[mod] rerank plugin: hide if bm25s is not installed & add to docs

docs/src/searx.plugins.rerank.rst

@@ -0,0 +1,9 @@
+.. _rerank plugin:
+
+================
+Rerank plugin
+================
+
+.. automodule:: searx.plugins.rerank
+  :members:
+

searx/plugins/rerank.py

@@ -1,8 +1,14 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 """Plugin which reranks the search results using the Okapi BM25 algorithm.
-Before enabling the Rerank plugin, you must the install the pip package ``bm25s``.
 
-Enable in ``settings.yml``:
+This plugin utilizes the `bm25s` library to reorder search results based on their relevance to the search query,
+potentially improving the quality of results.  Before enabling this plugin,
+ensure you have installed the ``bm25s`` pip package.  e.g. by installing it directly via pip or
+by adding it to the project's `requirements.txt` file.
+
+Configuration:
+--------------
+To enable the Rerank plugin, add it to the `enabled_plugins` list in your `settings.yml` file:
 
 .. code:: yaml
 
@@ -10,9 +16,13 @@ Enable in ``settings.yml``:
     ..
     - 'Rerank plugin'
 
-By default, the engine list is retained, so results found by multiple engines receive a score boost.
-The following setting can be used to ensure that the engine list only contains the first engine.
-This will prevent overlapping search engine results from affecting the ranking:
+By default, the plugin retains the information about which engines found a particular result.
+Results that appear in multiple engine results will receive a score boost.
+This approach might be relevant if you wish results found by different engines to be prioritized.
+You can modify this behaviour by configuring the ``remove_extra_engines`` setting.
+If ``remove_extra_engines`` is set to ``true``, the original engine list is reduced to only the first engine.
+This is useful when you prefer the reranking to not be affected by any potential overlap
+of results from different engines.
 
 .. code:: yaml
 
@@ -26,8 +36,8 @@ from searx import settings
 try:
     import bm25s
 except ImportError:
-    # Import error is ignored because the admin has to install bm25s manually to use the engine
-    pass
+    # Import error is ignored because the admin has to install bm25s manually to use the plugin
+    bm25s = None
 
 name = 'Rerank plugin'
 description = 'Rerank search results, ignoring original engine ranking'
@@ -41,13 +51,16 @@ remove_extra_engines = settings.get('rerank', {}).get('remove_extra_engines')
 
 
 def post_search(_request, search):
+    if not bm25s:
+        return True
+
     # pylint: disable=protected-access
     results = search.result_container._merged_results
     query = search.search_query.query
     locale = search.search_query.locale
 
     # Determine the stopwords based on the selected locale
-    stopwords = locale.language if locale and locale.language in stopword_langs else True
+    stopwords = locale.language if locale and locale.language in stopword_langs else 'en'
 
     retriever = bm25s.BM25()
     result_tokens = bm25s.tokenize(
@@ -75,3 +88,7 @@ def post_search(_request, search):
                 results[index]['positions'] = [position + 1] * len(results[index]['positions'])
 
     return True
+
+
+def is_allowed():
+    return bm25s is not None

searx/settings.yml

@@ -249,6 +249,7 @@ outgoing:
 #   - 'Hostnames plugin'  # see 'hostnames' configuration below
 #   - 'Open Access DOI rewrite'
 #   - 'Tor check plugin'
+#   - 'Rerank plugin'  # requires the bm25s python dependency to be installed
 
 # Configuration of the "Hostnames plugin":
 #