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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_binds(self, app=None):
 """Returns a dictionary with a table->engine mapping. This is suitable for use of sessionmaker(binds=db.get_binds(app)). """

        app = self.get_app(app)
        binds = [None] + list(app.config.get('SQLALCHEMY_BINDS') or ())
        retval = {}
        for bind in binds:
            engine = self.get_engine(app, bind)
            tables = self.get_tables_for_bind(bind)
            retval.update(dict((table, engine) for table in tables))
        return retval