Added register_default_json() function

Register a typecaster for PostgreSQL 9.2 json.

doc/src/extras.rst

@@ -155,6 +155,8 @@ versions the `simplejson`_ module is be used if available. Note that the last
 
 .. autofunction:: register_json
 
+.. autofunction:: register_default_json
+
 
 
 .. _adapt-hstore:

lib/_json.py

@@ -43,6 +43,10 @@ else:
         json = None
 
 
+# oids from PostgreSQL 9.2
+JSON_OID = 114
+JSONARRAY_OID = 199
+
 class Json(object):
     """A wrapper to adapt a Python object to :sql:`json` data type.
 
@@ -130,7 +134,7 @@ def register_json(conn_or_curs=None, globally=False, loads=None,
     if oid is None:
         oid, array_oid = _get_json_oids(conn_or_curs)
 
-    JSON, JSONARRAY = create_json_typecasters(oid, array_oid, loads)
+    JSON, JSONARRAY = _create_json_typecasters(oid, array_oid, loads)
 
     register_type(JSON, not globally and conn_or_curs or None)
 
@@ -139,7 +143,20 @@ def register_json(conn_or_curs=None, globally=False, loads=None,
 
     return JSON, JSONARRAY
 
-def create_json_typecasters(oid, array_oid, loads=None):
+def register_default_json(conn_or_curs=None, globally=False, loads=None):
+    """
+    Create and register :sql:`json` typecasters for PostgreSQL 9.2 and following.
+
+    Since PostgreSQL 9.2 :sql:`json` is a builtin type, hence its oid is known
+    and fixed. This function allows specifying a customized *loads* function
+    for the default :sql:`json` type without querying the database.
+    All the parameters have the same meaning of `register_json()`.
+    """
+    return register_json(conn_or_curs=conn_or_curs, globally=globally,
+        loads=loads, oid=JSON_OID, array_oid=JSONARRAY_OID)
+
+
+def _create_json_typecasters(oid, array_oid, loads=None):
     """Create typecasters for json data type."""
     if loads is None:
         if json is None:

lib/extensions.py

@@ -151,17 +151,14 @@ class NoneAdapter(object):
 
 
 # Create default json typecasters for PostgreSQL 9.2 oids
-from psycopg2._json import create_json_typecasters
+from psycopg2._json import register_default_json
 
 try:
-    JSON, JSONARRAY = create_json_typecasters(114, 199)
+    JSON, JSONARRAY = register_default_json()
 except ImportError:
     pass
-else:
-    register_type(JSON)
-    register_type(JSONARRAY)
 
-del create_json_typecasters
+del register_default_json
 
 
 # Add the "cleaned" version of the encodings to the key.

lib/extras.py

@@ -970,7 +970,7 @@ def register_composite(name, conn_or_curs, globally=False):
 
 
 # expose the json adaptation stuff into the module
-from psycopg2._json import json, Json, register_json
+from psycopg2._json import json, Json, register_json, register_default_json
 
 __all__ = filter(lambda k: not k.startswith('_'), locals().keys())
 

tests/test_types_extras.py

@@ -950,6 +950,24 @@ class JsonTestCase(unittest.TestCase):
             if olda:
                 psycopg2.extensions.register_type(olda)
 
+    @skip_if_no_json_module
+    @skip_before_postgres(9, 2)
+    def test_register_default(self):
+        curs = self.conn.cursor()
+
+        loads = lambda x: psycopg2.extras.json.loads(x, parse_float=Decimal)
+        psycopg2.extras.register_default_json(curs, loads=loads)
+
+        curs.execute("""select '{"a": 100.0, "b": null}'::json""")
+        data = curs.fetchone()[0]
+        self.assert_(isinstance(data['a'], Decimal))
+        self.assertEqual(data['a'], Decimal('100.0'))
+
+        curs.execute("""select array['{"a": 100.0, "b": null}']::json[]""")
+        data = curs.fetchone()[0]
+        self.assert_(isinstance(data[0]['a'], Decimal))
+        self.assertEqual(data[0]['a'], Decimal('100.0'))
+
 
 def test_suite():
     return unittest.TestLoader().loadTestsFromName(__name__)