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?

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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.