Allow selecting an alternative default database with postgres

sqlalchemy_utils/functions/database.py

@@ -453,10 +453,11 @@ def _sqlite_file_exists(database):
     return header[:16] == b'SQLite format 3\x00'
 
 
-def database_exists(url):
+def database_exists(url, default_db=None):
     """Check if a database exists.
 
     :param url: A SQLAlchemy engine URL.
+    :param default_db: The default database to use instead of requiring standard
 
     Performs backend-specific testing to quickly determine if a database
     exists on the server. ::
@@ -481,7 +482,7 @@ def database_exists(url):
     try:
         if dialect_name == 'postgresql':
             text = "SELECT 1 FROM pg_database WHERE datname='%s'" % database
-            for db in (database, 'postgres', 'template1', 'template0', None):
+            for db in (database, default_db or 'postgres', 'template1', 'template0', None):
                 url = _set_url_database(url, database=db)
                 engine = sa.create_engine(url)
                 try:
@@ -518,14 +519,15 @@ def database_exists(url):
             engine.dispose()
 
 
-def create_database(url, encoding='utf8', template=None):
+def create_database(url, encoding='utf8', template=None, default_db=None):
     """Issue the appropriate CREATE DATABASE statement.
 
     :param url: A SQLAlchemy engine URL.
     :param encoding: The encoding to create the database as.
     :param template:
         The name of the template from which to create the new database. At the
         moment only supported by PostgreSQL driver.
+    :param defualt_db: Overwrite the default database used when connecting.
 
     To create a database, you can pass a simple URL that would have
     been passed to ``create_engine``. ::
@@ -545,14 +547,17 @@ def create_database(url, encoding='utf8', template=None):
     dialect_name = url.get_dialect().name
     dialect_driver = url.get_dialect().driver
 
-    if dialect_name == 'postgresql':
-        url = _set_url_database(url, database="postgres")
-    elif dialect_name == 'mssql':
-        url = _set_url_database(url, database="master")
-    elif dialect_name == 'cockroachdb':
-        url = _set_url_database(url, database="defaultdb")
-    elif not dialect_name == 'sqlite':
-        url = _set_url_database(url, database=None)
+    if default_db != None:
+         if dialect_name == 'postgresql':
+             url = _set_url_database(url, database="postgres")
+         elif dialect_name == 'mssql':
+             url = _set_url_database(url, database="master")
+         elif dialect_name == 'cockroachdb':
+             url = _set_url_database(url, database="defaultdb")
+         elif not dialect_name == 'sqlite':
+             url = _set_url_database(url, database=None)
+    else:
+         url = _set_url_database(url, database=default_db)
 
     if (dialect_name == 'mssql' and dialect_driver in {'pymssql', 'pyodbc'}) \
             or (dialect_name == 'postgresql' and dialect_driver in {