Why I cannot make an insert to Python list?

Question

I'm trying to insert some item by index to existing list like this:

c = ['545646', 'text_item', '151561'].insert(1, '555')
print(c)

And I'm getting None in result.

Why I cannot make an insert to the Python list?

The needed output is:

['545646', '555', 'text_item', '151561']

Answer 1

By Python convention, all mutating functions return None. Nonmutating functions return the new value. insert is a mutating function (changes the object it operates on), so it returns None; you then assign it to c.

In fact, there is no way to do this in one statement in current Python. In the future (almost certainly in Python 3.8), there is a proposal for a walrus operator that will allow you to shorten this:

(c := ['545646', 'text_item', '151561']).insert(1, '555')

though I believe Pythonistas will frown on it :)

EDIT: With the question in the comments, how to do an insert as an expression? The easiest way is to define another function; for example:

def insert_and_return_list(lst, pos, val):
    lst.insert(pos, val)
    return lst

c = insert_and_return_list(['545646', 'text_item', '151561'], 1, '555')

You could also avoid insert altogether, and use slices and splats:

[*lst[:1], '555', *lst[2:]]

Answer 2

Try this

c = ['545646', 'text_item', '151561']
c.insert(1, '555')
print(c)

Explanation:

Every python function call will return some value or None. In case of list.insert() function call, the return value is None which you are saving as variable c which will be None.

Edit: for follow up question in comment: I'm doing list comprehension and I'm processing the list of the lists. So, every time I would like to change an item in the list using 2 methods insert and pop one by one. How to make these changes while list comprehension?

You should generally avoid list comp for it's side effect.

instead do:

for sublist in l:
    sublist.insert(1, '555') # or call sublist.pop

Answer 3

Insert method always returns null .You are trying to print the null type. Therefore do not map insert method to any variable.Use insert as a method and print the original list Try this

c=['545646', 'text_item', '151561']
c.insert(1,55)
print(c)