Provide a plugin_enabled Jinja test for themes

docs/themes.rst

@@ -140,6 +140,23 @@ your date according to the locale given in your settings::
 .. _datetime: https://docs.python.org/3/library/datetime.html#datetime-objects
 .. _strftime: https://docs.python.org/3/library/datetime.html#strftime-strptime-behavior
 
+Checking Loaded Plugins
+-----------------------
+
+Pelican provides a ``plugin_enabled`` Jinja test for checking if a certain plugin
+is enabled. This test accepts a plugin name as a string and will return a Boolean.
+Namespace plugins can be specified by full name (``pelican.plugins.plugin_name``)
+or short name (``plugin_name``). The following example uses the ``webassets`` plugin
+to minify CSS if the plugin is enabled and otherwise falls back to regular CSS::
+
+    {% if "webassets" is plugin_enabled %}
+        {% assets filters="cssmin", output="css/style.min.css", "css/style.scss" %}
+            <link rel="stylesheet" href="{{SITEURL}}/{{ASSET_URL}}">
+        {% endassets %}
+    {% else %}
+        <link rel="stylesheet" href="{{SITEURL}}/css/style.css}">
+    {% endif %}
+
 
 index.html
 ----------

pelican/generators.py

@@ -20,6 +20,7 @@
 from pelican.cache import FileStampDataCacher
 from pelican.contents import Article, Page, Static
 from pelican.plugins import signals
+from pelican.plugins._utils import plugin_enabled
 from pelican.readers import Readers
 from pelican.utils import (
     DateFormatter,
@@ -102,6 +103,9 @@ def __init__(
 
         # get custom Jinja tests from user settings
         custom_tests = self.settings["JINJA_TESTS"]
+        self.env.tests["plugin_enabled"] = partial(
+            plugin_enabled, plugin_list=self.settings["PLUGINS"]
+        )
         self.env.tests.update(custom_tests)
 
         signals.generator_init.send(self)

pelican/plugins/_utils.py

@@ -40,6 +40,22 @@ def list_plugins(ns_pkg=None):
         logger.info("No plugins are installed")
 
 
+def plugin_enabled(name, plugin_list=None):
+    if plugin_list is None or not plugin_list:
+        # no plugins are loaded
+        return False
+
+    if name in plugin_list:
+        # search name as is
+        return True
+
+    if "pelican.plugins.{}".format(name) in plugin_list:
+        # check if short name is a namespace plugin
+        return True
+
+    return False
+
+
 def load_legacy_plugin(plugin, plugin_paths):
     if "." in plugin:
         # it is in a package, try to resolve package first

pelican/tests/test_generators.py

@@ -1581,6 +1581,31 @@ def test_jinja2_filter(self):
 
         self._test_jinja2_helper(settings, content, expected)
 
+    def test_jinja2_filter_plugin_enabled(self):
+        """JINJA_FILTERS adds custom filters to Jinja2 environment"""
+        settings = {"PLUGINS": ["legacy_plugin", "pelican.plugins.ns_plugin"]}
+        jinja_template = (
+            "{plugin}: "
+            "{{% if '{plugin}' is plugin_enabled %}}yes"
+            "{{% else %}}no{{% endif %}}"
+        )
+        content = " / ".join(
+            (
+                jinja_template.format(plugin="ns_plugin"),
+                jinja_template.format(plugin="pelican.plugins.ns_plugin"),
+                jinja_template.format(plugin="legacy_plugin"),
+                jinja_template.format(plugin="unknown"),
+            )
+        )
+        expected = (
+            "ns_plugin: yes / "
+            "pelican.plugins.ns_plugin: yes / "
+            "legacy_plugin: yes / "
+            "unknown: no"
+        )
+
+        self._test_jinja2_helper(settings, content, expected)
+
     def test_jinja2_test(self):
         """JINJA_TESTS adds custom tests to Jinja2 environment"""
         content = "foo {{ foo is custom_test }}, bar {{ bar is custom_test }}"

pelican/tests/test_plugins.py

@@ -2,7 +2,12 @@
 from contextlib import contextmanager
 
 import pelican.tests.dummy_plugins.normal_plugin.normal_plugin as normal_plugin
-from pelican.plugins._utils import get_namespace_plugins, get_plugin_name, load_plugins
+from pelican.plugins._utils import (
+    get_namespace_plugins,
+    get_plugin_name,
+    load_plugins,
+    plugin_enabled,
+)
 from pelican.tests.support import unittest
 
