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 praw

    reddit = praw.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 to process all comments on Reddit, or comments belonging to
one or more specific subreddits, please see
:meth:`praw.models.reddit.subreddit.SubredditStream.comments`.

.. _extracting_comments:

Extracting comments with PRAW
-----------------------------

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

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

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:`~praw.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 praw.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 PRAW. This discrepancy is normal as that count
includes deleted, removed, and spam comments.
