Python 循環的技巧

當在字典中循環時，用 items() 方法可將關鍵字和對應的值同時取出

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

當在序列中循環時，用 enumerate() 函數能夠將索引位置和其對應的值同時取出

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

當同時在兩個或更多序列中循環時，能夠用 zip() 函數將其內元素一一匹配。

>>> 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.

當逆向循環一個序列時，先正向定位序列，而後調用 reversed() 函數

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

若是要按某個指定順序循環一個序列，能夠用 sorted() 函數，它能夠在不改動原序列的基礎上返回一個新的排好序的序列

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

有時可能會想在python循環時修改列表內容，通常來講改成建立一個新列表是比較簡單且安全的

>>> 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]