pandas as pd
from pandas import Period

def safe_get_ordinal_range(start, end, periods, freq):
    if freq is None and not isinstance(start, Period) and not isinstance(end, Period):
        raise ValueError('Pass an explicit freq; could not infer from start/end')
    return pd.core.arrays.period._get_ordinal_range(start, end, periods, freq)

def has_period_endpoint(start, end) -> bool:
    return isinstance(start, pd.Period) or isinstance(end, pd.Period)

try:
    data, freq = _get_ordinal_range(start, end, periods, freq)
except ValueError as e:
    if 'Could not infer freq' in str(e):
        freq = 'D'
        data, freq = _get_ordinal_range(start, end, periods, freq)
    else:
        raise

When it happens

Trigger: Reaching this requires freq=None plus both start and end being non-Period after construction (e.g. subclassing/monkeypatching Period, or a future refactor). Practically unreachable through the public period_range API for well-formed inputs.

Common situations: Custom subclasses overriding Period behavior; using private _get_ordinal_range directly with exotic objects; version regressions where the Period coercion path changes.

Related errors

• start and end must have same freq
• start and end must not be NaT
• Quarter must be 1 <= q <= 4
• cannot convert float NaN to integer
• Mismatched Period array lengths

AI-assisted analysis of pandas-dev/pandas@71959b8cb9 (2026-08-07). Data as JSON: /api/errors/537d6df941da430f. Report an issue: GitHub.