How to Make Web Scraping Faster – Python Tutorial

• How do you speed up web scraping in Python?
• Web scraping without optimization
• Preparation
• Writing unoptimized web scraper
• Web scraping using multiprocessing
• Web scraping using multithreading
• Asyncio for asynchronous programming

How do you speed up web scraping in Python?

There are a few possible approaches that can help increase the scraping speed:

• Multiprocessing

• Multithreading

• Asyncio

However, let’s first take a look at an unoptimized code to make sure the difference between all is clear.

Web scraping without optimization

We'll be scraping 1000 books from books.toscrape.com. This website is a dummy book store that's perfect for learning.

Preparation

The first step is to extract all 1000 links to the books and store them in a CSV file. Run this code file to create the links.csv file. You will need to install requests and Beautiful Soup packages for this code to work.

import requests
from bs4 import BeautifulSoup
from urllib.parse import urljoin


def fetch_links(url="https://books.toscrape.com/", links=[]):
    r = requests.get(url)
    print(r.url, flush=True)
    soup = BeautifulSoup(r.text, "html.parser")
    for link in soup.select("h3 a"):
        links.append(urljoin(url, link.get("href")))
    next_page = soup.select_one("li.next a")
    if next_page:
        return fetch_links(urljoin(url, next_page.get("href"), links))
    else:
        return links


def refresh_links():
    links = fetch_links()
    with open('links.csv', 'w') as f:
        for link in links:
            f.write(link + '\n')

refresh_links()

The fetch_links function will retrieve all the links, and refresh_links() will store the output in a file. We skipped sending the user agent as this is a test site. However, you can do so easily using the requests library.

Writing unoptimized web scraper

We'll focus on optimizing 1,000 pages of web scraping in Python.

First, install the requests library using pip:

pip install requests

To keep things simple, we'll use regular expressions to extract the title element of the page. Note the get_links functions that loads the urls we saved in the previous step.

import csv
import re
import time
import requests

def get_links():
	links = []
	with open("links.csv", "r") as f:
		reader = csv.reader(f)
		for i, row in enumerate(reader):
			links.append(row[0])
	return links

def get_response(session, url):
	with session.get(url) as resp:
		print('.', end='', flush=True)
		text = resp.text
		exp = r'(<title>).*(<\/title>)'
		return re.search(exp, text, flags=re.DOTALL).group(0)

def main():
	start_time = time.time()
	with requests.Session() as session:
		results = []
		for url in get_links():
			result = get_response(session, url)
			print(result)
	print(f"{(time.time() - start_time):.2f} seconds")


main()

The code without optimization code took 288.62 seconds.

Web scraping using multiprocessing

Multiprocessing, as the name suggests, is utilizing more than one processor. Most modern computers have more than one CPU core, if not multiple CPUs. Using the multiprocessing module, included with the Python standard library, we can write code that uses all these cores.

For example, if we have an 8-core CPU, we can essentially write code that can split the task into eight different processes where each process runs in a separate CPU core. Note that this approach is more suitable when the bottleneck is CPU or when the code is CPU-Bound. We'll still see some improvements in our case, though.

The first step is to import Pool and cpu_count from the multiprocessing module

from multiprocessing import Pool

The other change is required in both get_response and main functions.

def get_response(url):
	resp = requests.get(url)
	
	print('.', end='', flush=True)
	text = resp.text
	exp = r'(<title>).*(<\/title>)'
	return re.search(exp, text, flags=re.DOTALL).group(0)

def main():
	start_time = time.time()
	links = get_links()
	coresNr = multiprocessing.cpu_count()
	with Pool(coresNr) as p:
		results = p.map(get_response, links)
		for result in results:
			print(result)
	print(f"{(time.time() - start_time):.2f} seconds")

if __name__ == '__main__':
	main()

The most critical line of the code is where we create a Pool. Note that we're using cpu_count() function to get the count of CPU cores dynamically. This ensures that this code runs on every machine without any change.

In this example, the execution time was around 49 seconds. It's a better result compared to an unoptimized code, where the same process took around 126 seconds. Still, as expected, it's a slight improvement. As we mentioned, multiprocessing is more suitable when the code is CPU-Bound. Our code is I/O bound; thus, we can improve the performance of this code more with other methods.

Web scraping using multithreading

Multithreading is a great option to optimize web scraping code. Simply put, a thread is a separate flow of execution. Operating systems typically create hundreds of threads and switch the CPU time among them. The switching process is so fast that you can get the illusion of multitasking. It cannot be customized because the CPU controls the switching.

Using the concurrent.futures module of Python, we can customize how many threads we create to optimize our code. There is only one huge caveat: managing threads can become messy and error-prone as the code becomes more complex.

To change our code to utilize multithreading, minimal changes are needed.

First, import ThreadPoolExecutor.

from concurrent.futures import ThreadPoolExecutor

Next, instead of creating a Pool, create a ThreadPoolExecutor:

with ThreadPoolExecutor(max_workers=100) as p:
    results = p.map(get_response, links)

Note that you have to specify max workers. This number will depend on the complexity of the code. However, choosing a too-high number can overload your code, so you must be careful.

This script execution was completed in 7.02 seconds. For reference, the unoptimized code took around 126 seconds. This is a massive improvement.

Asyncio for asynchronous programming

Asynchronous coding using the asyncio module is essentially threading where the code controls the context switching. It also makes coding more effortless and less error-prone. Specifically, for web scraping projects, this is the most suitable approach.

This approach requires quite a lot of changes. First, the requests library will not work. Instead, we'll use the aiohttp library for web scraping in Python. This requires a separate installation:

python3 -m pip install aiohttp

Next, import asyncio and aiohttp modules.

import aiohttp 
import asyncio

The get_response() function now needs to change to a coroutine. Also, we'll be using the same session for every execution. Optionally, you can send the user agent if needed.

Note the use of async and await keywords.

async def get_response(session, url):
	async with session.get(url) as resp:
		text = await resp.text()
		exp = r'(<title>).*(<\/title>)'
		return re.search(exp, text, flags=re.DOTALL).group(0)

The most significant changes are in the main() function.

First, it needs to change to a coroutine. Next, we'll use aiohttp.ClientSession to create the session object. Most importantly, we'll need to create tasks for all the links. Finally, all the tasks will be sent to an event loop using the asyncio.gather method.

async def main():
	start_time = time.time()

	async with aiohttp.ClientSession() as session:
		tasks = []
		for url in get_links():
			tasks.append(asyncio.create_task(get_response(session, url)))
		results = await asyncio.gather(*tasks)
		for result in results:
			print(result)
	print(f"{(time.time() - start_time):.2f} seconds")

Lastly, to run the main() coroutine, we'd need to use asyncio.run(main())

In this example, the execution time was 15.61 seconds.

The asyncio approach, as expected, also showed great results compared to unoptimized script. Of course, this approach requires an entirely new way of thinking. If you have experience with async-await in any programming language, it won't be hard for you to use this approach for your web scraping jobs.