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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 session(connection_string=None):
 """Gets a SQLAlchemy session"""

    global _session_makers
    connection_string = connection_string or oz.settings["db"]

    if not connection_string in _session_makers:
        _session_makers[connection_string] = sessionmaker(bind=engine(connection_string=connection_string))

    return _session_makers[connection_string]()