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: sqlalchemy#4562
Closes: sqlalchemy#4563
Pull-request: sqlalchemy#4563
Pull-request-sha: 8a05c96

Change-Id: Ib6fca3c3c9eebeaf590d7f7fb0bc8cd4b6e4a55a

doc/build/changelog/unreleased_13/4562.rst

@@ -0,0 +1,10 @@
+.. change::
+    :tags: feature, postgresql
+    :tickets: 4562
+
+    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.

lib/sqlalchemy/dialects/postgresql/psycopg2.py

@@ -83,6 +83,33 @@
     `PQconnectdbParams \
     <http://www.postgresql.org/docs/9.1/static/libpq-connect.html#LIBPQ-PQCONNECTDBPARAMS>`_
 
+Empty DSN Connections / Environment Variable Connections
+---------------------------------------------------------
+
+The psycopg2 DBAPI can connect to PostgreSQL by passing an empty DSN to the
+libpq client library, which by default indicates to connect to a localhost
+PostgreSQL database that is open for "trust" connections.  This behavior can be
+further tailored using a particular set of environment variables which are
+prefixed with ``PG_...``, which are  consumed by ``libpq`` to take the place of
+any or all elements of the connection string.
+
+For this form, the URL can be passed without any elements other than the
+initial scheme::
+
+    engine = create_engine('postgresql+psycopg2://')
+
+In the above form, a blank "dsn" string is passed to the ``psycopg2.connect()``
+function which in turn represents an empty DSN passed to libpq.
+
+.. versionadded:: 1.3.2 support for parameter-less connections with psycopg2.
+
+.. seealso::
+
+    `Environment Variables\
+    <https://www.postgresql.org/docs/current/libpq-envars.html>`_ -
+    PostgreSQL documentation on how to use ``PG_...``
+    environment variables for connections.
+
 .. _psycopg2_execution_options:
 
 Per-Statement/Connection Execution Options
@@ -735,10 +762,16 @@ def _hstore_oids(self, conn):
 
     def create_connect_args(self, url):
         opts = url.translate_connect_args(username="user")
-        if "port" in opts:
-            opts["port"] = int(opts["port"])
-        opts.update(url.query)
-        return ([], opts)
+        if opts:
+            if "port" in opts:
+                opts["port"] = int(opts["port"])
+            opts.update(url.query)
+            # send individual dbname, user, password, host, port
+            # parameters to psycopg2.connect()
+            return ([], opts)
+        else:
+            # send a blank string for "dsn" to psycopg2.connect()
+            return ([''], opts)
 
     def is_disconnect(self, e, connection, cursor):
         if isinstance(e, self.dbapi.Error):

test/dialect/postgresql/test_dialect.py

@@ -27,6 +27,7 @@
 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 @@ def test_pg_dialect_use_native_unicode_from_config(self):
         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"