Add a singletom Metaclass

openlp/core/common/__init__.py

@@ -172,6 +172,21 @@ class SlideLimits(object):
     Next = 3
 
 
+class Singleton(type):
+    """
+    Provide a `Singleton` metaclass https://stackoverflow.com/questions/6760685/creating-a-singleton-in-python
+    """
+    _instances = {}
+
+    def __call__(cls, *args, **kwargs):
+        """
+        Create a new instance if one does not already exist.
+        """
+        if cls not in cls._instances:
+            cls._instances[cls] = super().__call__(*args, **kwargs)
+        return cls._instances[cls]
+
+
 def de_hump(name):
     """
     Change any Camel Case string to python string

openlp/core/common/i18n.py

@@ -29,7 +29,7 @@ from collections import namedtuple
 
 from PyQt5 import QtCore, QtWidgets
 
-from openlp.core.common import is_macosx, is_win
+from openlp.core.common import Singleton, is_macosx, is_win
 from openlp.core.common.applocation import AppLocation
 from openlp.core.common.settings import Settings
 
@@ -327,22 +327,11 @@ class LanguageManager(object):
         return LanguageManager.__qm_list__
 
 
-class UiStrings(object):
+class UiStrings(metaclass=Singleton):
     """
     Provide standard strings for objects to use.
     """
-    __instance__ = None
-
-    def __new__(cls):
-        """
-        Override the default object creation method to return a single instance.
-        """
-        if not cls.__instance__:
-            cls.__instance__ = super().__new__(cls)
-            cls.__instance__.load()
-        return cls.__instance__
-
-    def load(self):
+    def __init__(self):
         """
         These strings should need a good reason to be retranslated elsewhere.
         Should some/more/less of these have an & attached?

openlp/core/common/registry.py

@@ -23,29 +23,19 @@
 Provide Registry Services
 """
 import logging
-import sys
 
-from openlp.core.common import de_hump, trace_error_handler
+from openlp.core.common import Singleton, de_hump, trace_error_handler
 
 
 log = logging.getLogger(__name__)
 
 
-class Registry(object):
+class Registry(metaclass=Singleton):
     """
     This is the Component Registry.  It is a singleton object and is used to provide a look up service for common
     objects.
     """
     log.info('Registry loaded')
-    __instance__ = None
-
-    def __new__(cls):
-        """
-        Re-implement the __new__ method to make sure we create a true singleton.
-        """
-        if not cls.__instance__:
-            cls.__instance__ = object.__new__(cls)
-        return cls.__instance__
 
     @classmethod
     def create(cls):
@@ -57,20 +47,9 @@ class Registry(object):
         registry.service_list = {}
         registry.functions_list = {}
         registry.working_flags = {}
-        # Allow the tests to remove Registry entries but not the live system
-        registry.running_under_test = 'nose' in sys.argv[0] or 'pytest' in sys.argv[0]
         registry.initialising = True
         return registry
 
-    @classmethod
-    def destroy(cls):
-        """
-        Destroy the Registry.
-        """
-        if cls.__instance__.running_under_test:
-            del cls.__instance__
-            cls.__instance__ = None
-
     def get(self, key):
         """
         Extracts the registry value from the list based on the key passed in

openlp/core/display/screens.py

@@ -28,6 +28,7 @@ from functools import cmp_to_key
 
 from PyQt5 import QtCore, QtWidgets
 
+from openlp.core.common import Singleton
 from openlp.core.common.i18n import translate
 from openlp.core.common.registry import Registry
 from openlp.core.common.settings import Settings
@@ -142,24 +143,15 @@ class Screen(object):
                                                 screen_dict['custom_geometry']['height'])
 
 
-class ScreenList(object):
+class ScreenList(metaclass=Singleton):
     """
     Wrapper to handle the parameters of the display screen.
 
     To get access to the screen list call ``ScreenList()``.
     """
     log.info('Screen loaded')
-    __instance__ = None
     screens = []
 
-    def __new__(cls):
-        """
-        Re-implement __new__ to create a true singleton.
-        """
-        if not cls.__instance__:
-            cls.__instance__ = object.__new__(cls)
-        return cls.__instance__
-
     def __iter__(self):
         """
         Convert this object into an iterable, so that we can iterate over it instead of the inner list

openlp/core/state.py

@@ -28,6 +28,7 @@ contained within the openlp.core module.
 """
 import logging
 
+from openlp.core.common import Singleton
 from openlp.core.common.registry import Registry
 from openlp.core.common.mixins import LogMixin
 from openlp.core.lib.plugin import PluginStatus
@@ -52,17 +53,7 @@ class StateModule(LogMixin):
         self.text = None
 
 
-class State(LogMixin):
-
-    __instance__ = None
-
-    def __new__(cls):
-        """
-        Re-implement the __new__ method to make sure we create a true singleton.
-        """
-        if not cls.__instance__:
-            cls.__instance__ = object.__new__(cls)
-        return cls.__instance__
+class State(LogMixin, metaclass=Singleton):
 
     def load_settings(self):
         self.modules = {}

openlp/core/ui/icons.py

@@ -27,6 +27,7 @@ import logging
 import qtawesome as qta
 from PyQt5 import QtGui, QtWidgets
 
