Allow overriding codecs for builtin types on a connection.

Connection.set_type_codec now allows overriding codecs for builtin
types.

Per complaint in #73.

Author: elprans

asyncpg/protocol/codecs/base.pyx

@@ -474,10 +474,6 @@ cdef class DataCodecConfig:
                          encoder, decoder, binary):
         format = PG_FORMAT_BINARY if binary else PG_FORMAT_TEXT
 
-        if self.get_codec(typeoid, format) is not None:
-            raise ValueError('cannot override codec for type {}'.format(
-                typeoid))
-
         self._local_type_codecs[typeoid, format] = \
             Codec.new_python_codec(typeoid, typename, typeschema, typekind,
                                    encoder, decoder, format)
@@ -519,17 +515,17 @@ cdef class DataCodecConfig:
     cdef inline Codec get_codec(self, uint32_t oid, CodecFormat format):
         cdef Codec codec
 
-        codec = get_core_codec(oid, format)
-        if codec is not None:
-            return codec
-
         try:
-            return self._type_codecs_cache[oid, format]
+            return self._local_type_codecs[oid, format]
         except KeyError:
-            try:
-                return self._local_type_codecs[oid, format]
-            except KeyError:
-                return None
+            codec = get_core_codec(oid, format)
+            if codec is not None:
+                return codec
+            else:
+                try:
+                    return self._type_codecs_cache[oid, format]
+                except KeyError:
+                    return None
 
 
 cdef inline Codec get_core_codec(uint32_t oid, CodecFormat format):

tests/test_codecs.py

@@ -900,6 +900,30 @@ def hstore_encoder(obj):
                 DROP EXTENSION hstore
             ''')
 
+    async def test_custom_codec_override(self):
+        """Test overriding core codecs."""
+        import json
+
+        conn = await self.cluster.connect(database='postgres', loop=self.loop)
+        try:
+            def _encoder(value):
+                return json.dumps(value).encode('utf-8')
+
+            def _decoder(value):
+                return json.loads(value.decode('utf-8'))
+
+            await conn.set_type_codec(
+                'json', encoder=_encoder, decoder=_decoder,
+                schema='pg_catalog', binary=True
+            )
+
+            data = {'foo': 'bar', 'spam': 1}
+            res = await conn.fetchval('SELECT $1::json', data)
+            self.assertEqual(data, res)
+
+        finally:
+            await conn.close()
+
     async def test_composites_in_arrays(self):
         await self.con.execute('''
             CREATE TYPE t AS (a text, b int);