# Source code for scrapy.downloadermiddlewares.defaultheaders

    """
    DefaultHeaders downloader middleware

    See documentation in docs/topics/downloader-middleware.rst
    """

    from __future__ import annotations

    from typing import TYPE_CHECKING

    from scrapy.utils.decorators import _warn_spider_arg
    from scrapy.utils.python import without_none_values

    if TYPE_CHECKING:
        from collections.abc import Iterable

        # typing.Self requires Python 3.11
        from typing_extensions import Self

        from scrapy import Request, Spider
        from scrapy.crawler import Crawler
        from scrapy.http import Response

    class DefaultHeadersMiddleware:
        def __init__(self, headers: Iterable[tuple[str, str]]):
            self._headers: Iterable[tuple[str, str]] = headers

        @classmethod
        def from_crawler(cls, crawler: Crawler) -> Self:
            headers = without_none_values(crawler.settings["DEFAULT_REQUEST_HEADERS"])
            return cls(headers.items())

        @_warn_spider_arg
        def process_request(
            self, request: Request, spider: Spider | None = None
        ) -> Request | Response | None:
            for k, v in self._headers:
                request.headers.setdefault(k, v)
            return None