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"`.

1 files changed, 4 insertions, 0 deletions

diff --git a/test/DebugProtoTest.thrift b/test/DebugProtoTest.thrift
index de47ea717..1ab0f6aea 100644
--- a/test/DebugProtoTest.thrift
+++ b/test/DebugProtoTest.thrift
@@ -241,6 +241,10 @@ exception ExceptionWithAMap {
   2: map<string, string> map_field;
 }
 
+exception MutableException {
+  1: string msg;
+} (python.immutable = "false")
+
 service ServiceForExceptionWithAMap {
   void methodThatThrowsAnException() throws (1: ExceptionWithAMap xwamap);
 }