# Source code for scrapy.downloadermiddlewares.useragent

    """Set User-Agent header per spider or use a default value from settings"""

    from __future__ import annotations

    from typing import TYPE_CHECKING

    from scrapy import Request, Spider, signals
    from scrapy.utils.decorators import _warn_spider_arg
    from scrapy.utils.deprecate import warn_on_deprecated_spider_attribute

    if TYPE_CHECKING:
        # typing.Self requires Python 3.11
        from typing_extensions import Self

        from scrapy.crawler import Crawler
        from scrapy.http import Response

    class UserAgentMiddleware:
        """This middleware allows spiders to override the user_agent"""

        def __init__(self, user_agent: str = "Scrapy"):
            self.user_agent = user_agent

        @classmethod
        def from_crawler(cls, crawler: Crawler) -> Self:
            o = cls(crawler.settings["USER_AGENT"])
            crawler.signals.connect(o.spider_opened, signal=signals.spider_opened)
            return o

        def spider_opened(self, spider: Spider) -> None:
            if hasattr(spider, "user_agent"):  # pragma: no cover
                warn_on_deprecated_spider_attribute("user_agent", "USER_AGENT")

            self.user_agent = getattr(spider, "user_agent", self.user_agent)

        @_warn_spider_arg
        def process_request(
            self, request: Request, spider: Spider | None = None
        ) -> Request | Response | None:
            if self.user_agent:
                request.headers.setdefault(b"User-Agent", self.user_agent)
            return None