Use case:
I want to append a new item along with all the existing items in a list to the same list.
Ex:
list = [ 'a', 'b', 'c']
Appending 'd', expecting the output as: ['a', 'b', 'c', 'a', 'b', 'c', 'd']
My code:
list.append(list.append('d'))
Current output:
['a', 'b', 'c', 'd', None]
Why am I getting a None item here and how can I print the list as expected?
Use list.append('d') instead.
append function in list return nothing and so list.append(list.append('d')) will be added None.
To print the expected list (let the list be 'l'):
list_old = list(l)
l += l # ['a', 'b', 'c'] -> ['a', 'b', 'c', 'a', 'b', 'c']
l.append('d')
list_old.extend(l)
list.append returns None. This is because list.append is an in-place operation. In addition, you are shadowing a built-in, which is not recommended.
You can append to a copy, then extend your original list:
L = ['a', 'b', 'c']
L_to_append = L.copy()
L_to_append.append('d')
L.extend(L_to_append)
But this is verbose. You can just use the += operator:
L = ['a', 'b', 'c']
L += L + ['d']
print(L)
['a', 'b', 'c', 'a', 'b', 'c', 'd']
