Add support for @property for once_per_instance. (#22)

once/__init__.py

@@ -16,7 +16,7 @@
 
 def _is_method(func: collections.abc.Callable):
     """Determine if a function is a method on a class."""
-    if isinstance(func, (classmethod, staticmethod)):
+    if isinstance(func, (classmethod, staticmethod, property)):
         return True
     sig = inspect.signature(func)
     return "self" in sig.parameters
@@ -396,6 +396,8 @@ def __get__(self, obj, cls) -> collections.abc.Callable:
 class once_per_instance:  # pylint: disable=invalid-name
     """A version of once for class methods which runs once per instance."""
 
+    is_property: bool
+
     @classmethod
     def with_options(cls, per_thread: bool = False, retry_exceptions=False, allow_reset=False):
         return lambda func: cls(
@@ -432,6 +434,11 @@ def once_factory(self) -> _ONCE_FACTORY_TYPE:
     def _inspect_function(self, func: collections.abc.Callable):
         if isinstance(func, (classmethod, staticmethod)):
             raise SyntaxError("Must use @once.once_per_class on classmethod and staticmethod")
+        if isinstance(func, property):
+            func = func.fget
+            self.is_property = True
+        else:
+            self.is_property = False
         if not _is_method(func):
             raise SyntaxError(
                 "Attempting to use @once.once_per_instance method-only decorator "
@@ -450,4 +457,6 @@ def __get__(self, obj, cls) -> collections.abc.Callable:
                     bound_func, self.once_factory(), self.fn_type, self.retry_exceptions
                 )
                 self.callables[obj] = callable
+        if self.is_property:
+            return callable()
         return callable

once_test.py

@@ -882,6 +882,25 @@ def execute(i):
         self.assertEqual(min(results), 1)
         self.assertEqual(max(results), math.ceil(_N_WORKERS / 4))
 
+    def test_once_per_instance_property(self):
+        counter = Counter()
+
+        class _CallOnceClass:
+            @once.once_per_instance
+            @property
+            def value(self):
+                nonlocal counter
+                return counter.get_incremented()
+
+        a = _CallOnceClass()
+        b = _CallOnceClass()
+        self.assertEqual(a.value, 1)
+        self.assertEqual(b.value, 2)
+        self.assertEqual(a.value, 1)
+        self.assertEqual(b.value, 2)
+        self.assertEqual(_CallOnceClass().value, 3)
+        self.assertEqual(_CallOnceClass().value, 4)
+
     def test_once_per_class_classmethod(self):
         counter = Counter()