python 上下文管理器

# 上下文管理器的用法


# 1.资源清理************************************************************************

# 数据库连接
import pymysql
class DBConnection(object):

    def __init__(self,user=None,password=None,dbname=None,host='localhost'):
        self.user = user
        self.password = password
        self.host = host
        self.dbname = dbname

    def __enter__(self):
        self.connection = pymysql.connect(self.host,self.user,self.password,self.dbname )
        return self.connection.cursor()

    def __exit__(self, exc_type, exc_val, exc_tb):

        self.connection.close()


with DBConnection(user='root',password='root',dbname='test2') as db: #as 后边的变量接受__enter__返回的对象
    db.execute('select 1+1')
    a = db.fetchall()
    print(a)


# 2.避免重复(传播异常)************************************************************************

class BubbleException(object):

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        if exc_val:
            print('bubbling up eception :%s' %exc_val)
        return False

# with BubbleException():
#     a = 5+5


# with BubbleException():
#     a = 5/0


# 2.避免重复（终止异常）************************************************************************

class SupressException(object):

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        if exc_val:
            print('supressing up eception :%s' %exc_val)
        return True #尽量不要这么做

# 上述行为与以下示例是相似的
try:
    ['do something']
except:
    pass

# with SupressException():
#     a = 5+5
#     print(a)
#
# with SupressException():
#     a = 5/0
#     print(a)

# 2.避免重复（处理特定异常的类）************************************************************************

class HandleValueError(object):

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        if issubclass(exc_type,ValueError):
            return True

        return False

#特定的异常阻止抛出
with HandleValueError():
    raise ValueError('wrong value')

#剩余的异常继续抛出
# with HandleValueError():
#     raise TypeError('type error')

# 2.避免重复（不包括的子类，只捕获一个给定的异常类，不捕获他的子类）************************************************************************

class ValueErrorSubclass(ValueError):

    pass

class HandleValueError(object):

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        if exc_type == ValueError:
            return True

        return False

with HandleValueError():
    pass
    # raise ValueErrorSubclass('value error subclass')

# 3.更简单的用法************************************************************************
import contextlib
# @contextlib.contextmanager 装饰简单函数，将函数转换为上下文管理器类

from contextlib import contextmanager

@contextmanager
def file_open(path):
    try:
        f_obj = open(path,"w")
        yield f_obj
    except OSError:
        print("We had an error!")
    finally:
        print("Closing file")
        f_obj.close()


# with file_open("test.txt") as fobj:
#     fobj.write("Testing context managers")