Repository: pypa/pip
File: src/pip/_internal/req/req_file.py (Python)

Code before:
            if not line.is_requirement and (
                line.opts.requirements or line.opts.constraints
            ):
                # parse a nested requirements file
                if line.opts.requirements:
                    req_path = line.opts.requirements[0]
                    nested_constraint = False
                else:
                    req_path = line.opts.constraints[0]
                    nested_constraint = True

                # original file is over http
                if SCHEME_RE.search(filename):
                    # do a url join so relative paths work
                    req_path = urllib.parse.urljoin(filename, req_path)
                # original file and nested file are paths
                elif not SCHEME_RE.search(req_path):
                    # do a join so relative paths work
                    # and then abspath so that we can identify recursive references
                    req_path = os.path.abspath(
                        os.path.join(
                            os.path.dirname(filename),
                            req_path,
                        )
                    )
                if req_path in self._parsed_files.keys():
                    initial_file = self._parsed_files[req_path]
                    tail = (
                        f" and again in {initial_file}"
                        if initial_file is not None
                        else ""
                    )
                    raise RequirementsFileParseError(
                        f"{req_path} recursively references itself in {filename}{tail}"
                    )
                # Keeping a track where was each file first included in
                self._parsed_files[req_path] = filename
                yield from self._parse_and_recurse(req_path, nested_constraint)
            else:
                yield line

    def _parse_file(
        self, filename: str, constraint: bool
    ) -> Generator[ParsedLine, None, None]:
        _, content = get_file_content(filename, self._session)

        lines_enum = preprocess(content)

        for line_number, line in lines_enum:
            try:
                args_str, opts = self._line_parser(line)

Review comment (suggestion):
```suggestion
                if req_path in self._parsed_files:
```

Code after:
            if not line.is_requirement and (
                line.opts.requirements or line.opts.constraints
            ):
                # parse a nested requirements file
                if line.opts.requirements:
                    req_path = line.opts.requirements[0]
                    nested_constraint = False
                else:
                    req_path = line.opts.constraints[0]
                    nested_constraint = True

                # original file is over http
                if SCHEME_RE.search(filename):
                    # do a url join so relative paths work
                    req_path = urllib.parse.urljoin(filename, req_path)
                # original file and nested file are paths
                elif not SCHEME_RE.search(req_path):
                    # do a join so relative paths work
                    # and then abspath so that we can identify recursive references
                    req_path = os.path.abspath(
                        os.path.join(
                            os.path.dirname(filename),
                            req_path,
                        )
                    )
                if req_path in self._parsed_files:
                    initial_file = self._parsed_files[req_path]
                    tail = (
                        f" and again in {initial_file}"
                        if initial_file is not None
                        else ""
                    )
                    raise RequirementsFileParseError(
                        f"{req_path} recursively references itself in {filename}{tail}"
                    )
                # Keeping a track where was each file first included in
                self._parsed_files[req_path] = filename
                yield from self._parse_and_recurse(req_path, nested_constraint)
            else:
                yield line

    def _parse_file(
        self, filename: str, constraint: bool
    ) -> Generator[ParsedLine, None, None]:
        _, content = get_file_content(filename, self._session)

        lines_enum = preprocess(content)

        for line_number, line in lines_enum:
            try:
                args_str, opts = self._line_parser(line)