Index and Lists - Index out of range

You're trying to append items to a list, or more precisely to initialize a list with repeated copies of something. Here are Pythonic ways to do that:

# Use a list comprehension
>>> price = [100 for _ in range(3)]
[100, 100, 100]

# Use itertools.repeat
>>> import itertools
>>> list(itertools.repeat(100, 3))
[100, 100, 100]

These are both faster than (repeatedly) doing append(), which is O(N), so repeatedly doing append() is O(N^2) on a long list, which gets very slow.

(Btw if you know a priori the list will have at least N elements, and N is large, you could initialize it to price = [None] * N and you get [None, None, None...]. Now you can directly assign to them. But, explicitly doing append is better practice for beginners.)

The problem is you are assigning at an index too great for the length of the array.

for i in range(1, 3):

This initializes i to 1. Since arrays are zero-indexed, and the length of your array is 1 on the first pass, you will hit an assignment out of range error at the line you mentioned (when i=1).

Here is a minimal example showing the issue:

my_array = ["foo"]
my_array[1] = "bar" # throws assignment out of range error

You can't assign a value to a list directly why this list has note a previous size defined. You have to use append to add elements to the position you want.

Check:

# Check the value on our initial position
print(price[0])
for i in range(1, 3):
    price.append(price[i-1])
    print(price[i])

That will not print out:

100

100

You initialized a list with 1 element, the size of that list is 1. However your range starts at 1 for the for loop , what's really happening is:

price = 100  # assigns 'price' to 100
price = [price] # creates a 'price' list with 1 element: [100]

for i in range(1, 3): # The list is 0-indexed, meaning price[0] contains 100
    print(price[0]) # prints 100 as it should
    price[i] = price[i - 1] # i is 1, price[i] is not an assigned value, i.e: you never assigned price[1]
    price.append(price[i]) # This doesn't execute because an exception was thrown
    print(price[i]) # Neither does this

To get the result you're looking for, this would work:

price = [100] # creates a 'price' list with 1 element: [100]

for i in range(0, 2): # Start at index 0
    print(price[i]) # Print current index
    price.append(price[i]) # Append current value of price[i] to the price list

To ensure everything appended as you expected you can test it with len:

print(len(price))

Output:3

However, it is a preferred way of appending as @smci has shown in his/her answer.