Styjun

Shouldn't the following code print? 100 100

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

for i in range(1, 3):
 print(price[0]) # prints 100
 price[i] = price[i - 1]
 price.append(price[i])
 print(price[i])

Getting a IndexError: list assignment index out of range error at line price[i] = price[i - 1], but the line right before prints 100 successfully. Shouldnt price[i] simply be getting assigned price[0] value?

If you are just trying to append to a list, trying to do that with an index won't exactly work because that index isn't present in the list:

somelist = []
somelist[0] = 1
IndexError

So just use append

for i in range(1,3):
 price.append(price[i-1])

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.