pandas-dev/pandas · error · ValueError

Cannot pass both a timezone-aware dtype and tz=None

Error message

Cannot pass both a timezone-aware dtype and tz=None

What it means

Raised by _validate_tz_from_dtype when the dtype carries a tz (dtz is not None) and the caller explicitly passed `tz=None` (signaled by the explicit_tz_none flag, set when the user wrote tz=None literally rather than omitting it). This catches the contradictory request 'aware dtype but explicitly no tz'.

Solutions

  1. Drop the explicit `tz=None` and let the dtype's tz win.
  2. If you genuinely want a tz-naive result, also pass a tz-naive dtype like 'datetime64[ns]'.
  3. In library wrappers, treat `tz is None` as 'use dtype tz' (don't forward None literally when dtype carries tz).

Example fix

// before
dti = pd.DatetimeIndex(values, dtype='datetime64[ns, UTC]', tz=None)

// after
dti = pd.DatetimeIndex(values, dtype='datetime64[ns, UTC]')
Defensive patterns

Strategy: validation

Validate before calling

import pandas as pd

def drop_explicit_none(dtype_str, tz):
    dtz = None
    if dtype_str:
        try:
            d = pd.DatetimeTZDtype.construct_from_string(dtype_str)
            dtz = getattr(d, 'tz', None)
        except TypeError:
            pass
    if dtz is not None and tz is None:
        return dtype_str, 'omit'  # do not forward tz=None
    return dtype_str, tz

Type guard

def explicit_none_safe(dtype_str, explicit_tz_none) -> bool:
    import pandas as pd
    dtz = None
    if dtype_str:
        try:
            d = pd.DatetimeTZDtype.construct_from_string(dtype_str)
            dtz = getattr(d, 'tz', None)
        except TypeError:
            pass
    return not (dtz is not None and explicit_tz_none)

Try / catch

try:
    dti = pd.DatetimeIndex(values, dtype=dtype_str, tz=tz)
except ValueError as e:
    if 'timezone-aware dtype and tz=None' in str(e):
        kwargs = {'dtype': dtype_str}
        if tz is not None:
            kwargs['tz'] = tz
        dti = pd.DatetimeIndex(values, **kwargs)
    else:
        raise

Prevention

When it happens

Trigger: Calling `pd.DatetimeIndex(values, dtype='datetime64[ns, UTC]', tz=None)` — the literal tz=None conflicts with the tz embedded in the dtype. Reached when library code forwards a user-supplied tz=None alongside a tz-aware dtype.

Common situations: Wrapper functions that pass `tz=tz` where tz defaults to None, combined with a tz-aware default dtype. Refactoring that introduced explicit_tz_none semantics (older pandas would silently drop).

Related errors

AI-assisted analysis of pandas-dev/pandas@3b7651241d (2026-08-11). Data as JSON: /api/errors/b39df205f616898d. Report an issue: GitHub.

Appendix: source

Thrown at pandas/core/arrays/datetimes.py:3046

    ------
    ValueError : on tzinfo mismatch
    """
    if dtype is not None:
        if isinstance(dtype, str):
            try:
                dtype = DatetimeTZDtype.construct_from_string(dtype)
            except TypeError:
                # Things like `datetime64[ns]`, which is OK for the
                # constructors, but also nonsense, which should be validated
                # but not by us. We *do* allow non-existent tz errors to
                # go through
                pass
        dtz = getattr(dtype, "tz", None)
        if dtz is not None:
            if tz is not None and not timezones.tz_compare(tz, dtz):
                raise ValueError("cannot supply both a tz and a dtype with a tz")
            if explicit_tz_none:
                raise ValueError("Cannot pass both a timezone-aware dtype and tz=None")
            tz = dtz

        if tz is not None and lib.is_np_dtype(dtype, "M"):
            # We also need to check for the case where the user passed a
            #  tz-naive dtype (i.e. datetime64[ns])
            if tz is not None and not timezones.tz_compare(tz, dtz):
                raise ValueError(
                    "cannot supply both a tz and a "
                    "timezone-naive dtype (i.e. datetime64[ns])"
                )

    return tz


def _infer_tz_from_endpoints(
    start: Timestamp, end: Timestamp, tz: tzinfo | None
) -> tzinfo | None:
    """

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