a=["cat","dog"]
for x in a[:]:
不明白[:]是什么意思

http://docs.python.org/library/stdtypes.html#sequence-types-str-unicode-list-tuple-buffer-xrange

s[i:j] slice of s from i to j

1. If i or j is negative, the index is relative to the end of the string: len(s) + i or len(s) + j is substituted. But note that -0 is still 0.

2. The slice of s from i to j is defined as the sequence of items with index k such that i <= k < j. If i or j is greater than len(s), use len(s). If i is omitted or None, use 0. If j is omitted or None, use len(s). If i is greater than or equal to j, the slice is empty.

建议这位兄台 还是看看入门知识吧

你可以使用：取到数组的一个范围，：前留空则表示从第一个元素开始，：后留空则表示直到最后一个元素。负值表示从后索引（即-1是最后一个元素）。如下所示：
>>> mylist = ["List item 1", 2, 3.14]
>>> print mylist[:]
['List item 1', 2, 3.1400000000000001]
>>> print mylist[0:2]
['List item 1', 2]
>>> print mylist[-3:-1]
['List item 1', 2]
>>> print mylist[1:]
[2, 3.14]

python是一门实用性很强的语言，有些东西一试就明白了。

嗯 学习中...

李大师说的精辟

相当于自然语言中的省略号。

切片

for x in a 也可以吧。。。

python 好玩阿。