Merge pull request #633 from aspidites/sep-004.rst

sep 4 for #629

sep/sep-004.rst

+=======  ====================================
+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.

sep/sep-004.trac

-= SEP-004: Library API =
-
-[[PageOutline(2-5, Contents)]]
-
-||'''SEP'''||4||
-||'''Title'''||Library-like API for quick scraping||
-||'''Author'''||Pablo Hoffman||
-||'''Created'''||2009-07-21||
-||'''Status'''||Archived||
-
-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. 
-
-See these snippets for some examples:
- * http://snippets.scrapy.org/snippets/8/
- * http://snippets.scrapy.org/snippets/9/
-
-== 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:
-
-{{{
-#!python
-#!/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.
-