Question

I have a nested list that looks like this:

mylist = [['A;B', 'C'], ['D;E', 'F']]

I’d like to have it in the following form:

[['A', 'B', 'C'], ['D', 'E', 'F']]

Figured I’d write a simple list comprehension to do the task:

>>> newlist = [item[0].split(';').append(item[1]) for item in mylist]
>>> newlist
[None, None]

After some experimenting, I found that the error was in trying to use append() on anonymous lists:

>>> type(['A', 'B'])
<class 'list'>
>>> type(['A', 'B'].append('C'))
<class 'NoneType'>

Which seems like a gotcha, considering that you can do things like this:

>>> 'abc'.upper()
'ABC'

Obviously in most cases you could get around this by binding ['A', 'B'] to a variable before calling append(), but how would I make this work inside of a list comprehension? Furthermore, can anyone explain this unintuitive behavior?

Answer 1

As you found out, mutating methods aren’t of much use inside a list comprehension because the transient objects disappear immediately.

What works instead is to build-up a list through concatenation:

>>> mylist = [['A;B', 'C'], ['D;E', 'F']]
>>> [first.split(';') + [second] for first, second in mylist]
[['A', 'B', 'C'], ['D', 'E', 'F']]