When looping through dictionaries, the key and corresponding value can be retrieved at the same time using the items()
method.
>>>
>>> knights = {‘gallahad‘: ‘the pure‘, ‘robin‘: ‘the brave‘} >>> for k, v in knights.items(): ... print(k, v) ... gallahad the pure robin the brave
When looping through a sequence, the position index and corresponding value can be retrieved at the same time using the enumerate()
function.
>>>
>>> for i, v in enumerate([‘tic‘, ‘tac‘, ‘toe‘]): ... print(i, v) ... 0 tic 1 tac 2 toe
To loop over two or more sequences at the same time, the entries can be paired with the zip()
function.
>>>
>>> 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.
To loop over a sequence in reverse, first specify the sequence in a forward direction and then call the reversed()
function.
>>>
>>> for i in reversed(range(1, 10, 2)): ... print(i) ... 9 7 5 3 1
To loop over a sequence in sorted order, use the sorted()
function which returns a new sorted list while leaving the source unaltered.
>>>
>>> basket = [‘apple‘, ‘orange‘, ‘apple‘, ‘pear‘, ‘orange‘, ‘banana‘] >>> for f in sorted(set(basket)): ... print(f) ... apple banana orange pear
To change a sequence you are iterating over while inside the loop (for example to duplicate certain items), it is recommended that you first make a copy. Looping over a sequence does not implicitly make a copy. The slice notation makes this especially convenient:
>>>
>>> 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‘]