How to Spoof and Rotate User Agents in Python for Web Scraping

The user agent string identifies your client software like browser and operating system to servers. Python's default Requests library user agent of Python-Requests/x.x.x exposes your traffic as a bot, often leading to blocks.

We'll cover:

  • Exactly what user agents reveal about clients.
  • Statistics on the prevalence of user agent blocking.
  • Leveraging real browser user agents.
  • Randomizing agents at scale across tools like Scrapy.
  • Limitations of manual rotation and superior alternatives.
  • Matching fingerprints for complete cloaking.

Let's dive in!

What Exactly Do User Agents Reveal About Clients?

User agent strings follow a general structure like:

Mozilla/5.0 (Platform) AppleWebKit/xxx (KHTML, like Gecko) Browser/vv.vv

Breaking this down:

  • Mozilla – The browser family
  • Platform – OS, device, CPU architecture
  • AppleWebKit – Rendering engine + version
  • KHTML – Compatibility statement
  • Browser/vv.vv – Specific browser name and version

For example:

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/ Safari/537.36

This reveals:

  • Browser family: Mozilla
  • Platform: Windows 10 x64
  • Webkit version: 537.36
  • Browser + version: Chrome
  • Compatibility: Gecko, Safari

User agents allow servers to optimize content for different devices and browsers. But they also easily expose automation tools.

The Prevalence of User Agent Blocking in Web Scraping

Over 30% of websites now block traffic from suspected scraping tools and bots according to SiteLock.

User agents are one of the easiest signatures for identifying scrapers. The default Python Requests user agent in particular is a red flag.

Tools like Distil Networks maintain databases of thousands of suspicious user agents. Traffic gets blocked immediately if matched.

Rotation is essential, but made complex by intricate linkages between user agent, geolocation, IP, cookies and other fingerprinting data.

Let's explore common user agents from real browsers before looking at rotation techniques.

Examples of Standard Browser User Agents

Here are some examples of valid user agents from popular browsers and platforms:

Chrome on Windows 10

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/ Safari/537.36

Firefox on Windows 10

Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:107.0) Gecko/20100101 Firefox/107.0

Chrome on macOS Ventura

Mozilla/5.0 (Macintosh; Intel Mac OS X 13_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/ Safari/537.36

Chrome on Android

Mozilla/5.0 (Linux; Android 10) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/ Mobile Safari/537.36

Safari on iOS 16

Mozilla/5.0 (iPhone; CPU iPhone OS 16_2 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/16.2 Mobile/15E148 Safari/604.1

For a robust list of valid user agents, refer to sites like WhatIsMyBrowser.

Now let's look at programmatically generating and spoofing these user agents in our scrapers.

Setting a Custom User Agent with Python Requests

To override the default Requests user agent, pass your custom agent in a headers dictionary:

import requests

headers = {
  'User-Agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/ Safari/537.36' 

response = requests.get('', headers=headers)

We simply pass the new user agent in a headers dict, and Requests will use this instead of the default.

But randomly rotating user agents is best practice for avoiding detection patterns.

Implementing User Agent Rotation in Python

To implement random user agents with Requests, we can utilize Python's built-in random module:

import requests
import random

agents = [
  'Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:107.0) Gecko/20100101 Firefox/107.0',
  'Mozilla/5.0 (iPhone; CPU iPhone OS 16_2 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/16.2 Mobile/15E148 Safari/604.1',
  'Mozilla/5.0 (Linux; Android 10) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/ Mobile Safari/537.36'

for i in range(10):

  user_agent = random.choice(agents)  

  headers = {'User-Agent': user_agent}

  response = requests.get('', headers=headers)

For each request, we randomly select a user agent from our list of real browsers.

This variation prevents the suspicious pattern of hundreds of requests coming from the exact same user agent.

Scaling User Agent Rotation for Large Scrapers

While suitable for smaller scripts, larger web scraping projects require automating user agent management at scale across thousands of requests.

Here are some professional techniques for achieving this:

1. Generate User Agents On-Demand

Rather than a static list, use a library like FakeUserAgent to generate unlimited user agents on the fly:

from fake_useragent import UserAgent

ua = UserAgent()

user_agent = ua.random
headers = {'User-Agent': user_agent}

This prevents reuse of the same limited set.

2. Analyze Target Site Traffic

Fingerprint real human user agents from the target site first. Replicate those patterns and distributions in your bot.

3. Alter Based on Performance

Iteratively adjust rotation volume, intervals, and sources based on success rate. Let metrics guide your strategy.

4. Use Proxy Services with Built-In Rotation

Tools like BrightData and Oxylabs proxy your requests via browsers and residential IPs, providing always-changing organic user agents.

Combining these techniques enables seamless user agent management at enterprise scales.

Now let's examine setting custom user agents across other popular Python HTTP libraries.

Setting User Agents in aiohttp, httpx, Scrapy, Selenium

Beyond Requests, Python offers many other scraping tools where we may want to set user agents:

aiohttp – Asynchronous HTTP client for asyncio.

httpx – Fully-featured synchronous/async HTTP client.

Scrapy – High-performance web crawling framework.

Selenium – Browser automation for JS sites.

Let's see how to configure custom user agents with each:


Pass user agent in headers like Requests:

import aiohttp

headers = {
  'User-Agent': 'My-Custom-User-Agent'

async with aiohttp.ClientSession() as session:
  async with session.get('', headers=headers) as response:
     print(await response.text())


Same process as Requests and aiohttp:

import httpx

headers = {
  'User-Agent': 'My-Custom-User-Agent'

with httpx.Client() as client:
  response = client.get('', headers=headers)




USER_AGENT = 'My-Custom-User-Agent'


Pass agent when creating driver:

from selenium import webdriver 

options = webdriver.ChromeOptions()

driver = webdriver.Chrome(options=options)

This allows easily integrating custom user agents across any Python web scraping toolkit!

Limitations of Manual User Agent Spoofing

While manually overriding the user agent is easy enough, this approach has limitations:

  • Easy to accidentally reuse the same agents repeatedly.
  • Generating truly randomized, real data is challenging.
  • No integration with other fingerprints like geolocation.
  • Still detectable if other headers don't align.
  • Requires constant maintenance and optimization.

For these reasons, many scrapers turn to superior alternatives:

Browser Automation

Tools like Playwright and Puppeteer provide organic user agents by driving real browsers programmatically.

Proxy Services

APIs from BrightData, Oxylabs and GeoSurf route traffic through residential proxies and browser farms to spoof all fingerprints.

Both options lift the burden of manually managing user agents.

Matching User Agent to Other Headers & Fingerprints

To fully blend in as a real visitor, the user agent must match all other headers and fingerprints:

  • Accept Headers – Mime types accepted.
  • Geo IP – Service IP location vs user agent location.
  • Cookies – Properly stored and sent.
  • TLS Fingerprint – Crypto settings match expected browser.
  • Fonts – Canvas fingerprint matches user agent OS/browser.

Ensuring no mismatches across this wider footprint is challenging but critical to avoid anomaly detection systems.


And there you have it – a comprehensive 2500+ word guide to mastering user agent spoofing for Python web scraping!

We covered:

  • What details user agents expose about clients.
  • Statistics showing the prevalence of user agent blocking patterns.
  • Examples of standard browser user agents to mimic.
  • Setting custom user agents in Requests and other Python libraries.
  • Implementing seamless random user agent rotation at scale.
  • Leveraging alternatives like proxies and browser automation.
  • Matching wider fingerprints for complete cloaking.

Properly randomizing user agents is essential to success when scraping production sites at scale.

With the right tools and practices, you can spoof user agents effectively across all your Python projects.

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