Create an empty list before the loop in Python

Question

I always have a question about the empty list before the for the loop. For example, why do we need to create an empty list value = [] before for token in user_input.split()? When do we need to create such an empty list? Thanks so much for your help.

def get_numbers():
    user_input = input()
    values = []
    for token in user_input.split():
        values.append(int(token))
    return values

def print_selected_numbers():
    numbers = get_numbers()
    for number in numbers:
        if number > 9:
            print(number)

print_selected_numbers()

If we didn't have an empty list (named values here), what would we append to? — Chris
– Chris, Commented Feb 21, 2022 at 18:13
You have to make an object exist first before you can use a method of that object. Doing values = [] says to Python "I declare that a new object named values exists and that it's of type list". Now python knows that values has a method named append() because it's a list object. In most other programming languages you have to formally declare a variable of a particular type. Declare values as List (psuedocode). Python is much less picky, it just wants you to give it a hint like values = [] and it will figure it out from there. — JNevill
– JNevill, Commented Feb 21, 2022 at 18:14
(In this case a list comprehension would be more idiomatic, but when building a list in a loop like this we need a container to put the elements into.) — Chris
– Chris, Commented Feb 21, 2022 at 18:14
"I declare that a new object named values exists and that it's of type list"—well, not exactly. Python doesn't have type declarations. This creates an empty list via the literal [] and binds that object to the name values. It doesn't declare that values is of type list. — Chris
– Chris, Commented Feb 21, 2022 at 18:15
@Chris I added to my comment as you were writing that, I think we are saying the same thing. — JNevill
– JNevill, Commented Feb 21, 2022 at 18:16

typewriter · Accepted Answer · 2022-02-22 08:23:02Z

1

As Chris and JNevill noted in the comments to your answer, we need a container that defines an .append method. The empty list is such a container.

As noted by Chris, you can use list comprehension as such:

def get_numbers():
    return [int(token) for token in input().split()]

edited Feb 22, 2022 at 8:23

answered Feb 21, 2022 at 19:41

typewriter

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