Python 循环的技巧

当在字典中循环时，用 items() 方法可将关键字和对应的值同时取出

>>> knights = {\'gallahad\': \'the pure\', \'robin\': \'the brave\'}
>>> for k, v in knights.items():
...     print(k, v)
...
gallahad the pure
robin the brave

当在序列中循环时，用 enumerate() 函数可以将索引位置和其对应的值同时取出

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

当同时在两个或更多序列中循环时，可以用 zip() 函数将其内元素一一匹配。

>>> 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.

当逆向循环一个序列时，先正向定位序列，然后调用 reversed() 函数

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

如果要按某个指定顺序循环一个序列，可以用 sorted() 函数，它可以在不改动原序列的基础上返回一个新的排好序的序列

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

有时可能会想在python循环时修改列表内容，一般来说改为创建一个新列表是比较简单且安全的

>>> import math
>>> raw_data = [56.2, float(\'NaN\'), 51.7, 55.3, 52.5, float(\'NaN\'), 47.8]
>>> filtered_data = []
>>> for value in raw_data:
...     if not math.isnan(value):
...         filtered_data.append(value)
...
>>> filtered_data
[56.2, 51.7, 55.3, 52.5, 47.8]