Repository: pandas-dev/pandas
File: pandas/io/parsers/base_parser.py (Python)

Code before:
        if not isinstance(mapping, dict):
            return mapping
        clean = {}
        # for mypy
        assert self.orig_names is not None

        for col, v in mapping.items():
            if isinstance(col, int) and col not in self.orig_names:
                col = self.orig_names[col]
            clean[col] = v
        if isinstance(mapping, defaultdict):
            remaining_cols = set(self.orig_names) - set(clean.keys())
            clean.update({col: mapping[col] for col in remaining_cols})
        return clean

    @final
    def _agg_index(self, index) -> Index:
        arrays = []
        converters = self._clean_mapping(self.converters)
        clean_dtypes = self._clean_mapping(self.dtype)

        if self.index_names is not None:
            names: Iterable = self.index_names
        else:
            names = itertools.cycle([None])
        for i, (arr, name) in enumerate(zip(index, names, strict=True)):
            if self._should_parse_dates(i):
                arr = date_converter(
                    arr,
                    col=self.index_names[i] if self.index_names is not None else None,
                    dayfirst=self.dayfirst,
                    cache_dates=self.cache_dates,
                    date_format=self.date_format,
                )

            if self.na_filter:
                col_na_values = self.na_values
                col_na_fvalues = self.na_fvalues
            else:
                col_na_values = set()
                col_na_fvalues = set()

            if isinstance(self.na_values, dict):
                assert self.index_names is not None
                col_name = self.index_names[i]
                if col_name is not None:
                    col_na_values, col_na_fvalues = get_na_values(
                        col_name, self.na_values, self.na_fvalues, self.keep_default_na
                    )
                else:
                    col_na_values, col_na_fvalues = set(), set()

Review comment (suggestion):
```suggestion
            names = itertools.cycle([None])
            strict = False
```

Code after:
        if not isinstance(mapping, dict):
            return mapping
        clean = {}
        # for mypy
        assert self.orig_names is not None

        for col, v in mapping.items():
            if isinstance(col, int) and col not in self.orig_names:
                col = self.orig_names[col]
            clean[col] = v
        if isinstance(mapping, defaultdict):
            remaining_cols = set(self.orig_names) - set(clean.keys())
            clean.update({col: mapping[col] for col in remaining_cols})
        return clean

    @final
    def _agg_index(self, index) -> Index:
        arrays = []
        converters = self._clean_mapping(self.converters)
        clean_dtypes = self._clean_mapping(self.dtype)

        if self.index_names is not None:
            names: Iterable = self.index_names
            zip_strict = True
        else:
            names = itertools.cycle([None])
            zip_strict = False
        for i, (arr, name) in enumerate(zip(index, names, strict=zip_strict)):
            if self._should_parse_dates(i):
                arr = date_converter(
                    arr,
                    col=self.index_names[i] if self.index_names is not None else None,
                    dayfirst=self.dayfirst,
                    cache_dates=self.cache_dates,
                    date_format=self.date_format,
                )

            if self.na_filter:
                col_na_values = self.na_values
                col_na_fvalues = self.na_fvalues
            else:
                col_na_values = set()
                col_na_fvalues = set()

            if isinstance(self.na_values, dict):
                assert self.index_names is not None
                col_name = self.index_names[i]
                if col_name is not None:
                    col_na_values, col_na_fvalues = get_na_values(
                        col_name, self.na_values, self.na_fvalues, self.keep_default_na
                    )