Fix lazyproperty to cache a legitimately None return value

src/pptx/util.py

@@ -183,14 +183,12 @@ def __get__(self, obj: Any, type: Any = None) -> _T:
 
         # --- when accessed on instance, start by checking instance __dict__ for
         # --- item with key matching the wrapped function's name
-        value = obj.__dict__.get(self._name)
-        if value is None:
+        if self._name not in obj.__dict__:
             # --- on first access, the __dict__ item will be absent. Evaluate fget()
             # --- and store that value in the (otherwise unused) host-object
             # --- __dict__ value of same name ('fget' nominally)
-            value = self._fget(obj)
-            obj.__dict__[self._name] = value
-        return cast(_T, value)
+            obj.__dict__[self._name] = self._fget(obj)
+        return cast(_T, obj.__dict__[self._name])
 
     def __set__(self, obj: Any, value: Any) -> None:
         """Raises unconditionally, to preserve read-only behavior.

tests/test_util.py

@@ -4,7 +4,41 @@
 
 import pytest
 
-from pptx.util import Centipoints, Cm, Emu, Inches, Length, Mm, Pt
+from pptx.util import Centipoints, Cm, Emu, Inches, Length, Mm, Pt, lazyproperty
+
+
+class DescribeLazyproperty(object):
+    def it_only_calls_the_decorated_method_once(self):
+        class Obj(object):
+            def __init__(self):
+                self.call_count = 0
+
+            @lazyproperty
+            def fget(self):
+                self.call_count += 1
+                return self.call_count
+
+        obj = Obj()
+
+        assert obj.fget == 1
+        assert obj.fget == 1
+        assert obj.call_count == 1
+
+    def it_caches_a_None_return_value_too(self):
+        class Obj(object):
+            def __init__(self):
+                self.call_count = 0
+
+            @lazyproperty
+            def fget(self):
+                self.call_count += 1
+                return None
+
+        obj = Obj()
+
+        assert obj.fget is None
+        assert obj.fget is None
+        assert obj.call_count == 1
 
 
 class DescribeLength(object):