Extract generic decorator into a reusable class (#569)

The only current use of the decorator pattern in colcon-core is the
argument parser, but the concept can be applied generically in a variety
of scenarios.

colcon_core/argument_parser/__init__.py

@@ -5,6 +5,7 @@
 import types
 import warnings
 
+from colcon_core.generic_decorator import GenericDecorator
 from colcon_core.logging import colcon_logger
 from colcon_core.plugin_system import instantiate_extensions
 from colcon_core.plugin_system import order_extensions_by_priority
@@ -85,7 +86,7 @@ def decorate_argument_parser(parser):
     return parser
 
 
-class ArgumentParserDecorator:
+class ArgumentParserDecorator(GenericDecorator):
     """
     Decorate an argument parser as well as all recursive subparsers.
 
@@ -107,15 +108,13 @@ def __init__(self, parser, **kwargs):
           instance
         """
         assert '_parser' not in kwargs
-        kwargs['_parser'] = parser
         assert '_nested_decorators_' not in kwargs
         kwargs['_nested_decorators_'] = []
         assert '_group_decorators' not in kwargs
         kwargs['_group_decorators'] = []
         assert '_recursive_decorators' not in kwargs
         kwargs['_recursive_decorators'] = []
-        for k, v in kwargs.items():
-            self.__dict__[k] = v
+        super().__init__(parser, **kwargs)
 
     @property
     def _nested_decorators(self):  # pragma: no cover
@@ -125,40 +124,14 @@ def _nested_decorators(self):  # pragma: no cover
             'deprecated', stacklevel=2)
         return self._nested_decorators_
 
-    def __getattr__(self, name):
-        """
-        Get an attribute from this decorator if it exists or the decoree.
-
-        :param str name: The name of the attribute
-        :returns: The attribute value
-        :raises AttributeError: if the attribute doesn't exist in either of the
-          two instances
-        """
-        # when argcomplete changes self.__class__ at runtime
-        # the instance might not have a _parser attribute anymore
-        if '_parser' not in self.__dict__:
-            raise AttributeError(name)
-        # get attribute from decoree
-        return getattr(self.__dict__['_parser'], name)
-
-    def __setattr__(self, name, value):
+    @property
+    def _parser(self):
         """
-        Set an attribute value on this decorator if it exists or the decoree.
+        Get the parser that this instance decorates (the decoree).
 
-        :param str name: The name of the attribute
-        :param value: The attribute value
+        TODO: Deprecate _parser in favor of _decoree
         """
-        # overwrite existing attribute
-        if name in self.__dict__:
-            self.__dict__[name] = value
-            return
-        # when argcomplete changes self.__class__ at runtime
-        # the instance might not have a _parser attribute anymore
-        if '_parser' not in self.__dict__:
-            self.__dict__[name] = value
-            return
-        # get attribute on decoree
-        setattr(self.__dict__['_parser'], name, value)
+        return self._decoree
 
     def add_argument(self, *args, **kwargs):
         """

colcon_core/generic_decorator.py

@@ -0,0 +1,50 @@
+# Copyright 2023 Open Source Robotics Foundation, Inc.
+# Licensed under the Apache License, Version 2.0
+
+
+class GenericDecorator:
+    """A generic class decorator."""
+
+    def __init__(self, decoree, **kwargs):
+        """
+        Create a new decorated class instance.
+
+        :param decoree: The instance to decorate
+        :param **kwargs: The keyword arguments are set as attributes on this
+          instance
+        """
+        assert '_decoree' not in kwargs
+        kwargs['_decoree'] = decoree
+        for k, v in kwargs.items():
+            self.__dict__[k] = v
+
+    def __getattr__(self, name):
+        """
+        Get an attribute from this decorator if it exists or the decoree.
+
+        :param str name: The name of the attribute
+        :returns: The attribute value
+        :raises AttributeError: if the attribute doesn't exist in either of the
+          two instances
+        """
+        if '_decoree' not in self.__dict__:
+            raise AttributeError(name)
+        return getattr(self.__dict__['_decoree'], name)
+
+    def __setattr__(self, name, value):
+        """
+        Set an attribute value on this decorator if it exists or the decoree.
+
+        :param str name: The name of the attribute
+        :param value: The attribute value
+        """
+        assert name != '_decoree'
+        # overwrite existing attribute
+        if name in self.__dict__:
+            self.__dict__[name] = value
+            return
+        if '_decoree' not in self.__dict__:
+            self.__dict__[name] = value
+            return
+        # set attribute on decoree
+        setattr(self.__dict__['_decoree'], name, value)

test/spell_check.words

@@ -1,6 +1,5 @@
 addopts
 apache
-argcomplete
 argparse
 asyncio
 autouse

test/test_argument_parser.py

@@ -94,7 +94,7 @@ def test_argument_parser_decorator():
     decorator = ArgumentParserDecorator(parser)
     assert decorator.format_help == parser.format_help
 
-    del decorator.__dict__['_parser']
+    del decorator.__dict__['_decoree']
     with pytest.raises(AttributeError):
         decorator.format_help
 
@@ -108,7 +108,7 @@ def test_argument_parser_decorator():
     assert parser.add_argument is True
 
     assert 'bar' not in decorator.__dict__
-    del decorator.__dict__['_parser']
+    del decorator.__dict__['_decoree']
     decorator.bar = 'baz'
     assert 'bar' in decorator.__dict__
     assert decorator.__dict__['bar'] == 'baz'