pandas-dev/pandas · error · TypeError

only list-like objects are allowed to be passed to isin(), y

Error message

only list-like objects are allowed to be passed to isin(), you passed a `{type(comps).__name__}`

What it means

Raised by pandas.core.algorithms.isin when the comps argument (the values being tested for membership) is not list-like. Series/Index.isin route their underlying values through this function; a non-list-like comps cannot be tested element-wise, so pandas rejects it up front.

Source

Thrown at pandas/core/algorithms.py:525

_FLOAT64_INT_EXACT_MAX = 2**53


def isin(comps: ListLike, values: ListLike) -> npt.NDArray[np.bool_]:
    """
    Compute the isin boolean array.

    Parameters
    ----------
    comps : list-like
    values : list-like

    Returns
    -------
    ndarray[bool]
        Same length as `comps`.
    """
    if not is_list_like(comps):
        raise TypeError(
            "only list-like objects are allowed to be passed "
            f"to isin(), you passed a `{type(comps).__name__}`"
        )
    if not is_list_like(values):
        raise TypeError(
            "only list-like objects are allowed to be passed "
            f"to isin(), you passed a `{type(values).__name__}`"
        )

    if isinstance(values, (set, frozenset)) and len(values) > 0:
        # GH#25507: for a set of values, membership can be tested directly
        # via the set, avoiding an O(len(values)) materialization that
        # otherwise dominates when comps is much smaller than values.
        # Restrict to integer/bool comps (i.e. dtypes that cannot contain
        # NaN), since Python set membership would mis-handle the case where
        # both sides contain NaN values that are not identical.
        if isinstance(comps, (ABCSeries, ABCIndex)):
            comps_arr = comps._values

View on GitHub (pinned to 71959b8cb9)

Solutions

Pass a list-like for comps (wrap a scalar in a list).
Prefer the public Series.isin(values) / Index.isin(values) which validate at the boundary.
Add an is_list_like check before invoking the internal function.

Example fix

# before
from pandas.core.algorithms import isin
mask = isin(5, [1, 2, 5])
# after
mask = isin([5], [1, 2, 5])

Defensive patterns

Strategy: type-guard

Validate before calling

from pandas.api.types import is_list_like

def safe_isin(comps, values):
    if not is_list_like(comps):
        raise TypeError(f'comps must be list-like, got {type(comps).__name__}')
    return pd.core.algorithms.isin(comps, values)

Type guard

from pandas.api.types import is_list_like

def is_listlike(x) -> bool:
    return is_list_like(x)

Prevention

Always pass a list-like for the comps (left) side of isin.
Prefer the public Series.isin / Index.isin over the internal function.
Validate with is_list_like at boundaries.

When it happens

Trigger: Calling the internal pd.core.algorithms.isin with a scalar comps; indirectly, paths that build the comps from a scalar before reaching isin.

Common situations: Calling isin via the internal API with accidentally-scalar data; refactors that pass a single value instead of a sequence as the left-hand side.

Related errors

AI-assisted analysis of pandas-dev/pandas@71959b8cb9 (2026-08-07). Data as JSON: /api/errors/74ed64d417ef2f17. Report an issue: GitHub.