Mastering Python Second Edition Release Code

CH_08_metaclasses/plugins/__init__.py

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+from .base import Plugin
+from .base import Plugins
+from .base import PluginsOnDemand
+from .base import PluginsThroughConfiguration
+from .base import PluginsThroughFilesystem
+
+__all__ = [
+    'Plugin', 'Plugins', 'PluginsOnDemand',
+    'PluginsThroughConfiguration', 'PluginsThroughFilesystem']
+

CH_08_metaclasses/plugins/a.py

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+from . import base
+
+
+class A(base.Plugin):
+    pass
+

CH_08_metaclasses/plugins/b.py

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+from . import base
+
+
+class B(base.Plugin):
+    pass
+

CH_08_metaclasses/plugins/base.py

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+import re
+import abc
+import pathlib
+import importlib
+
+
+CURRENT_FILE = pathlib.Path(__file__)
+PLUGINS_DIR = CURRENT_FILE.parent
+MODULE_NAME_RE = re.compile('[a-z][a-z0-9_]*', re.IGNORECASE)
+
+
+class Plugins(abc.ABCMeta):
+
+    plugins = dict()
+
+    def __new__(metaclass, name, bases, namespace):
+        cls = abc.ABCMeta.__new__(
+            metaclass, name, bases, namespace)
+        metaclass.plugins[name.lower()] = cls
+        return cls
+
+    @classmethod
+    def get(cls, name):
+        return cls.plugins[name]
+
+
+class Plugin(metaclass=Plugins):
+    pass
+
+
+class PluginsOnDemand(Plugins):
+
+    @classmethod
+    def get(cls, name):
+        if name not in cls.plugins:
+            print('Loading plugins from plugins.%s' % name)
+            importlib.import_module('plugins.%s' % name)
+        return cls.plugins[name]
+
+
+class PluginsThroughConfiguration(PluginsOnDemand):
+
+    @classmethod
+    def load(cls, *plugin_names):
+        for plugin_name in plugin_names:
+            cls.get(plugin_name)
+
+
+class PluginsThroughFilesystem(PluginsThroughConfiguration):
+
+    @classmethod
+    def autoload(cls):
+        for filename in PLUGINS_DIR.glob('*.py'):
+            # Skip __init__.py and other non-plugin files
+            if not MODULE_NAME_RE.match(filename.stem):
+                continue
+                cls.get(filename.stem)
+
+            # Skip this file
+            if filename == CURRENT_FILE:
+                continue
+
+            # Load the plugin
+            cls.get(filename.stem)