THRIFT-4002: Make generated exception classes immutable by default

Currently, the generated exception classes are not hashable under
Python 3 because of the generated `__eq__` method.  Exception objects
are generally expected to be hashable by the Python standard library.
Post-construction mutation of an exception object seems like a very
unlikely case, so enable hashing for all exceptions by making them
immutable by default.  This also adds a way to opt-out of immutability
by setting the `python.immutable` annotation to `"false"`.

2 files changed, 12 insertions, 2 deletions

diff --git a/lib/py/src/protocol/TBase.py b/lib/py/src/protocol/TBase.py
index 9ae1b1182..6c6ef18e8 100644
--- a/lib/py/src/protocol/TBase.py
+++ b/lib/py/src/protocol/TBase.py
@@ -80,3 +80,7 @@ class TFrozenBase(TBase):
                                       [self.__class__, self.thrift_spec])
         else:
             return iprot.readStruct(cls, cls.thrift_spec, True)
+
+
+class TFrozenExceptionBase(TFrozenBase, TExceptionBase):
+    pass
diff --git a/lib/py/src/protocol/TProtocol.py b/lib/py/src/protocol/TProtocol.py
index 3456e8f0e..339a2839d 100644
--- a/lib/py/src/protocol/TProtocol.py
+++ b/lib/py/src/protocol/TProtocol.py
@@ -303,8 +303,14 @@ class TProtocolBase(object):
 
     def readContainerStruct(self, spec):
         (obj_class, obj_spec) = spec
-        obj = obj_class()
-        obj.read(self)
+
+        # If obj_class.read is a classmethod (e.g. in frozen structs),
+        # call it as such.
+        if getattr(obj_class.read, '__self__', None) is obj_class:
+            obj = obj_class.read(self)
+        else:
+            obj = obj_class()
+            obj.read(self)
         return obj
 
     def readContainerMap(self, spec):