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
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__()方法返回的值,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