List comprehension not working [duplicate]

问题

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Closed 3 years ago.

I want to put the unique items from one list to another list, i.e eliminating duplicate items. When I do it by the longer method I am able to do it see for example.

>>>new_list = []
>>>a = ['It', 'is', 'the', 'east', 'and', 'Juliet', 'is', 'the', 'sun']

>>> for word in a:
    if word not in a:
        new_list.append(word)

>>> new_list
['It', 'is', 'the', 'east', 'and', 'Juliet', 'sun']

But when try to accomplish this using list comprehension in a single line the each iteration returns value "None"

>>> new_list = []
>>> a = ['It', 'is', 'the', 'east', 'and', 'Juliet', 'is', 'the', 'sun']
>>> new_list = [new_list.append(word) for word in a if word not in new_list]

Can someone please help in understanding whats going wrong in the list comprehension.

Thanks in Advance Umesh

回答1:

If you want a unique list of words, you can use set().

list(set(a))
# returns:
# ['It', 'is', 'east', 'and', 'the', 'sun', 'Juliet']

If the order is important, try:

new_list = []
for word in a:
    if not a in new_list:
        new_list.append(word)

回答2:

List comprehensions provide a concise way to create lists. Common applications are to make new lists where each element is the result of some operations applied to each member of another sequence or iterable, or to create a subsequence of those elements that satisfy a certain condition.

Maybe you can try this:

>>> new_list = []
>>> a = ['It', 'is', 'the', 'east', 'and', 'Juliet', 'is', 'the', 'sun']
>>> unused=[new_list.append(word) for word in a if word not in new_list]
>>> new_list
['It', 'is', 'the', 'east', 'and', 'Juliet', 'sun']
>>> unused
[None, None, None, None, None, None, None]

Notice:

append() returns None if the inserted operation is successful.

Another way, you can try to use set to remove duplicate item:

>>> a = ['It', 'is', 'the', 'east', 'and', 'Juliet', 'is', 'the', 'sun']
>>> list(set(a))
['and', 'sun', 'is', 'It', 'the', 'east', 'Juliet']

来源：https://stackoverflow.com/questions/43152140/list-comprehension-not-working

标签

python

for-loop

list-comprehension