将新值附加到python列表

Question

Lets say I have two lists:

x = [1,2,3,4]
y = [1,4,7,8]

I want to append to x any values in y that are not already in x. I can do this easily with a loop:

for value in y:
    if value not in x:
        x.append(value)

But I am wondering if there a more Pythonic way of doing this.

Answer 1

Something like this:

In [22]: x = [1,2,3,4]

In [23]: y = [1,4,7,8]

In [24]: x += [ item for item in y if item not in x]

In [25]: x
Out[25]: [1, 2, 3, 4, 7, 8]

+= acts as list.extend , so the above code is equivalent to :

In [26]: x = [1,2,3,4]

In [27]: lis = [ item for item in y if item not in x]

In [28]: x.extend(lis)

In [29]: x
Out[29]: [1, 2, 3, 4, 7, 8]

Note that if the size of list x is huge and your list x/y contain only immutable(hashable) items then you must use sets here, as they will improve the time complexity to O(N) .

Answer 2

>>> x = [1,2,3,4]
>>> y = [1,4,7,8]
>>> set_x = set(x) # for fast O(1) amortized lookup
>>> x.extend(el for el in y if el not in set_x)
>>> x
[1, 2, 3, 4, 7, 8]

Answer 3

If you did not care about the order of the result you could do:

>>> x=[1,2,3,4]
>>> y=[1,4,7,8]
>>> x = list(set(x + y))
>>> x
[1, 2, 3, 4, 7, 8]

Answer 4

x = [1,2,3,4]
y = [1,4,7,8]

Now we want to append to x any values in y that are not already in x. This can be done easily as:

temp = [item for item in y if item not in x]
x.extend(temp)

This will first put all the elements which are not in x but are present in y in a list called temp. Now we shall extend list x to include elements from temp. Hope it helps.