Fixed #3747 -- cache extensions on x.509 objects (#3769)

* Fixed #3747 -- cache extensions on x.509 objects

* be kind to cpython, save a dict lookup

* flake8

* changelog

1 files changed, 14 insertions, 0 deletions

diff --git a/src/cryptography/utils.py b/src/cryptography/utils.py
index d28dc71d..efb12e21 100644
--- a/src/cryptography/utils.py
+++ b/src/cryptography/utils.py
@@ -145,3 +145,17 @@ def deprecated(value, module_name, message, warning_class):
     if not isinstance(module, _ModuleWithDeprecations):
         sys.modules[module_name] = _ModuleWithDeprecations(module)
     return _DeprecatedValue(value, message, warning_class)
+
+
+def cached_property(func):
+    cached_name = "_cached_{0}".format(func)
+    sentinel = object()
+
+    def inner(instance):
+        cache = getattr(instance, cached_name, sentinel)
+        if cache is not sentinel:
+            return cache
+        result = func(instance)
+        setattr(instance, cached_name, result)
+        return result
+    return property(inner)