Issue 4970 - Document the library API of python-abp

README.md

 # python-abp
 
-This repository contains the script that is used for building Adblock Plus
-filter lists from the form in which they are authored into the format suitable
-for consumption by the adblocking software.
+This repository contains a library for working with Adblock Plus filter lists
+and the script that is used for building Adblock Plus filter lists from the
+form in which they are authored into the format suitable for consumption by the
+adblocking software.
 
 ## Installation
 
 Prerequisites:
 
 * Linux, Mac OS X or Windows (any modern Unix should work too),
-* Python (2.7 or 3.5),
+* Python (2.7 or 3.5+),
 * pip.
 
 To install:
 
     $ pip install -U python-abp
 
 ## Rendering of filter lists
 
 The filter lists are originally authored in relatively smaller parts focused
 on a particular type of filters, related to a specific topic or relevant
 for particular geographical area.
 We call these parts _filter list fragments_ (or just _fragments_)
 to distinguish them from full filter lists that are
 consumed by the adblocking software such as Adblock Plus.
 
 Rendering is a process that combines filter list fragments into a filter list.
 It starts with one fragment that can include other ones and so forth.
 The produced filter list is marked with a version, a timestamp and 
-a [checksum](https://adblockplus.org/filters#special-comments).
+a [checksum][1].
 
 Python-abp contains a script that can do this called `flrender`:
 
     $ flrender fragment.txt filterlist.txt
 
 This will take the top level fragment in `fragment.txt`, render it and save into
 `filterlist.txt`.
 
 Fragments might reference other fragments that should be included into them.
 The references come in two forms: http(s) includes and local includes:
 
     %include http://www.server.org/dir/list.txt%
-    %include easylist:easylist/easylist_general_block.txt
+    %include easylist:easylist/easylist_general_block.txt%
 
 The first instruction contains a URL that will be fetched and inserted at the
 point of reference. 
 The second one contains a path inside easylist repository.
 `flrender` needs to be able to find a copy of the repository on the local
 filesystem. We use `-i` option to point it to to the right directory:
 
     $ flrender -i easylist=/home/abc/easylist input.txt output.txt
 
 Now the second reference above will be resolved to
 `/home/abc/easylist/easylist/easylist_general_block.txt` and the fragment will
-be read from this file.
+be loaded from this file.
 
 Directories that contain filter list fragments that are used during rendering
 are called sources.
 They are normally working copies of the repositories that contain filter list
 fragments.
 Each source is identified by a name: that's the part that comes before ":"
 in the include instruction and it should be the same as what comes before "="
 in the `-i` option.
 
 Commonly used sources have generally accepted names. For example the main
 EasyList repository is referred to as `easylist`.
 If you don't know all the source names that are needed to render some list,
 just run `flrender` and it will report what it's missing:
 
     $ flrender easylist.txt output/easylist.txt
     Unknown source: 'easylist' when including 'easylist:easylist/easylist_gener
     al_block.txt' from 'easylist.txt'
 
 You can clone the necessary repositories to a local directory and add `-i`
 options accordingly.
 
+## Library API
+
+Python-abp can also be used as a library for parsing filter lists. For example
+to read a filter list (we use Python 3 syntax here but the API is the same):
+
+    from abp.filters import parse_filterlist
+
+    with open('filterlist.txt') as filterlist:
+        for line in parse_filterlist(filterlist):
+            print(line)
+
+If `filterlist.txt` contains a filter list:
+
+    [Adblock Plus 2.0]
+    ! Title: Example list
+
+    abc.com,cdf.com##div#ad1
+    abc.com/ad$image
+    @@/abc\.com/
+    ...
+
+the output will look something like:
+
+    Header(version='Adblock Plus 2.0')
+    Metadata(key='Title', value='Example list')
+    EmptyLine()
+    Filter(text='abc.com,cdf.com##div#ad1', selector={'type': 'css', 'value': 'div#ad1'}, action='hide', options=[('domain', [('abc .com', True), ('cdf.com', True)])])
+    Filter(text='abc.com/ad$image', selector={'type': 'url-pattern', 'value': 'abc.com/ad'}, action='block', options=[('image', True)])
+    Filter(text='@@/abc\\.com/', selector={'type': 'url-regexp', 'value': 'abc\\.com'}, action='allow', options=[])
+    ...
+
+`abp.filters` module also exports a lower-level function for parsing individual
+lines of a filter list: `parse_line`. It returns a parsed line object just like
+the items in the iterator returned by `parse_filterlist`. 
+
+For further information on the library API use `help()` on `abp.filters` and
+its contents in interactive Python session, read the docstrings or look at the
+tests for some usage examples.
+
 ## Testing
 
 Unit tests for `python-abp` are located in the `/tests` directory.
-[Pytest](http://pytest.org/) is used for quickly running the tests
+[Pytest][3] is used for quickly running the tests
 during development.
-[Tox](https://tox.readthedocs.org/) is used for testing in different
-environments (Python 2.7, Python 3.5 and PyPy) and code quality
+[Tox][4] is used for testing in different
+environments (Python 2.7, Python 3.5+ and PyPy) and code quality
 reporting.
 
 In order to execute the tests, first create and activate development
 virtualenv:
 
     $ python setup.py devenv
     $ . devenv/bin/activate
 
 With the development virtualenv activated use pytest for a quick test run:
 
-    (devenv) $ py.test tests
+    (devenv) $ pytest tests
 
 and tox for a comprehensive report:
 
     (devenv) $ tox
+
+## Development
+
+When adding new functionality, add tests for it (preferably first). Code
+coverage (as measured by `tox -e qa`) should not decrease and the tests
+should pass in all Tox environments.
+
+All public functions, classes and methods should have docstrings compliant with
+[NumPy/SciPy documentation guide][5]. One exception is the constructors of
+classes that the user is not expected to instantiate (such as exceptions).
+
+ [1]: https://adblockplus.org/filters#special-comments
+ [2]: https://adblockplus.org/filters#options
+ [3]: http://pytest.org/
+ [4]: https://tox.readthedocs.org/
+ [5]: https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt