Support with keyword in session (#451)

This PR enables with keyword in session, which can automatically release resourced when the context manager exits.
Moreover, use with will automatically set the session as the default session.

Two simple test cases has been added that can fully demonstrate its usage.

python/graphscope/client/session.py

@@ -695,6 +695,10 @@ def info(self):
         info["engine_config"] = self._engine_config
         return info
 
+    @property
+    def closed(self):
+        return self._closed
+
     def eager(self):
         return self._config_params["mode"] == "eager"
 
@@ -781,6 +785,30 @@ def __del__(self):
         except Exception:  # pylint: disable=broad-except
             pass
 
+    def _check_closed(self, msg=None):
+        """Internal: raise a ValueError if session is closed"""
+        if self.closed:
+            raise ValueError("Operation on closed session." if msg is None else msg)
+
+    # Context manager
+    def __enter__(self):
+        """Context management protocol.
+        Returns self and register self as default session.
+        """
+        self._check_closed()
+        self.as_default()
+        return self
+
+    def __exit__(self, type, value, traceback):
+        """Deregister self from the default session,
+        close the session and release the resources, ignore all exceptions in close().
+        """
+        try:
+            self._deregister_default()
+            self.close()
+        except Exception:
+            pass
+
     def as_default(self):
         """Obtain a context manager that make this object as default session.
 

python/tests/test_session.py

@@ -255,3 +255,13 @@ def test_border_cases():
                 "e0": "twitter_property_e_0#header_row=true",
             }
         )
+
+
+def test_with():
+    with graphscope.session(cluster_type="hosts") as sess:
+        assert graphscope.get_default_session() == sess
+
+    sess = graphscope.session(cluster_type="hosts")
+    with sess:
+        pass
+    assert sess.info["status"] == "closed"