 
@@ -183,3 +188,78 @@ def register(self):
             get_plugin_name(NoopPlugin()),
             "PluginTest.test_get_plugin_name.<locals>.NoopPlugin",
         )
+
+    def test_plugin_enabled(self):
+        def get_plugin_names(plugins):
+            return [get_plugin_name(p) for p in plugins]
+
+        with tmp_namespace_path(self._NS_PLUGIN_FOLDER):
+            # with no `PLUGINS` setting, load namespace plugins
+            SETTINGS = {}
+            plugins = get_plugin_names(load_plugins(SETTINGS))
+            self.assertTrue(plugin_enabled("ns_plugin", plugins))
+            self.assertTrue(plugin_enabled("pelican.plugins.ns_plugin", plugins))
+            self.assertFalse(plugin_enabled("normal_plugin", plugins))
+            self.assertFalse(plugin_enabled("unknown", plugins))
+
+            # disable namespace plugins with `PLUGINS = []`
+            SETTINGS = {"PLUGINS": []}
+            plugins = get_plugin_names(load_plugins(SETTINGS))
+            self.assertFalse(plugin_enabled("ns_plugin", plugins))
+            self.assertFalse(plugin_enabled("pelican.plugins.ns_plugin", plugins))
+            self.assertFalse(plugin_enabled("normal_plugin", plugins))
+            self.assertFalse(plugin_enabled("unknown", plugins))
+
+            # with `PLUGINS`, load only specified plugins
+
+            # normal plugin
+            SETTINGS = {
+                "PLUGINS": ["normal_plugin"],
+                "PLUGIN_PATHS": [self._NORMAL_PLUGIN_FOLDER],
+            }
+            plugins = get_plugin_names(load_plugins(SETTINGS))
+            self.assertFalse(plugin_enabled("ns_plugin", plugins))
+            self.assertFalse(plugin_enabled("pelican.plugins.ns_plugin", plugins))
+            self.assertTrue(plugin_enabled("normal_plugin", plugins))
+            self.assertFalse(plugin_enabled("unknown", plugins))
+
+            # normal submodule/subpackage plugins
+            SETTINGS = {
+                "PLUGINS": [
+                    "normal_submodule_plugin.subplugin",
+                    "normal_submodule_plugin.subpackage.subpackage",
+                ],
+                "PLUGIN_PATHS": [self._NORMAL_PLUGIN_FOLDER],
+            }
+            plugins = get_plugin_names(load_plugins(SETTINGS))
+            self.assertFalse(plugin_enabled("ns_plugin", plugins))
+            self.assertFalse(plugin_enabled("pelican.plugins.ns_plugin", plugins))
+            self.assertFalse(plugin_enabled("normal_plugin", plugins))
+            self.assertFalse(plugin_enabled("unknown", plugins))
+
+            # namespace plugin short
+            SETTINGS = {"PLUGINS": ["ns_plugin"]}
+            plugins = get_plugin_names(load_plugins(SETTINGS))
+            self.assertTrue(plugin_enabled("ns_plugin", plugins))
+            self.assertTrue(plugin_enabled("pelican.plugins.ns_plugin", plugins))
+            self.assertFalse(plugin_enabled("normal_plugin", plugins))
+            self.assertFalse(plugin_enabled("unknown", plugins))
+
+            # namespace plugin long
+            SETTINGS = {"PLUGINS": ["pelican.plugins.ns_plugin"]}
+            plugins = get_plugin_names(load_plugins(SETTINGS))
+            self.assertTrue(plugin_enabled("ns_plugin", plugins))
+            self.assertTrue(plugin_enabled("pelican.plugins.ns_plugin", plugins))
+            self.assertFalse(plugin_enabled("normal_plugin", plugins))
+            self.assertFalse(plugin_enabled("unknown", plugins))
+
+            # normal and namespace plugin
+            SETTINGS = {
+                "PLUGINS": ["normal_plugin", "ns_plugin"],
+                "PLUGIN_PATHS": [self._NORMAL_PLUGIN_FOLDER],
+            }
+            plugins = get_plugin_names(load_plugins(SETTINGS))
+            self.assertTrue(plugin_enabled("ns_plugin", plugins))
+            self.assertTrue(plugin_enabled("pelican.plugins.ns_plugin", plugins))
+            self.assertTrue(plugin_enabled("normal_plugin", plugins))
+            self.assertFalse(plugin_enabled("unknown", plugins))