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The code is a step-by-step copy from sqlahcmey's orm tutorial , except the last line, I intended to drop all tables after the query. But the program blocked on Base.metadata.drop_all(bind=engine) , below is the status of MySQL at that time(taken from MySQL Workbench): As the marked line shows, the drop table process was hanged due to the table metadata lock , I suggest the metadata lock was caused by result = session.query(User).all() since the program did not block if that line was removed, but I still don't know the reason. So my question is: Why did this happen, how to avoid the blocking #!/usr/bin/env python # -*- coding: utf-8 -*- from sqlalchemy import create_engine from sqlalchemy.orm import sessionmaker from sqlalchemy.ext.declarative import declarative_base from sqlalchemy import Column, Integer, String Base = declarative_base() class User(Base): __tablename__ = 'users' id = Column(Integer, primary_key=True) name = Column(String(16)) fullname = Column(String(16)) password = Column(String(16)) def __init__(self, name, fullname, password): self.name = name self.fullname = fullname self.password = password def __repr__(self): return "<User('%s','%s', '%s')>" % (self.name, self.fullname, self.password) uri = 'mysql://root:zxsaqw21@localhost/test_sa' engine = create_engine(uri, echo=False) Base.metadata.create_all(engine) Session = sessionmaker(bind=engine) session = Session() user = User('ed', 'Ed Jones', 'edspassword') session.add(user) session.commit() result = session.query(User).all() print len(result) Base.metadata.drop_all(bind=engine)

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