+from openlp.core.common import Singleton
 from openlp.core.common.applocation import AppLocation
 from openlp.core.lib import build_icon
 
@@ -34,22 +35,11 @@ from openlp.core.lib import build_icon
 log = logging.getLogger(__name__)
 
 
-class UiIcons(object):
+class UiIcons(metaclass=Singleton):
     """
     Provide standard icons for objects to use.
     """
-    __instance__ = None
-
-    def __new__(cls):
-        """
-        Override the default object creation method to return a single instance.
-        """
-        if not cls.__instance__:
-            cls.__instance__ = super().__new__(cls)
-            cls.__instance__.load()
-        return cls.__instance__
-
-    def load(self):
+    def __init__(self):
         """
         These are the font icons used in the code.
         """
@@ -165,6 +155,7 @@ class UiIcons(object):
             'volunteer': {'icon': 'fa.group'}
         }
         self.load_icons(icon_list)
+        self.main_icon = build_icon(':/icon/openlp-logo.svg')
 
     def load_icons(self, icon_list):
         """
@@ -184,7 +175,6 @@ class UiIcons(object):
                     setattr(self, key, qta.icon('fa.plus-circle', color='red'))
             except Exception:
                 setattr(self, key, qta.icon('fa.plus-circle', color='red'))
-        self.main_icon = build_icon(':/icon/openlp-logo.svg')
 
     @staticmethod
     def _print_icons():

openlp/plugins/bibles/lib/__init__.py

@@ -26,6 +26,7 @@ plugin.
 import logging
 import re
 
+from openlp.core.common import Singleton
 from openlp.core.common.i18n import translate
 from openlp.core.common.settings import Settings
 
@@ -64,20 +65,10 @@ class LanguageSelection(object):
     English = 2
 
 
-class BibleStrings(object):
+class BibleStrings(metaclass=Singleton):
     """
     Provide standard strings for objects to use.
     """
-    __instance__ = None
-
-    def __new__(cls):
-        """
-        Override the default object creation method to return a single instance.
-        """
-        if not cls.__instance__:
-            cls.__instance__ = object.__new__(cls)
-        return cls.__instance__
-
     def __init__(self):
         """
         These strings should need a good reason to be retranslated elsewhere.

tests/functional/openlp_core/common/test_common.py

@@ -26,7 +26,7 @@ from pathlib import Path
 from unittest import TestCase
 from unittest.mock import MagicMock, call, patch
 
-from openlp.core.common import clean_button_text, de_hump, extension_loader, is_linux, is_macosx, is_win, \
+from openlp.core.common import Singleton, clean_button_text, de_hump, extension_loader, is_linux, is_macosx, is_win, \
     normalize_str, path_to_module, trace_error_handler
 
 
@@ -163,6 +163,48 @@ class TestCommonFunctions(TestCase):
             mocked_logger.error.assert_called_with(
                 'OpenLP Error trace\n   File openlp.fake at line 56 \n\t called trace_error_handler_test')
 
+    def test_singleton_metaclass_multiple_init(self):
+        """
+        Test that a class using the Singleton Metaclass is only initialised once despite being called several times and
+        that the same instance is returned each time..
+        """
+        # GIVEN: The Singleton Metaclass and a test class using it
+        class SingletonClass(metaclass=Singleton):
+            def __init__(self):
+                pass
+
+        with patch.object(SingletonClass, '__init__', return_value=None) as patched_init:
+
+            # WHEN: Initialising the class multiple times
+            inst_1 = SingletonClass()
+            inst_2 = SingletonClass()
+
+        # THEN: The __init__ method of the SingletonClass should have only been called once, and both returned values
+        #       should be the same instance.
+        assert inst_1 is inst_2
+        assert patched_init.call_count == 1
+
+    def test_singleton_metaclass_multiple_classes(self):
+        """
+        Test that multiple classes using the Singleton Metaclass return the different an appropriate instances.
+        """
+        # GIVEN: Two different classes using the Singleton Metaclass
+        class SingletonClass1(metaclass=Singleton):
+            def __init__(self):
+                pass
+
+        class SingletonClass2(metaclass=Singleton):
+            def __init__(self):
+                pass
+
+        # WHEN: Initialising both classes
+        s_c1 = SingletonClass1()
+        s_c2 = SingletonClass2()
+
+        # THEN: The instances  should be an instance of the appropriate class
+        assert isinstance(s_c1, SingletonClass1)
+        assert isinstance(s_c2, SingletonClass2)
+
     def test_is_win(self):
         """
         Test the is_win() function

tests/functional/openlp_core/ui/test_icons.py

@@ -33,7 +33,7 @@ from tests.helpers.testmixin import TestMixin
 
 class TestIcons(TestCase, TestMixin):
 
-    @patch('openlp.core.ui.icons.UiIcons.load')
+    @patch('openlp.core.ui.icons.UiIcons.__init__', return_value=None)
     def test_simple_icon(self, _):
         # GIVEN: an basic set of icons
         icons = UiIcons()