CODEWITHSUNDEEP
pythonpython-3.x
mauscope
mauscope

Reputation: 318

Copying and Appending to an empty list

In a piece of code I need to copy a list and append a new value. I have found the following behaviour:

a=[]
b=[n for n in a].append('val')
print(b)
None

While, the following works as I require:

b=[n for n in a]
b.append('val')
print(b)
['val']

Any insight why it is so?

Upvotes: 2

Views: 1655

Answers (3)

Maroun
Maroun

Reputation: 96016

append modifies the list in place, it doesn't return a new list, that's why you're getting None.

See the documentation:

.. The methods that add, subtract, or rearrange their members in place, and don’t return a specific item, never return the collection instance itself but None.

Upvotes: 4

shahkalpesh
shahkalpesh

Reputation: 33484

That is because b=[n for n in a].append('val') does not return anything. In specific append('val') does not return any value.

Upvotes: 1

awesoon
awesoon

Reputation: 33701

Method append returns None, because it modifies list in place, that is why b in your first example is None. You could use list concatenation in order to copy a list and append an element to it:

In [238]: a = [1, 2, 3]

In [239]: b = a + [4]

In [240]: b
Out[240]: [1, 2, 3, 4]

Upvotes: 2

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