将项目附加到 Python 中列表的所有子列表

Question

Here are my datasets:

R = [["yyy", "3"],["www", "4"],["eee","3"],["zzz", "2"]]

I am trying to append one item ('-1') to all sublists of a list (R) to get a new list (new_R), and keep R unchanged:

print(new_R)
output:
[['yyy', '3', '-1'],
 ['www', '4', '-1'],
 ['eee', '3', '-1'],
 ['zzz', '2', '-1']]

print(R)
output:
[["yyy", "3"],["www", "4"],["eee","3"],["zzz", "2"]]

I tried:

new_R = [x.append('-1') for x in R]

and

new_R = list(map(lambda x: x.append('-1'), R))

However, the results of R and new_R are not expected:

print(new_R)
output:
[None, None, None, None]

print(R)
output:
[['yyy', '3', '-1'],
 ['www', '4', '-1'],
 ['eee', '3', '-1'],
 ['zzz', '2', '-1']]

Looking for explanation and solution! Thanks!

Answer 1

append operates in place and returns None , so you don't want that (it's why your solutions create lots of None s in the new list while modifying the original list ). Simplest solution is just list concatenation, eg:

new_R = [x + ['-1'] for x in R]

or (with newer syntax for 3.5 and higher , and potentially fewer temporaries) construct a new list by unpacking the old one plus an extra value:

new_R = [[*x, '-1'] for x in R]