# Source code for scrapy.downloadermiddlewares.defaultheaders

    """
    DefaultHeaders downloader middleware

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

    from scrapy.utils.python import without_none_values

    class DefaultHeadersMiddleware:
        def __init__(self, headers):
            self._headers = headers

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

        def process_request(self, request, spider):
            for k, v in self._headers:
                request.headers.setdefault(k, v)