python上下文管理器小记

April 11, 2019

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2513 words

1.实现了上下文协议的对象可以使用with关键字
2.实现了上下文协议的对象通常称为上下文管理器
3.只要对象实现了__enter__和__exit__就是实现了上下文协议
4.上下文表达式必须要返回一个上下文管理器（with xxx():就是上下文表达式）

class MyResource:
        def __enter__(self):
            print("connect to resource")
            return self
    
        def __exit__(self, exc_type, exc_val, exc_tb):
            """
            :param exc_type: 错误类型
            :param exc_val: 错误信息
            :param exc_tb: traceback对象
            :return:
            """
            if exc_tb:
                # 可以在这里处理异常
                print("process exception")
            else:
                print("no exception")
    
            print("close resource connection")
    
            # __exit__方法必须要返回一个True或者False，如果什么都不返回，就会返回False
            # 当with中发生了异常，若返回True则在with语句外部则不会在抛出异常；若是False则会再次抛出异常
            return True
    
        def query(self):
            print("query data")
    
    
    with MyResource() as resource:
        # as 后面的变量是__enter__中返回的值
        resource.query()
    
    # 执行顺序 __enter__ -> query -> __exit__

使用contextmanager来简化MyResource定义

from contextlib import contextmanager
    # contextmanager好处是将不是上下管理器的类变成了一个管理器
    
    
    class MyResource:
        def query(self):
            print("query data")
    
    
    @contextmanager
    def make_my_resource():
        # __enter__
        print("connect to resource")
        yield MyResource()
    
        # __exit__
        print("close resource connection")
    
    
    with make_my_resource() as resource:
        resource.query()

eg1：实现给一个字符串前后加上书名号

from contextlib import contextmanager
    
    
    @contextmanager
    def book_mark():
        print("《", end="")
        yield
        print("》", end="")
    
    
    with book_mark():
        print("唐诗三百首", end="")
    
    # output: 《唐诗三百首》

eg2:给SQLAlchemy添加一个auto_commit

from contextlib import contextmanager
    
    from flask_sqlalchemy import SQLAlchemy as _SQLAlchemy
    
    
    class SQLAlchemy(_SQLAlchemy):
        @contextmanager
        def auto_commit(self):
            try:
                yield
                self.session.commit()
            except Exception as e:
                db.session.rollback()
                raise e
    
    
    db = SQLAlchemy()
    
    with db.auto_commit():
        user = User()
    
        user.name = "xiaoming"
        user.password = "hello"
    
        db.session.add(user)

Last modification：April 12th, 2019 at 12:04 am

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python上下文管理器小记