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How to override plone.app.discussion's comments viewlet
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Override the comments template through-the web
----------------------------------------------

    * go to the ZMI
    * click on portal_view_customizations
    * customize plone.comments (plone.app.discussion.interfaces.IDiscussionLayer)


Override the comments template with z3c.jbot on the file system
---------------------------------------------------------------

http://pypi.python.org/pypi/z3c.jbot

setup.py::

    install_requires=[
        ...
        'z3c.jbot',
    ],

configure.zcml::

    <include package="z3c.jbot" file="meta.zcml" />

    <browser:jbot
        directory="overrides"
        layer="<layer>" />

Replace <layer> with a custom browserlayer of your package.

Create template directory::

    $ mkdir overrides

Copy comments viewlet template to overrides directory::
    
    $ cp ../parts/omelette/plone/app/discussion/browser/comments.pt overrides/plone.app.discussion.browser.comments.pt

Restart your Plone instance and you can start to customize your copy of the comments viewlet.


Override the comments viewlet class on the file system
------------------------------------------------------

interfaces.py::

    from plone.app.discussion.interfaces import IDiscussionLayer

    class IMyDiscussionLayer(IDiscussionLayer):
        """Marker interface for browser layer
        """

profiles/default/browserlayer.xml::

    <?xml version="1.0"?>
    <layers>
      <layer name="my.discussion"
             interface="my.discussion.interfaces.IMyDiscussionLayer" />
    </layers>


configure.zcml::

  <!-- Override plone.app.discussion's comments viewlet -->
  <browser:viewlet
    name="plone.comments"
    for="Products.CMFCore.interfaces.IContentish"
    layer="plone.app.discussion.interfaces.IDiscussionLayer"
    manager="plone.app.layout.viewlets.interfaces.IBelowContent"
    view="plone.app.layout.globals.interfaces.IViewView"
    class=".comments.CommentsViewlet"
    permission="zope2.View"
    />

comments.py::

    from Products.Five.browser.pagetemplatefile import ViewPageTemplateFile
    
    from plone.app.discussion.browser.comments import CommentsViewlet as PloneAppDiscussionCommentsViewlet
    from plone.app.discussion.browser.comments import CommentForm
    
    
    class CommentsViewlet(PloneAppDiscussionCommentsViewlet):
    
        form = CommentForm
        index = ViewPageTemplateFile('comments.pt')
        
        def get_commenter_home_url(self, username=None):
            if username is None:
                return None
            else:
                return "%s/memberhome/%s" % (self.context.portal_url(), username)


comment.pt::

    ...