the for statement in python differs a bit from what you may be used to in c or pascal. rather than always iterating over an arithmetic progression of numbers (like in pascal), or giving the user the ability to define both the iteration step and halting condition (as c), python’s for statement iterates over the items of any sequence (a list or a string), in the order that they appear in the sequence. for example (no pun intended):python中的for语句与c或pascal中使用的不同。而不是总是在算术级数（例如pascal）上迭代，或者给用户定义迭代步骤和停止条件（如c）的能力，python 中 for语句在任意序列（列表或字符串）的项上迭代，按照它们所提出的顺序。序列中的r。例如（没有双关语）：
>>> # measure some strings: 测量字符串长度（例子）
... words = ['cat', 'window', 'defenestrate'] #创建一个列表
>>> for w in words:
... print(w, len(w))
...
cat 3
window 6
defenestrate 12
if you need to modify the sequence you are iterating over while inside the loop (for example to duplicate selected items), it is recommended that youfirst make a copy. iterating over a sequence does not implicitly make a copy. the slice notation makes this especially convenient:如果需要修改循环内循环的顺序（例如复制选定项），建议您先复制。迭代一个序列不会隐式复制。切片符号使这特别方便：
>>> for w in words[:]: # loop over a slice copy of the entire list.循环遍历整个列表的切片(截取段)副本
... if len(w) >6:
... words.insert(0, w)
...
>>>words
['defenestrate', 'cat', 'window', 'defenestrate']
with for w in words:, the example would attempt to create an infinite list, inserting defenestrate over and over again.
以w为词：示例将尝试创建一个无限列表，一次又一次插入缺省的列表。