Merge pull request #23 from piotrmaslanka/issue-15

fixes #15

CHANGELOG.md

 ## v2.0.22rc3
 
+* fixes #15
 * fixes #20
 * fixes #21
 

satella/coding/__init__.py

@@ -12,6 +12,8 @@ from .typecheck import typed, Callable, Sequence, \
     TypeVar, Mapping, Iterable, Any, Optional, CallSignature, \
     Number, coerce, Set, Dict, List, Tuple, checked_coerce, for_argument, \
     precondition, PreconditionError
+from .structures import TimeBasedHeap, Heap, typednamedtuple, OmniHashableMixin
+from .singleton import Singleton
 
 __all__ = [
     'typednamedtuple', 'OmniHashableMixin'
@@ -22,6 +24,7 @@ __all__ = [
     'CallSignature', 'Number',
     'Set', 'Dict', 'List', 'Tuple', 'checked_coerce', 'for_argument',
     'precondition', 'PreconditionError',
-    'rethrow_as', 'silence_excs'
+    'rethrow_as', 'silence_excs',
+    'Singleton'
 ]
 

satella/coding/singleton.py

+# coding=UTF-8
+from __future__ import print_function, absolute_import, division
+
+import functools
+
+import six
+
+__all__ = [
+    'Singleton',
+]
+
+if six.PY3:
+
+    # Taken from https://wiki.python.org/moin/PythonDecoratorLibrary
+    def Singleton(cls):
+        """
+        Make a singleton out of decorated class.
+
+        Usage:
+
+            @Singleton
+            class MyClass(object):
+                ...
+        """
+
+        cls.__new_old__ = cls.__new__
+
+        @functools.wraps(cls.__new__)
+        def singleton_new(cls, *args, **kw):
+            it = cls.__dict__.get('__it__')
+            if it is not None:
+                return it
+
+            cls.__it__ = it = cls.__new_old__(cls, *args, **kw)
+            it.__init_old__(*args, **kw)
+            return it
+
+        cls.__new__ = singleton_new
+        cls.__init_old__ = cls.__init__
+        cls.__init__ = object.__init__
+
+        return cls
+else:
+    class _SingletonWrapper:
+        """
+        A singleton wrapper class. Its instances would be created
+        for each decorated class.
+        """
+
+        def __init__(self, cls):
+            self.__wrapped__ = cls
+            self._instance = None
+
+        def __call__(self, *args, **kwargs):
+            """Returns a single instance of decorated class"""
+            if self._instance is None:
+                self._instance = self.__wrapped__(*args, **kwargs)
+            return self._instance
+
+
+    # taken from https://pypi.python.org/pypi/singleton-decorator/1.0.0
+    def Singleton(cls):
+        """
+        A singleton decorator. Returns a wrapper objects. A call on that object
+        returns a single instance object of decorated class. Use the __wrapped__
+        attribute to access decorated class directly in unit tests.
+        """
+        return _SingletonWrapper(cls)

tests/test_coding/test_singleton.py

+# coding=UTF-8
+from __future__ import print_function, absolute_import, division
+import unittest
+from satella.coding import Singleton
+
+
+class TestSingleton(unittest.TestCase):
+    def test_singleton(self):
+
+        @Singleton
+        class MyClass(object):
+            def __init__(self):
+                self.a = 5
+
+        a = MyClass()
+        b = MyClass()
+
+        a.a = 6
+
+        self.assertEqual(b.a, 6)
+