Method _handle_usecols

pandas/io/parsers/python_parser.py:767–813

Sets self._col_indices usecols_key is used if there are string usecols.

(
        self,
        columns: list[list[Scalar | None]],
        usecols_key: list[Scalar | None],
        num_original_columns: int,
    )

        return line

    def _handle_usecols(
        self,
        columns: list[list[Scalar | None]],
        usecols_key: list[Scalar | None],
        num_original_columns: int,
    ) -> list[list[Scalar | None]]:
        """
        Sets self._col_indices

        usecols_key is used if there are string usecols.
        """
        col_indices: set[int] | list[int]
        if self.usecols is not None:
            if callable(self.usecols):
                col_indices = evaluate_callable_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:
                            self._validate_usecols_names(self.usecols, usecols_key)
                    else:
                        col_indices.append(col)
            else:
                missing_usecols = [
                    col for col in self.usecols if col >= num_original_columns
                ]
                if missing_usecols:
                    raise ParserError(
                        "Defining usecols with out-of-bounds indices is not allowed. "
                        f"{missing_usecols} are out-of-bounds.",
                    )
                col_indices = self.usecols

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

    def _buffered_line(self) -> list[Scalar]:
        """