diff --git a/mock.py b/mock.py
index 46d0fcd..8ab2b7c 100644
--- a/mock.py
+++ b/mock.py
@@ -238,13 +238,27 @@
 def _copy_func_details(func, funcopy):
     funcopy.__name__ = func.__name__
     funcopy.__doc__ = func.__doc__
-    #funcopy.__dict__.update(func.__dict__)
-    funcopy.__module__ = func.__module__
+    try:
+        funcopy.__text_signature__ = func.__text_signature__
+    except AttributeError:
+        pass
+    # we explicitly don't copy func.__dict__ into this copy as it would
+    # expose original attributes that should be mocked
+    try:
+        funcopy.__module__ = func.__module__
+    except AttributeError:
+        pass
+    try:
+        funcopy.__defaults__ = func.__defaults__
+    except AttributeError:
+        pass
+    try:
+        funcopy.__kwdefaults__ = func.__kwdefaults__
+    except AttributeError:
+        pass
     if not inPy3k:
         funcopy.func_defaults = func.func_defaults
         return
-    funcopy.__defaults__ = func.__defaults__
-    funcopy.__kwdefaults__ = func.__kwdefaults__
 
 
 def _callable(obj):