# Buggy code
def _drop_axis(self, labels, axis, level=None, errors="raise"):
    """
    Drop labels from specified axis. Used in the ``drop`` method
    internally.

    Parameters
    ----------
    labels : single label or list-like
    axis : int or axis name
    level : int or level name, default None
        For MultiIndex
    errors : {'ignore', 'raise'}, default 'raise'
        If 'ignore', suppress error and existing labels are dropped.

    """
    axis = self._get_axis_number(axis)
    axis_name = self._get_axis_name(axis)
    axis, axis_ = self._get_axis(axis), axis

    if axis.is_unique:
        if level is not None:
            if not isinstance(axis, MultiIndex):
                raise AssertionError("axis must be a MultiIndex")
            new_axis = axis.drop(labels, level=level, errors=errors)
        else:
            new_axis = axis.drop(labels, errors=errors)
        dropped = self.reindex(**{axis_name: new_axis})
        try:
            dropped.axes[axis_].set_names(axis.names, inplace=True)
        except AttributeError:
            pass
        result = dropped

    else:
        labels = _ensure_object(com._index_labels_to_array(labels))
        if level is not None:
            if not isinstance(axis, MultiIndex):
                raise AssertionError("axis must be a MultiIndex")
            indexer = ~axis.get_level_values(level).isin(labels)
        else:
            indexer = ~axis.isin(labels)

        slicer = [slice(None)] * self.ndim
        slicer[self._get_axis_number(axis_name)] = indexer

        result = self.loc[tuple(slicer)]

    return result


# Traceback (ValueError)
items.drop('abcd',axis=1)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/local/lib/python3.6/site-packages/pandas/core/generic.py", line 2530, in drop
obj = obj._drop_axis(labels, axis, level=level, errors=errors)
File "/usr/local/lib/python3.6/site-packages/pandas/core/generic.py", line 2562, in _drop_axis
new_axis = axis.drop(labels, errors=errors)
File "/usr/local/lib/python3.6/site-packages/pandas/core/indexes/base.py", line 3744, in drop
labels[mask])
ValueError: labels ['abcd'] not contained in axis

# Fixed code
def _drop_axis(self, labels, axis, level=None, errors="raise"):
    """
    Drop labels from specified axis. Used in the ``drop`` method
    internally.

    Parameters
    ----------
    labels : single label or list-like
    axis : int or axis name
    level : int or level name, default None
        For MultiIndex
    errors : {'ignore', 'raise'}, default 'raise'
        If 'ignore', suppress error and existing labels are dropped.

    """
    axis = self._get_axis_number(axis)
    axis_name = self._get_axis_name(axis)
    axis, axis_ = self._get_axis(axis), axis

    if axis.is_unique:
        if level is not None:
            if not isinstance(axis, MultiIndex):
                raise AssertionError("axis must be a MultiIndex")
            new_axis = axis.drop(labels, level=level, errors=errors)
        else:
            new_axis = axis.drop(labels, errors=errors)
        dropped = self.reindex(**{axis_name: new_axis})
        try:
            dropped.axes[axis_].set_names(axis.names, inplace=True)
        except AttributeError:
            pass
        result = dropped

    else:
        labels = _ensure_object(com._index_labels_to_array(labels))
        if level is not None:
            if not isinstance(axis, MultiIndex):
                raise AssertionError("axis must be a MultiIndex")
            indexer = ~axis.get_level_values(level).isin(labels)
        else:
            indexer = ~axis.isin(labels)

        if errors == "raise" and indexer.all():
            raise KeyError("{} not found in axis".format(labels))

        slicer = [slice(None)] * self.ndim
        slicer[self._get_axis_number(axis_name)] = indexer

        result = self.loc[tuple(slicer)]

    return result