port replication initialisation to Python

nominatim/db/connection.py

@@ -0,0 +1,58 @@
+"""
+Specialised connection and cursor functions.
+"""
+import logging
+
+import psycopg2
+import psycopg2.extensions
+import psycopg2.extras
+
+class _Cursor(psycopg2.extras.DictCursor):
+    """ A cursor returning dict-like objects and providing specialised
+        execution functions.
+    """
+
+    def execute(self, query, args=None): # pylint: disable=W0221
+        """ Query execution that logs the SQL query when debugging is enabled.
+        """
+        logger = logging.getLogger()
+        logger.debug(self.mogrify(query, args).decode('utf-8'))
+
+        super().execute(query, args)
+
+    def scalar(self, sql, args=None):
+        """ Execute query that returns a single value. The value is returned.
+            If the query yields more than one row, a ValueError is raised.
+        """
+        self.execute(sql, args)
+
+        if self.rowcount != 1:
+            raise ValueError("Query did not return a single row.")
+
+        return self.fetchone()[0]
+
+
+class _Connection(psycopg2.extensions.connection):
+    """ A connection that provides the specialised cursor by default and
+        adds convenience functions for administrating the database.
+    """
+
+    def cursor(self, cursor_factory=_Cursor, **kwargs):
+        """ Return a new cursor. By default the specialised cursor is returned.
+        """
+        return super().cursor(cursor_factory=cursor_factory, **kwargs)
+
+    def table_exists(self, table):
+        """ Check that a table with the given name exists in the database.
+        """
+        with self.cursor() as cur:
+            num = cur.scalar("""SELECT count(*) FROM pg_tables
+                                WHERE tablename = %s""", (table, ))
+            return num == 1
+
+
+def connect(dsn):
+    """ Open a connection to the database using the specialised connection
+        factory.
+    """
+    return psycopg2.connect(dsn, connection_factory=_Connection)