def event_before_read_docs(app, env, _):
    """Called by Sphinx before phase 1 (reading).

    * Verify config.
    * Initialize the cache dict before reading any docs with an empty dictionary if not exists.
    * Prune old/invalid data from cache.

    :param sphinx.application.Sphinx app: Sphinx application object.
    :param sphinx.environment.BuildEnvironment env: Sphinx build environment.
    :param _: Not used.
    """
    client_id = app.config['imgur_client_id']
    if not client_id:
        raise ExtensionError('imgur_client_id config value must be set for Imgur API calls.')
    if not RE_CLIENT_ID.match(client_id):
        raise ExtensionError('imgur_client_id config value must be 5-30 lower case hexadecimal characters only.')

    imgur_album_cache = getattr(env, 'imgur_album_cache', None)
    imgur_image_cache = getattr(env, 'imgur_image_cache', None)
    env.imgur_album_cache, env.imgur_image_cache = initialize(imgur_album_cache, imgur_image_cache, (), ())
    prune_cache(env.imgur_album_cache, env.imgur_image_cache, app)