NoneType List

[Alan Gauld]

On 31/12/2022 05:45, Goran Ikac wrote:

> b = a.append(3)


> I mean: why b = a.append(something) is the None type, and how to make a new
> list that contains all the items from a and some new items?

append() like many other collection methods in Python works
in place and returns None. But the action has succeeded
and 3 will have been appended to list 'a'.

So, to create a new list that contains all the old items you could do:

newlist = []   # an empty list
for item in oldlist:
    newlist.append(item)

This is so common Python has a shorthand form to do this:

newlist = [item for item in oldlist]

called a list comprehension.

And there is an even shorter way using something called slicing:

newlist = oldlist[:]    # copy oldlist to new.


However, as an ex-Smalltalk programmer, I do wish that Python
returned self from these methods rather than None so that
we could chain them. But sadly it doesn't.

-- 
Alan G
Author of the Learn to Program web site
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