Function _resolve_group

xarray/core/groupby.py:459–509

(
    obj: T_DataWithCoords, group: T_Group | Hashable | IndexVariable
)

def _resolve_group(
    obj: T_DataWithCoords, group: T_Group | Hashable | IndexVariable
) -> T_Group:
    from xarray.core.dataarray import DataArray

    error_msg = (
        "the group variable's length does not "
        "match the length of this variable along its "
        "dimensions"
    )

    newgroup: T_Group
    if isinstance(group, DataArray):
        try:
            align(obj, group, join="exact", copy=False)
        except ValueError as err:
            raise ValueError(error_msg) from err

        newgroup = group.copy(deep=False)
        newgroup.name = group.name or "group"

    elif isinstance(group, IndexVariable):
        # This assumption is built in to _ensure_1d.
        if group.ndim != 1:
            raise ValueError(
                "Grouping by multi-dimensional IndexVariables is not allowed."
                "Convert to and pass a DataArray instead."
            )
        (group_dim,) = group.dims
        if len(group) != obj.sizes[group_dim]:
            raise ValueError(error_msg)
        newgroup = DataArray(group)

    else:
        if not hashable(group):
            raise TypeError(
                "`group` must be an xarray.DataArray or the "
                "name of an xarray variable or dimension. "
                f"Received {group!r} instead."
            )
        group_da: DataArray = obj[group]
        if group_da.name not in obj._indexes and group_da.name in obj.dims:
            # DummyGroups should not appear on groupby results
            newgroup = _DummyGroup(obj, group_da.name, group_da.coords)
        else:
            newgroup = group_da

    if newgroup.size == 0:
        raise ValueError(f"{newgroup.name} must not be empty")

    return newgroup


@dataclass