add an cache.underscore_memoization decorator to make some recurring patterns easier to read

jedi/cache.py

@@ -102,6 +102,37 @@ def cache_call_signatures(stmt):
     return None if module_path is None else (module_path, stmt.start_pos)
 
 
+def underscore_memoization(func):
+    """
+    Decorator for methods::
+
+        class A(object):
+            def x(self):
+                if self._x:
+                    self._x = 10
+                return self._x
+
+    Becomes::
+
+        class A(object):
+            @underscore_memoization
+            def x(self):
+                return 10
+
+    A now has an attribute ``_x`` written by this decorator.
+    """
+    def wrapper(self):
+        name = '_' + func.__name__
+        try:
+            return getattr(self, name)
+        except AttributeError:
+            result = func(self)
+            setattr(self, name, result)
+            return result
+
+    return wrapper
+
+
 def cache_star_import(func):
     def wrapper(evaluator, scope, *args, **kwargs):
         with common.ignored(KeyError):