前言

with 语句适用于对资源进行访问的场合,确保不管使用过程中是否发生异常都会执行必要的“清理”操作,释放资源,

比如文件使用后自动关闭/线程中锁的自动获取和释放等。

问题引出

如下代码:

file = open("1.txt")
data = file.read()
file.close()


上面代码存在2个问题: 

(1)文件读取发生异常,但没有进行任何处理; 

(2)可能忘记关闭文件句柄;

改进

try:
f = open('xxx')
except:
print('fail to open')
exit(-1)
try:
do something
except:
do something
finally:
f.close()


虽然这段代码运行良好,但比较冗长。 

而使用with的话,能够减少冗长,还能自动处理上下文环境产生的异常。如下面代码:

with open("1.txt") as file:
data = file.read()


with 工作原理

(1)紧跟with后面的语句被求值后,返回对象的“–enter–()”方法被调用,这个方法的返回值将被赋值给as后面的变量; 

(2)当with后面的代码块全部被执行完之后,将调用前面返回对象的“–exit–()”方法。 

with工作原理代码示例:

class Sample:
def __enter__(self):
print "in __enter__"
return "Foo"
def __exit__(self, exc_type, exc_val, exc_tb):
print "in __exit__"
def get_sample():
return Sample()
with get_sample() as sample:
print "Sample: ", sample


代码的运行结果如下:

in __enter__
Sample: Foo
in __exit__


可以看到,整个运行过程如下: 

(1)enter()方法被执行; 

(2)enter()方法的返回值,在这个例子中是”Foo”,赋值给变量sample; 

(3)执行代码块,打印sample变量的值为”Foo”; 

(4)exit()方法被调用;

【注:】exit()方法中有3个参数, exc_type, exc_val, exc_tb,这些参数在异常处理中相当有用。 

exc_type: 错误的类型 

exc_val: 错误类型对应的值 

exc_tb: 代码中错误发生的位置 

示例代码:

class Sample():
def __enter__(self):
print('in enter')
return self
def __exit__(self, exc_type, exc_val, exc_tb):
print "type: ", exc_type
print "val: ", exc_val
print "tb: ", exc_tb
def do_something(self):
bar = 1 / 0
return bar + 10
with Sample() as sample:
sample.do_something()


 

程序输出结果:

in enter
Traceback (most recent call last):
type: <type 'exceptions.ZeroDivisionError'>
val: integer division or modulo by zero
File "/home/user/cltdevelop/Code/TF_Practice_2017_06_06/with_test.py", line 36, in <module>
tb: <traceback object at 0x7f9e13fc6050>
sample.do_something()
File "/home/user/cltdevelop/Code/TF_Practice_2017_06_06/with_test.py", line 32, in do_something
bar = 1 / 0
ZeroDivisionError: integer division or modulo by zero

Process finished with exit code 1


 

总结

实际上,在with后面的代码块抛出异常时,exit()方法被执行。开发库时,清理资源,关闭文件等操作,都可以放在exit()方法中。 

总之,with-as表达式极大的简化了每次写finally的工作,这对代码的优雅性是有极大帮助的。 

如果有多项,可以这样写:

With open('1.txt') as f1, open('2.txt') as  f2:
do something