DB api base class (#2787)

* Add query to db.py

* change delete with truncate

* DB interface base class

Author: lhw362950217

python/runtime/dbapi/__init__.py

@@ -0,0 +1,12 @@
+# Copyright 2020 The SQLFlow Authors. All rights reserved.
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License

python/runtime/dbapi/connection.py

@@ -0,0 +1,120 @@
+# Copyright 2020 The SQLFlow Authors. All rights reserved.
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License
+
+from abc import ABC, abstractmethod
+
+
+class ResultSet(ABC):
+    """Base class for DB query result, caller can iteratable this object
+    to get all result rows"""
+    def __init__(self):
+        self._generator = None
+
+    def __iter__(self):
+        return self
+
+    def _gen(self):
+        fetch_size = 128
+        while True:
+            rows = self._fetch(fetch_size) or []
+            for r in rows:
+                yield r
+            if len(rows) < fetch_size:
+                break
+
+    def __next__(self):
+        if self._generator is None:
+            self._generator = self._gen()
+        return next(self._generator)
+
+    @abstractmethod
+    def _fetch(self, fetch_size):
+        """Fetch given count of records in the result set
+
+        Args:
+            fetch_size: max record to retrive
+
+        Returns:
+            A list of records, each record is a list 
+            represent a row in the result set
+        """
+        pass
+
+    @abstractmethod
+    def column_info(self):
+        """Get the result column meta, type in the meta maybe DB-specific
+
+        Returns:
+            A list of column metas, like [(field_a, INT), (field_b, STRING)]
+        """
+        pass
+
+    @abstractmethod
+    def success(self):
+        """Return True if the query is success"""
+        return False
+
+    @abstractmethod
+    def close(self):
+        """Close the ResultSet explicitly, release any resource incurred by this query"""
+        pass
+
+
+class Connection(ABC):
+    """Base class for DB connection
+
+    Args:
+        conn_uri: a connection uri in the schema://name:passwd@host/path?params format
+
+    """
+    def __init__(self, conn_uri):
+        self.conn_uri = conn_uri
+
+    @abstractmethod
+    def _get_result_set(self, statement):
+        """Get the ResultSet for given statement
+
+        Args:
+            statement: the statement to execute
+
+        Returns:
+            A ResultSet object
+        """
+        pass
+
+    def query(self, statement):
+        """Execute given statement and return a ResultSet
+        Typical usage will be:
+
+        rs = conn.query("SELECT * FROM a;")
+        result_rows = [r for r in rs]
+        rs.close()
+
+        Args:
+            statement: the statement to execute
+
+        Returns:
+            A ResultSet object which is iteratable, each generated 
+            record in the iterator is a result-row wrapped by list
+        """
+        return self._get_result_set(statement)
+
+    def exec(self, statement):
+        """Execute given statement and return True on success"""
+        try:
+            rs = self._get_result_set(statement)
+            return rs.success()
+        except:
+            return False
+        finally:
+            rs.close()