initial implementation of cachedproperty and tests, addressing #66. docs soon.

boltons/cacheutils.py

@@ -572,6 +572,22 @@ def cachedmethod(cache, typed=False, selfish=True):
     return cached_method_decorator
 
 
+class cachedproperty(object):
+    def __init__(self, func):
+        self.func = func
+
+    def __get__(self, obj, objtype=None):
+        if obj is None:
+            return self
+        value = self.func(obj)
+        setattr(obj, self.func.__name__, value)
+        return value
+
+    def __repr__(self):
+        cn = self.__class__.__name__
+        return '<%s func=%s>' % (cn, self.func)
+
+
 class ThresholdCounter(object):
     """A **bounded** dict-like Mapping from keys to counts. The
     ThresholdCounter automatically compacts after every (1 /

tests/test_cacheutils.py

@@ -2,7 +2,7 @@
 
 import string
 
-from boltons.cacheutils import LRU, LRI, cached, cachedmethod
+from boltons.cacheutils import LRU, LRI, cached, cachedmethod, cachedproperty
 
 
 class CountingCallable(object):
@@ -242,3 +242,31 @@ def test_cachedmethod():
     print(repr(car_two.door))
     print(repr(Car.door))
     return
+
+
+def test_cachedproperty():
+    class Proper(object):
+        def __init__(self):
+            self.expensive_func = CountingCallable()
+
+        @cachedproperty
+        def useful_attr(self):
+            return self.expensive_func()
+
+    prop = Proper()
+
+    assert prop.expensive_func.call_count == 0
+    assert prop.useful_attr == 1
+    assert prop.expensive_func.call_count == 1
+    assert prop.useful_attr == 1
+    assert prop.expensive_func.call_count == 1
+
+    prop.useful_attr += 1  # would not be possible with normal properties
+    assert prop.useful_attr == 2
+
+    delattr(prop, 'useful_attr')
+    assert prop.expensive_func.call_count == 1
+    assert prop.useful_attr
+    assert prop.expensive_func.call_count == 2
+
+    repr(Proper.useful_attr)