A middleware for falcon, python web framework, to handle multipart form requests. It stores files in `Request.files` and other form parameters in `Request.form_params`

falcon_multipart.py

FieldValue = Union[FieldStorage, str]

class MultipartMiddleware(Middleware):
    MEDIA_FORM = "multipart/form-data"
    METHODS_ALLOWED = {"PUT", "POST"}

    def _get_field_val(self, field: FieldStorage) -> FieldValue:
        """
        Returns input value for input text fields and FileStream object
        """

        if field.file and "filename" in field.disposition_options:
            return field

        return unescape(str(field.value))

    def process_request(self, req: Request, resp: Response, **kwargs):
        files = {}
        form_params = {}
        content_type: Optional[str] = req.content_type
        is_media_form = content_type is not None and self.MEDIA_FORM in content_type

        if req.method not in self.METHODS_ALLOWED:
            return

        if not is_media_form:
            return

        try:
            # This must be done to avoid a bug in cgi.FieldStorage.
            req.env.setdefault("QUERY_STRING", "")
            field_storage = FieldStorage(fp=req.stream, environ=req.env)
        except ValueError as e:
            raise HTTPBadRequest("Error parsing file", str(e))

        for field_key in field_storage.keys():
            field = field_storage[field_key]

            if isinstance(field, list):
                param_list = []
                file_list = []

                for item in field:
                    field_val = self._get_field_val(item)

                    if isinstance(field_val, str):
                        param_list.append(field_val)
                    elif field_val is not None:
                        file_list.append(field_val)

                if param_list:
                    form_params[field_key] = (
                        tlz.first(param_list) if len(param_list) == 1 else param_list
                    )

                if file_list:
                    files[field_key] = (
                        tlz.first(file_list) if len(file_list) == 1 else file_list
                    )
            else:
                field_val = self._get_field_val(field)

                if isinstance(field_val, str):
                    form_params[field_key] = field_val
                elif field_val is not None:
                    files[field_key] = field_val

        req.files = files
        req.form_params = form_params