diff --git a/implementations/python/preserves/schema.py b/implementations/python/preserves/schema.py
index 5b52b52..5baae6b 100644
--- a/implementations/python/preserves/schema.py
+++ b/implementations/python/preserves/schema.py
@@ -40,12 +40,12 @@ class SchemaEntity:
     def decode(cls, v):
         i = cls.try_decode(v)
         if i is None:
-            raise Exception('Could not decode ' + str(cls))
+            raise ValueError('Could not decode ' + str(cls))
         return i
 
     @classmethod
     def try_decode(cls, v):
-        raise Exception('Subclass responsibility')
+        raise NotImplementedError('Subclass responsibility')
 
     @classmethod
     def parse(cls, p, v, args):
@@ -128,10 +128,10 @@ class SchemaEntity:
                 if k not in v: return None
                 if cls.parse(pp, v[k], args) is None: return None
             return ()
-        raise Exception('Bad schema')
+        raise ValueError('Bad schema')
 
     def _encode(self):
-        raise Exception('Subclass responsibility')
+        raise NotImplementedError('Subclass responsibility')
 
     def __repr__(self):
         n = self._constructor_name()
@@ -144,13 +144,13 @@ class SchemaEntity:
             return n + ' ' + repr(self._as_dict())
 
     def _as_dict(self):
-        raise Exception('Subclass responsibility')
+        raise NotImplementedError('Subclass responsibility')
 
 class Enumeration(SchemaEntity):
     VARIANTS = None
 
     def __init__(self):
-        raise Exception('Cannot create instance of Enumeration')
+        raise TypeError('Cannot create instance of Enumeration')
 
     @classmethod
     def _set_schema(cls, rootns, module_path, name, schema, _variant, _enumeration):
@@ -174,7 +174,7 @@ class Enumeration(SchemaEntity):
         return None
 
     def _encode(self):
-        raise Exception('Cannot encode instance of Enumeration')
+        raise TypeError('Cannot encode instance of Enumeration')
 
 def safesetattr(o, k, v):
     if keyword.iskeyword(k):
@@ -199,14 +199,14 @@ class Definition(SchemaEntity):
         if self.SIMPLE:
             if self.EMPTY:
                 if len(args) != 0:
-                    raise Exception('%s takes no arguments' % (self._constructor_name(),))
+                    raise TypeError('%s takes no arguments' % (self._constructor_name(),))
             else:
                 if len(args) != 1:
-                    raise Exception('%s needs exactly one argument' % (self._constructor_name(),))
+                    raise TypeError('%s needs exactly one argument' % (self._constructor_name(),))
                 self.value = args[0]
         else:
             if len(args) != len(self.FIELD_NAMES):
-                raise Exception('%s needs argument(s) %r' % (self._constructor_name(), self.FIELD_NAMES))
+                raise TypeError('%s needs argument(s) %r' % (self._constructor_name(), self.FIELD_NAMES))
             i = 0
             for k in self.FIELD_NAMES:
                 safesetattr(self, k, args[i])
@@ -255,7 +255,7 @@ class Definition(SchemaEntity):
         return None
 
     def _encode(self):
-        raise Exception('Not yet implemented')
+        raise NotImplementedError('Not yet implemented')
 
     def _as_dict(self):
         return dict((k, getattr(self, k)) for k in self.FIELD_NAMES)
@@ -298,7 +298,7 @@ def gather_defined_field_names(s, acc):
     elif s.key == DICT:
         gather_defined_field_names(tuple(s[0].values()), acc)
     else:
-        raise Exception('Bad schema')
+        raise ValueError('Bad schema')
 
 def pretty_subclass(C, module_name, class_name):
     class S(C): pass
@@ -317,7 +317,7 @@ def lookup(ns, module_path, name):
     return ns[name]
 
 def definition_not_found(module_path, name):
-    raise Exception('Definition not found: ' + module_path_str(module_path + (name,)))
+    raise KeyError('Definition not found: ' + module_path_str(module_path + (name,)))
 
 class Namespace:
     def __init__(self, prefix):
@@ -336,7 +336,7 @@ class Namespace:
     def __setitem__(self, name, value):
         name = Symbol(name)
         if name in self._items:
-            raise Exception('Name conflict: ' + module_path_str(self._prefix + (name.name,)))
+            raise ValueError('Name conflict: ' + module_path_str(self._prefix + (name.name,)))
         self._items[name] = value
 
     def __contains__(self, name):
@@ -361,7 +361,7 @@ class Compiler:
 
     def load_schema(self, module_path, schema):
         if schema[0][VERSION] != 1:
-            raise Exception('Unsupported Schema version')
+            raise NotImplementedError('Unsupported Schema version')
         ns = self.root
         for e in module_path:
             if not e in ns: