Python 进阶 之 else块 巧(慎)用

Python 的 else 模块和其他语言的else模块相比有那么一丢丢的特殊用法,有些用法虽然不推荐用,但是别人如果用了,最起码能看懂是不是?

1:快捷返回值: 

格式:
    value1 if expression else value1
  #
expression 为真,返回value1,expression为假,返回value2

>>> print 1 if 1 else 0
1
>>> print 1 if 0 else 0
0
>>>

2: if - elif - else 表达式:

 格式:

if expression1:
  value1
elif expression2:
   value2 
else:
   value3
#expression1 为真,返回value1,expression2为真,返回value2,否则返回value3
>>> if 0 == 2: 
...
print 2
...
elif 0 == 1:
...
print 1
...
else:
...
print 0
...
0
>>>

3:for - else 表达式:

for expression:
    if not loop-expression:
     value1
break else: value2
#
expression 为假,跳出循环,执行value2,loop-expression为假,跳出循环,执行value1
>>> for i in xrange(3):
...     if i > 1:
...         print "loop-expression"
...         break
... else:
...     print "else block"
... 
loop-expression
>>> for i in xrange(3):
...     if i > 4:
...         print "loop-expression"
...         break
...else:
...    print "else block"
... 
else block

 

3:while - else 表达式:

   与for - else 类似,不再赘述。

4:try - except - else - finally 模块

try:
expression
except:
value1
else:
value2
finally:
value3
#执行try内expression
1:执行异常,输出value1,value3
2:执行正常,输出value2,value3

>>> try: ... 1 == 1 ... except: ... print "error" ... else: ... print "else block" ... finally: ... print "finally block" ... else block finally block
>>> try:
...     x == 1
... except:
...     import traceback
...     print "------- except start -------"
...     traceback.print_exc()
...     print "------- except stop  -------"
... else:
...     print "else block"
... finally:
...     print "finally block"
... 
------- except start -------
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
NameError: name 'x' is not defined
------- except stop  -------
finally block

 

 

posted @ 2017-09-01 09:58  Jansora  阅读(352)  评论(0编辑  收藏  举报