=======  ====================================
SEP      4
Title    Library-Like API for quick scraping
Author   Pablo Hoffman
Created  2009-07-21
Status   Archived
=======  ====================================

====================
SEP-004: Library API
====================
.. note:: the library API has been implemented, but slightly different from
          proposed in this SEP. You can run a Scrapy crawler inside a Twisted
          reactor, but not outside it. 

Introduction
============

It would be desirable for Scrapy to provide a quick, "light-weight" mechanism
for implementing crawlers by just using callback functions. That way you could
use Scrapy as any standard library (like you would use os.walk) in a script
without the overhead of having to create an entire project from scratch.

Proposed API
============

Here's a simple proof-of-concept code of such script:

::

   #!/usr/bin/env python
   from scrapy.http import Request
   from scrapy import Crawler

   # a container to hold scraped items
   scraped_items = []

   def parse_start_page(response):
       # collect urls to follow into urls_to_follow list
       requests = [Request(url, callback=parse_other_page) for url in urls_to_follow]
       return requests

   def parse_other_page(response):
       # ... parse items from response content ...
       scraped_items.extend(parsed_items)

   start_urls = ["http://www.example.com/start_page.html"]

   cr = Crawler(start_urls, callback=parse_start_page)
   cr.run() # blocking call - this populates scraped_items

   print "%d items scraped" % len(scraped_items)
   # ... do something more interesting with scraped_items ...

The behaviour of the Scrapy crawler would be controller by the Scrapy settings,
naturally, just like any typical scrapy project. But the default settings
should be sufficient so as to not require adding any specific setting. But, at
the same time, you could do it if you need to, say, for specifying a custom
middleware.

It shouldn't be hard to implement this API as all this functionality is a
(small) subset of the current Scrapy functionality. At the same time, it would
provide an additional incentive for newcomers.

Crawler class
=============

The Crawler class would have the following instance arguments (most of them
have been singletons so far):

- engine
- settings
- spiders
- extensions

Spider Manager
==============

The role of the spider manager will be to "resolve" spiders from URLs and
domains. Also, it should be moved outside scrapy.spider (and only BaseSpider
left there).

There is also the ``close_spider()`` method which is called for all closed
spiders, even when they weren't resolved first by the spider manager. We need
to decide what to do with this method.