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