Why use list(x) if I'm already working with a list?

Courses
,
1

In the exemple is used “list(double_list)” to print the list, but why? double_list alone is not a list?
I tried to use “print(double_list)” and I know it doesn’t work, I got a weird number, but why is it? And that number, is it the ID?
One more question, when I try to use just “list” instead of “list_1”, I got an error “something is not defined”, why that happens? Do I need to always use a underscore?

num_list = [0, 1, 2, 3, 4, 5]
double_list = map(lambda n: n * 2, num_list)
print(list(double_list))

thanks :slight_smile:

Course: Educative: Interactive Courses for Software Developers
Lesson: https://www.educative.io/courses/learn-python-3-from-scratch/mELQq49yJ2n

Course: https://www.educative.io/courses/learn-python-3-from-scratch
Lesson: https://www.educative.io/courses/learn-python-3-from-scratch/mELQq49yJ2n

2

Hi @Lugumedeiros !!
In the given example,

num_list = [0, 1, 2, 3, 4, 5]
double_list = map(lambda n: n * 2, num_list)
print(list(double_list))  # Convert the iterator to a list and then print it

We have a list num_list containing some numbers, and we’re using the map function with a lambda function to double each element in num_list. The result of the map operation is an iterator, not a list. That’s why we need to use list(double_list) to convert the iterator back into a list before printing.

Let’s break down your questions:

  1. Using list(double_list) instead of just double_list
    The map function returns an iterator in Python. An iterator is a type of object that produces values one at a time as you iterate over it. It doesn’t generate all the values at once, which can be memory-efficient. In order to see the actual values produced by the map operation, you need to convert the iterator into a list. The list() function is used to consume the iterator and create a list from its values. So, list(double_list) is used to convert the iterator double_list into a list and then print it.

  2. print(double_list) not working and produce a weird number
    When you directly try to print the double_list iterator, Python displays its memory address, which is represented as a weird number. This is not the intended behavior because you’re seeing the internal representation of the iterator, not the actual values it produces.

  3. Encountering error when using just list instead of list(double_list)
    In this code, we are using the map function to create an iterator double_list. When we use the function list (like list(double_list)), we are converting this iterator into a list. If you try to use list without parentheses like list, Python interprets it as you trying to access a variable named list, which is not defined in your code. Adding an underscore or another identifier after list prevents this confusion and ensures you’re calling the built-in list() function.
    I hope it helps. Happy Learning :blush:

1 Like