Support DNS-less connections for psycopg2

Added support for parameter-less connection URLs for the psycopg2 dialect,
meaning, the URL can be passed to :func:`.create_engine` as
``"postgresql+psycopg2://"`` with no additional arguments to indicate an
empty DSN passed to libpq, which indicates to connect to "localhost" with
no username, password, or database given. Pull request courtesy Julian
Mehnle.

Fixes: #4562
Closes: #4563
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4563
Pull-request-sha: 8a05c96944a0523b06e9772cfe1832e500a43641

Change-Id: Ib6fca3c3c9eebeaf590d7f7fb0bc8cd4b6e4a55a

1 files changed, 15 insertions, 0 deletions

diff --git a/test/dialect/postgresql/test_dialect.py b/test/dialect/postgresql/test_dialect.py
index cadcbdc1c..8c19e31bb 100644
--- a/test/dialect/postgresql/test_dialect.py
+++ b/test/dialect/postgresql/test_dialect.py
@@ -27,6 +27,7 @@ from sqlalchemy import testing
 from sqlalchemy import text
 from sqlalchemy import TypeDecorator
 from sqlalchemy.dialects.postgresql import base as postgresql
+from sqlalchemy.dialects.postgresql import psycopg2 as psycopg2_dialect
 from sqlalchemy.engine import engine_from_config
 from sqlalchemy.engine import url
 from sqlalchemy.testing import engines
@@ -114,6 +115,20 @@ class DialectTest(fixtures.TestBase):
         e = engine_from_config(config, _initialize=False)
         eq_(e.dialect.use_native_unicode, True)
 
+    def test_psycopg2_empty_connection_string(self):
+        dialect = psycopg2_dialect.dialect()
+        u = url.make_url("postgresql://")
+        cargs, cparams = dialect.create_connect_args(u)
+        eq_(cargs, [''])
+        eq_(cparams, {})
+
+    def test_psycopg2_nonempty_connection_string(self):
+        dialect = psycopg2_dialect.dialect()
+        u = url.make_url("postgresql://host")
+        cargs, cparams = dialect.create_connect_args(u)
+        eq_(cargs, [])
+        eq_(cparams, {"host": "host"})
+
 
 class BatchInsertsTest(fixtures.TablesTest):
     __only_on__ = "postgresql+psycopg2"