Python学习笔记（二）

if判断

if <判断条件1>：
    <执行1>
elif <判断条件2>：
      <执行2>
else
      <执行3>

只要x是非零数值、非空字符串、非空list等，就判断为True，否则为False。

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循环#

enumerate()

当需要显示序列index时，使用enumrate函数 enumrate(iterable[,start]) 示例如下：

>>> for i, v in enumerate(['tic', 'tac', 'toe']):
...     print i, v
...
0 tic
1 tac
2 toe

zip()

当同时遍历两个或多个sequence时，使用zip函数，示例如下：

>>> questions = ['name', 'quest', 'favorite color']
>>> answers = ['lancelot', 'the holy grail', 'blue']
>>> for q, a in zip(questions, answers):
...     print 'What is your {0}?  It is {1}.'.format(q, a)
...
What is your name?  It is lancelot.
What is your quest?  It is the holy grail.
What is your favorite color?  It is blue.

reversed()和 a[::-1]

倒序遍历sequence ，使用reversed函数：

>>> for i in reversed(xrange(1,10,2)):
...     print i
...
9
7
5
3
1

另一种优雅的倒序实现方式 a[::-1]：

>>> a = [1,2,3,4,5]
>>> for i in a[::-1]
        print i

sorted()

要按顺序遍历一个序列，使用sorted()函数返回一个新的排序列表而源列表不变。

>>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
>>> for f in sorted(set(basket)):
...     print f
...
apple
banana
orange
pear

L[:] copy List

当遍历一个sequence时，需要改变sequence元素，推荐使用copy。示例代码如下：

>>> words = ['cat', 'window', 'defenestrate']
>>> for w in words[:]:  # Loop over a slice copy of the entire list.
         if len(w) > 6:
...         words.insert(0, w)
...
>>> words
['defenestrate', 'cat', 'window', 'defenestrate']

List 拷贝代码：

>>> word = ['cat','dog']
>>> word_copy = word
>>> word.append('pig')
>>> word_copy
['cat', 'dog', 'pig']
>>> word_copy = word[:] # list copy
>>> word_copy
['cat', 'dog', 'pig']
>>> del word[0]
>>> word
['dog', 'pig']
>>> word_copy
['cat', 'dog', 'pig']