What is the `connection` parameter of MapperExtension in sqlalchemy?

The class MapperExtension has some methods, and before_insert, before_update, ... all have a parameter connection.

def before_insert(self, mapper, co开发者_JAVA百科nnection, instance):

I've read the documents of MapperExtension, but found nothing about this connection. What is it? And how to use it?

It's an instance of Connection class. Recent versions of SQLAlchemy distribution have examples directory, see examples/nested_sets/nested_sets.py to get an idea on how to use it in mapper extension.