Let's walk through what's causing that error and how to get your desired output [2, 4, 6, 8], plus break down why list comprehensions are such a handy tool here.

Why You Got the AttributeError #

The error AttributeError: 'int' object has no attribute 'append' spells out exactly what's wrong: somewhere before your loop, you likely assigned new_list to an integer value (like new_list = 5 for example). When you try to call .append() on an integer, Python throws this error because integers don't have an append method—only lists do.

Even if you didn't explicitly assign it to an integer, if you never initialized new_list as an empty list at all, Python might be picking up a variable named new_list from elsewhere in your code that's an integer type.

Fixing the Loop Approach #

To get your original loop working, you just need to initialize new_list as an empty list before the loop runs. Here's the corrected code:

some_list = [1, 2, 3, 4]
new_list = []  # Start with an empty list
for x in some_list:
    new_list.append(x * 2)

print(new_list)  # Output: [2, 4, 6, 8]

This works because we start with an empty list, then add each doubled value to it one by one using the .append() method.

Simplifying with List Comprehensions #

List comprehensions are Python's concise way to create new lists by transforming or filtering existing iterables (like your some_list). The line new_list = [x*2 for x in some_list] does exactly the same thing as the loop above, but in a single, readable line.

Let's break down the list comprehension:

• x*2: The expression we want to apply to each element in the original list
• for x in some_list: The loop that iterates over every element in some_list
• The square brackets []: Tell Python we're building a new list from this logic

Running this code will directly give you your desired output:

some_list = [1, 2, 3, 4]
new_list = [x*2 for x in some_list]
print(new_list)  # Output: [2, 4, 6, 8]

List comprehensions aren't just shorter—they're often faster and more readable than equivalent for loops when you're creating a new list from scratch.

内容的提问来源于stack exchange，提问作者Vishnu Gautam