How to understand the with statement in Python

This article shares with you how to understand the operation of the with statement in Python. The content is quite good. I hope it can help friends in need

Num01–>The source of the with statement

The with keyword was added in Python2.5. It is used to replace the try...except...finally... pattern.

Num02–>The role of the with statement

The with statement is used when accessing resources. Make sure that no matter whether an exception occurs during use, the necessary "cleaning" operations will be performed and resources will be released. For example, files are automatically closed after use, and locks in threads are automatically acquired and released.

Num03–>The syntax format of the with statement

with EXPR [ as  VAR ]:
    BLOCK
    
    简单说明：
    1，EXPR可以是任意表达式。
    2，as  VAR是可选的。
    3，BLOCK是with语句的语句体

will be explained:

1, calculate EXPR and obtain a context manager.

2, the exit() method of the context manager is saved for subsequent calls.

3, call the enter() method of the context manager

4, if the with expression contains as VAR, then the return value of EXPR is assigned to VAR.

5, execute the expression in BLOCK

6, and call the exit() method of the context manager. If an exception occurs during the execution of BLOCK and causes the program to exit, the type, value, and traceback in the exception (that is, the return value of sys.exc_info()) will be passed as parameters to exit() method and then the exception is thrown in the console. Otherwise three None values ​​will be passed.

The above process is expressed in code as follows:

mgr = (EXPR)
exit = type(mgr).__exit__  # 这里没有执行
value = type(mgr).__enter__(mgr)
exc = True

try: 
   try:     
      VAR = value  # 如果有 as VAR
        BLOCK
    except:
        exc = False
        if not exit(mgr, *sys.exc_info()):        
           raise
 finally:   
    if exc:     
       exit(mgr, None, None, None)

Note:

1, if there is no enter()# in the context manager ## or any method in exit(), then the interpreter will throw an AttributeError.

2. After an exception occurs in BLOCK, if

exit()The method returns a value that can be regarded as True, then this exception will not be thrown, and the subsequent code will continue to execute.

Num04–>No exception case

class my_name:
    def __enter__(self):
        print("调用__enter__()方法")        
        return "xiaoke"

    def __exit__(self, type, value, trace):
        print("调用__exit__()方法")
        
        def get_name():   
            return my_name()
            
    with get_name() as name:
    print("my  name  is :", name)
    # 结果如下：
    # 调用__enter__()方法
    # my  name  is : xiaoke
    # 调用__exit__()方法

Num05–>Exception case

class number:
    # with语句执行的时候调用
    def __enter__(self):
        print("调用_enter_()方法")        return self    # with语句执行完毕，结束的时候调用
    def __exit__(self, type, value, trace):
        print("type:", type)
        print("value:", value)
        print("trace:", trace)    # 当有异常的时候，那么就不会有返回值
    def do_number(self):
        num = 10 / 0
        return num + 100with number()as result:
    res = result.do_number()
    print(res)    # 结果如下：
    # 调用_enter_()方法
    # type: <class &#39;ZeropisionError&#39;>
    # value: pision by zero
    # trace: <traceback object at 0x00000224182AE388>
    # Traceback (most recent call last):
    #   File "E:/pycharmProject/Test49.py", line 56, in <module>
    #     res = result.do_number()
    #   File "E:/pycharmProject/Test49.py", line 51, in do_number
    #     num = 10 / 0
    # ZeropisionError: pision by zero

Num06–>Added in Python3.5, coroutine When using with, add the async keyword

官网代码：import asyncio

async def coro(name, lock):
    print('coro {}: waiting for lock'.format(name))
    async with lock:
        print('coro {}: holding the lock'.format(name))
        await asyncio.sleep(1)
        print('coro {}: releasing the lock'.format(name))

loop = asyncio.get_event_loop()
lock = asyncio.Lock()
coros = asyncio.gather(coro(1, lock), coro(2, lock))try:
    loop.run_until_complete(coros)finally:
    loop.close()

Output code:

coro 1: waiting for lock
coro 1: holding the lock
coro 2: waiting for lock
coro 1: releasing the lock
coro 2: holding the lock
coro 2: releasing the lock

The article references are as follows: 1. Official website: https://www. python.org/dev/peps/pep-0343/2

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