Comment Extraction and Parsing
==============================

A common use for Reddit's API is to extract comments from submissions and use
them to perform keyword or phrase analysis.

As always, you need to begin by creating an instance of :class:`.Reddit`:

.. code-block:: python

   import asyncpraw

   reddit = asyncpraw.Reddit(user_agent='Comment Extraction (by /u/USERNAME)',
                        client_id='CLIENT_ID', client_secret="CLIENT_SECRET",
                        username='USERNAME', password='PASSWORD')

.. note:: If you are only analyzing public comments, entering a username and
   password is optional.

In this document we will detail the process of finding all the comments for a
given submission. If you instead want process all comments on Reddit, or
comments belonging to one or more specific subreddits, please see
:meth:`asyncpraw.models.reddit.subreddit.SubredditStream.comments`.

.. _extracting_comments:

Extracting comments with asyncpraw
-----------------------------

Assume we want to process the comments for this submission:
https://www.reddit.com/r/funny/comments/3g1jfi/buttons/

We first need to obtain a submission object. We can do that either with the
entire URL:

.. code-block:: python

   submission = reddit.submission(url='https://www.reddit.com/r/funny/comments/3g1jfi/buttons/')

or with the submission's ID which comes after ``comments/`` in the URL:

.. code-block:: python

   submission = reddit.submission(id='3g1jfi')

With a submission object we can then interact with its :class:`.CommentForest`
through the submission's :attr:`~asyncpraw.models.Submission.comments` attribute. A
:class:`.CommentForest` is a list of top-level comments each of which contains
a :class:`.CommentForest` of replies.

If we wanted to output only the ``body`` of the top level comments in the
thread we could do:

.. code-block:: python

   for top_level_comment in submission.comments:
       print(top_level_comment.body)

While running this you will most likely encounter the exception
``AttributeError: 'MoreComments' object has no attribute 'body'``. This
submission's comment forest contains a number of :class:`.MoreComments`
objects. These objects represent the "load more comments", and "continue this
thread" links encountered on the website. While we could ignore
:class:`.MoreComments` in our code, like so:

.. code-block:: python

   from asyncpraw.models import MoreComments
   for top_level_comment in submission.comments:
       if isinstance(top_level_comment, MoreComments):
           continue
       print(top_level_comment.body)

The ``replace_more`` method
---------------------------

In the previous snippet, we used ``isinstance`` to check whether the item
in the comment list was a :class:`.MoreComments` so that we could ignore it.
But there is a better way: the :class:`.CommentForest` object has a method
called :meth:`.replace_more`, which replaces or removes :class:`.MoreComments`
objects from the forest.

Each replacement requires one network request, and its response may yield
additional :class:`.MoreComments` instances. As a result, by default,
:meth:`.replace_more` only replaces at most thirty-two :class:`.MoreComments`
instances -- all other instances are simply removed. The maximum number of
instances to replace can be configured via the ``limit`` parameter.
Additionally a ``threshold`` parameter can be set to only perform replacement
of :class:`.MoreComments` instances that represent a minimum number of
comments; it defaults to 0, meaning all :class:`.MoreComments` instances will
be replaced up to ``limit``.

A ``limit`` of 0 simply removes all :class:`.MoreComments` from the forest.
The previous snippet can thus be simplified:

.. code-block:: python

   submission.comments.replace_more(limit=0)
   for top_level_comment in submission.comments:
       print(top_level_comment.body)

.. note:: Calling :meth:`.replace_more` is destructive. Calling it again on the
   same submission instance has no effect.

Meanwhile, a ``limit`` of ``None`` means that all :class:`.MoreComments`
objects will be replaced until there are none left, as long as they satisfy
the ``threshold``.

.. code-block:: python

   submission.comments.replace_more(limit=None)
   for top_level_comment in submission.comments:
       print(top_level_comment.body)

Now we are able to successfully iterate over all the top-level comments. What
about their replies? We could output all second-level comments like so:

.. code-block:: python

   submission.comments.replace_more(limit=None)
   for top_level_comment in submission.comments:
       for second_level_comment in top_level_comment.replies:
           print(second_level_comment.body)

However, the comment forest can be arbitrarily deep, so we'll want a more
robust solution. One way to iterate over a tree, or forest, is via a
breadth-first traversal using a queue:

.. code-block:: python

   submission.comments.replace_more(limit=None)
   comment_queue = submission.comments[:]  # Seed with top-level
   while comment_queue:
       comment = comment_queue.pop(0)
       print(comment.body)
       comment_queue.extend(comment.replies)

The above code will output all the top-level comments, followed by
second-level, third-level, etc. While it is awesome to be able to do your own
breadth-first traversals, :class:`.CommentForest` provides a convenience
method, :meth:`.list`, which returns a list of comments traversed in the same
order as the code above. Thus the above can be rewritten as:

.. code-block:: python

   submission.comments.replace_more(limit=None)
   for comment in submission.comments.list():
       print(comment.body)

You can now properly extract and parse all (or most) of the comments
belonging to a single submission. Combine this with :ref:`submission iteration
<submission-iteration>` and you can build some really cool stuff.

Finally, note that the value of ``submission.num_comments`` may not match up
100% with the number of comments extracted via asyncpraw. This discrepancy is
normal as that count includes deleted, removed, and spam comments.
