Sets self._col_indices

        usecols_key is used if there are string usecols.

def _handle_usecols(self, columns, usecols_key):
        """
        Sets self._col_indices

        usecols_key is used if there are string usecols.
        """
        if self.usecols is not None:
            if callable(self.usecols):
                col_indices = _evaluate_usecols(self.usecols, usecols_key)
            elif any(isinstance(u, str) for u in self.usecols):
                if len(columns) > 1:
                    raise ValueError("If using multiple headers, usecols must "
                                     "be integers.")
                col_indices = []

                for col in self.usecols:
                    if isinstance(col, str):
                        try:
                            col_indices.append(usecols_key.index(col))
                        except ValueError:
                            _validate_usecols_names(self.usecols, usecols_key)
                    else:
                        col_indices.append(col)
            else:
                col_indices = self.usecols

            columns = [[n for i, n in enumerate(column) if i in col_indices]
                       for column in columns]
            self._col_indices = col_indices
        return columns