for语句和shell编程的风格很象，是对一个sequence来进行iteration。比如枚举一个List或是String。

>>> # Measure some strings:
... a = ['cat', 'window', 'defenestrate']
>>> for x in a:
...     print x, len(x)
... 
cat 3
window 6
defenestrate 12

为了避免在循环体内更改正在被Iterating的sequence的内容，从而带来不可预料的结果，采用的方法是用sequence的拷贝来做iteration:

>>> for x in a[:]: # make a slice copy of the entire list
...    if len(x) > 6: a.insert(0, x)
... 
>>> a
['defenestrate', 'cat', 'window', 'defenestrate']