gain

gain

Async web crawling framework for everyone.

Built on asyncio, aiohttp, and lxml/pyquery. Declare items and parsers; gain handles the concurrency, retries, and persistence.

Install

pip install gain

Linux users can opt into uvloop for an extra speed bump:

pip install "gain[uvloop]"

Requires Python 3.10+.

Quickstart

import aiofiles
from gain import Css, Item, Parser, Spider


class Post(Item):
    title = Css(".entry-title")
    content = Css(".entry-content")

    async def save(self):
        async with aiofiles.open("scrapinghub.txt", "a+") as f:
            await f.write(self.results["title"] + "\n")


class MySpider(Spider):
    concurrency = 5
    headers = {"User-Agent": "Google Spider"}
    start_url = "https://blog.scrapinghub.com/"
    parsers = [
        Parser(r"https://blog.scrapinghub.com/page/\d+/"),
        Parser(r"https://blog.scrapinghub.com/\d{4}/\d{2}/\d{2}/[a-z0-9\-]+/", Post),
    ]


MySpider.run()

Run it:

python spider.py

XPath parsers

from gain import Css, Item, Parser, Spider, XPathParser


class Post(Item):
    title = Css(".breadcrumb_last")

    async def save(self):
        print(self.title)


class MySpider(Spider):
    start_url = "https://mydramatime.com/europe-and-us-drama/"
    concurrency = 5
    headers = {"User-Agent": "Google Spider"}
    parsers = [
        XPathParser('//span[@class="category-name"]/a/@href'),
        XPathParser('//div[contains(@class, "pagination")]/ul/li/a[contains(@href, "page")]/@href'),
        XPathParser('//div[@class="mini-left"]//div[contains(@class, "mini-title")]/a/@href', Post),
    ]
    proxy = "https://localhost:1234"


MySpider.run()

How it works

   ┌────────────┐    ┌────────────┐    ┌────────────┐    ┌────────────┐
   │  start_url │ ─▶ │  Parser    │ ─▶ │  Item      │ ─▶ │ save()     │
   │            │    │  (follow)  │    │  (extract) │    │  (persist) │
   └────────────┘    └────────────┘    └────────────┘    └────────────┘
                          ▲                                      │
                          └──────────── new urls ────────────────┘

1. Spider kicks off from start_url under a concurrency budget.
2. Parsers either follow (one argument) — discovering more URLs to queue — or extract (two arguments) — instantiating an Item from each matching page.
3. Items use Css / Xpath / Regex selectors to pull fields out of HTML.
4. save() is your async hook to persist results — write a file, push to a queue, insert into a database.

Examples

See the example/ directory for runnable scripts against Scrapinghub, V2EX, and Sciencenet.

Development

git clone https://github.com/elliotgao2/gain.git
cd gain
uv sync                 # install deps into .venv
uv run pytest           # run tests
uv run ruff check .     # lint

We use uv for packaging and ruff for lint + format. Install the pre-commit hooks:

uv run pre-commit install

Contributing

Pull requests are welcome. For non-trivial changes, please open an issue first to discuss. Make sure pytest and ruff check pass before submitting.

License

MIT © Elliot Gao