Understanding and using the context manager Python

Understanding and using the context manager Python

What is the context manager?

• Manager is a context object wrapper arbitrary code block.
• The context manager ensure access to context manager, every time the code consistency of execution.
• When you exit the context manager, related resources can be properly recycled.

Simply put context manager is with 语句

The benefits of using a context manager

• It may be in a more elegant way, the operation (create / acquire / release) resources, such as file operations, database connection;

• Must be able to ensure the implementation of exit steps, with statements by writing code, make the code more concise, do not go to close the file.

• It may be in a more elegant way to handle exceptions.

Use the context manager

To achieve their management of such a context, you must first know the context management protocol.

Simply put, that is in a class, instance of this class is a context manager.实现了 enter 和__exit__的方法

# 上下文管理协议
class Sample:
    def __enter__(self):
        print("enter")
        # 获取资源
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        # 释放资源
        print("exit")

    def do_something(self):
        print("doing something")


with Sample() as sample:
    sample.do_something()

Output:

enter
doing something
exit

Can clearly be seen from this example, when writing code, or can be acquired in connection __enter__ resources, and the resource is written off in the __exit__.

contextlib simplified context manager

# 让普通函数也可以使用上下文管理, 前提条件是add函数必须是生成器函数，且只有yield语句
from contextlib import contextmanager


@contextmanager
def add(x, y):
    print('__enter__')  # 第一步：调用__enter__
    yield x + y  # 第二步：返回__enter__函数的返回值给obj对象
    print('__exit__')  # 第四步：调用__exit__


with add(1, 2) as obj:
    print(obj)  # 第三步：执行上下文语句块

Output:

__enter__
3
__exit__

• contextlib Python standard library module is to provide a more user-friendly context management tool module;
• It is implemented by the decorator, do not need to create a class and use __enter__ and __exit__ these two methods, it is more convenient than with the statement;
• @contextmanager is a decorator decorator, generator which receives a generator, the generator in the yield with the value assigned to the variable ... as, and performed with a normal sentence;
• closing after @contextmanager also a decorative decorator, closing () can become the object context object, then use the with statement (provided that this object can call close () method!);

Reference: context manager