CODEWITHSUNDEEP
pythonlistlist-comprehension
Dilshad Abduwali
Dilshad Abduwali

Reputation: 1458

Python List Comprehension is not returning a new List

I am surprised by the behavior of List Comprehension on a list of lists. I would expect List comprehension to return a new list to me ALWAYS. For example:

>>> L = [1,2,3,4,5,6]
>>> M = [ x * 2 for x in L]
>>> L
>>> [1,2,3,4,5,6]
>>> M
>>> [2,4,6,8,10,12]

So L is not changed.

However,


>>> L = [[1], [2], [3]]
>>> M = [x.append(100) for x in L]
>>> M
>>> [None, None, None]
>>> L
>>> [[1,100], [2,100], [3,100]]

Now L is changed and list comprehension does not return a new List.

I am expecting a new List by the list comprehension. Any explanation would help me to understand this behavior of list comprehension

Upvotes: 0

Views: 808

Answers (3)

Pedro Rodrigues
Pedro Rodrigues

Reputation: 2648

You are appending to x, is an element in L, that is why L changes. If you want a new list do not mutate a older, create a new one.

You don't specify a result you expect, but I believe this what your looking for:

>>> L = [[1], [2], [3]]
>>> M = [[*x, 100] for x in L]
>>> L
[[1], [2], [3]]
>>> M
[[1, 100], [2, 100], [3, 100]]

Upvotes: 1

AnsFourtyTwo
AnsFourtyTwo

Reputation: 2518

x * 2 is an expression which evaluates to a result and its result will be stored in the list M.

x.append(100) on the other hand applies function append() on the object x, which is an element of list L and returns None.

It is the same, why you do y = x * 2, but not y = x.append(100).

Upvotes: 2

user3180528
user3180528

Reputation: 51

x.append(100)

Return nothing(None), this why you have only None's inside M.

Upvotes: 0

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