python之循环

喜欢ヅ旅行 2022-12-19 13:23 158阅读 0赞

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

在遍历同一个序列时修改该集合的代码可能很难获得正确的结果。通常，更直接的做法是循环遍历该集合的副本或创建新集合

# Strategy: Iterate over a copy
    for user, status in users.copy().items():
    	if status == 'inactive':
    		del users[user]
    # Strategy: Create a new collection
    active_users = { }
    for user, status in users.items():
    if status == 'active':
    active_users[user] = status

参考：python官方文档[https://download.csdn.net/download/qq\_38048756/13096127][https_download.csdn.net_download_qq_38048756_13096127]

[https_download.csdn.net_download_qq_38048756_13096127]: https://download.csdn.net/download/qq_38048756/13096127