Refine exception invoking which on non-union type

Related to https://github.com/capnproto/pycapnp/issues/254

capnp/lib/capnp.pyx

@@ -1147,8 +1147,10 @@ cdef class _DynamicStructReader:
     cpdef _which_str(self):
         try:
             return <char *>helpers.fixMaybe(self.thisptr.which()).getProto().getName().cStr()
-        except:
-            raise KjException("Attempted to call which on a non-union type")
+        except RuntimeError as e:
+            if str(e) == "Member was null.":
+                raise KjException("Attempted to call which on a non-union type")
+            raise
 
     cpdef _DynamicEnumField _which(self):
         """Returns the enum corresponding to the union in this struct
@@ -1161,8 +1163,10 @@ cdef class _DynamicStructReader:
         try:
             which = _DynamicEnumField()._init(
                 _StructSchemaField()._init(helpers.fixMaybe(self.thisptr.which()), self).proto)
-        except:
-            raise KjException("Attempted to call which on a non-union type")
+        except RuntimeError as e:
+            if str(e) == "Member was null.":
+                raise KjException("Attempted to call which on a non-union type")
+            raise
 
         return which
 
@@ -1445,8 +1449,10 @@ cdef class _DynamicStructBuilder:
     cpdef _which_str(self):
         try:
             return <char *>helpers.fixMaybe(self.thisptr.which()).getProto().getName().cStr()
-        except:
-            raise KjException("Attempted to call which on a non-union type")
+        except RuntimeError as e:
+            if str(e) == "Member was null.":
+                raise KjException("Attempted to call which on a non-union type")
+            raise
 
     cpdef _DynamicEnumField _which(self):
         """Returns the enum corresponding to the union in this struct
@@ -1459,8 +1465,10 @@ cdef class _DynamicStructBuilder:
         try:
             which = _DynamicEnumField()._init(
                 _StructSchemaField()._init(helpers.fixMaybe(self.thisptr.which()), self).proto)
-        except:
-            raise KjException("Attempted to call which on a non-union type")
+        except RuntimeError as e:
+            if str(e) == "Member was null.":
+                raise KjException("Attempted to call which on a non-union type")
+            raise
 
         return which
 

test/test_struct.py

@@ -4,6 +4,8 @@ import os
 import tempfile
 import sys
 
+from capnp.lib.capnp import KjException
+
 this_dir = os.path.dirname(__file__)
 
 
@@ -37,9 +39,13 @@ def test_which_builder(addressbook):
     assert bob.employment.which == addressbook.Person.Employment.unemployed
     assert bob.employment.which == "unemployed"
 
-    with pytest.raises(Exception):
-        addresses.which
-    with pytest.raises(Exception):
+    with pytest.raises(KjException):
+        addresses._which()
+
+    with pytest.raises(KjException):
+        addresses._which_str()
+
+    with pytest.raises(KjException):
         addresses.which
 
 
@@ -71,9 +77,13 @@ def test_which_reader(addressbook):
     bob = people[1]
     assert bob.employment.which == "unemployed"
 
-    with pytest.raises(Exception):
-        addresses.which
-    with pytest.raises(Exception):
+    with pytest.raises(KjException):
+        addresses._which_str()
+
+    with pytest.raises(KjException):
+        addresses._which()
+
+    with pytest.raises(KjException):
         addresses.which