How to add headers to every or some scrapy requests?

There are several ways to add headers in scrapy spiders. This can be done for each request manually:

class MySpider(scrapy.Spider):
    def parse(self, response):
        yield scrapy.Request(..., headers={"x-token": "123"})

However to automatically add headers to every or specific outgoing scrapy requests the DEAFAULT_REQUEST_HEADERS setting can be used:

# settings.py
DEFAULT_REQUEST_HEADERS = {
    "User-Agent": "my awesome scrapy robot",
}

In case more complex logic is needed like adding headers only to some requests or random User-Agent header a request middleware is the best option:

# middlewares.py
import random


class RandomUserAgentMiddleware:
    def __init__(self, user_agents):
        self.user_agents = user_agents

    @classmethod
    def from_crawler(cls, crawler):
        """retrieve user agent list from settings.USER_AGENTS"""
        user_agents = crawler.settings.get('USER_AGENTS', [])
        if not user_agents:
            raise ValueError('No user agents found in settings. Please provide a list of user agents in the USER_AGENTS setting.')
        return cls(user_agents)

    def process_request(self, request, spider):
        """attach random user agent to every outgoing request"""
        user_agent = random.choice(self.user_agents)
        request.headers.setdefault('User-Agent', user_agent)
        spider.logger.debug(f'Using User-Agent: {user_agent}')

# settings.py
MIDDLEWARES = {
    # ...
    'myproject.middlewares.RandomUserAgentMiddleware': 760,
    # ...
}

USER_AGENTS = [
    'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36',
    'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36',
    'Mozilla/5.0 (Windows NT 6.1; WOW64; rv:54.0) Gecko/20100101 Firefox/54.0',
    'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.100 Safari/537.36',
    # ...
]

Note that if you're using Scrapfly's scrapy SDK some headers like User-Agent string are automatically by the smart anti-blocking API.