["python","list","loops","for-loop"]
Strange result when removing item from a list while iterating over it in Python

I've got this code: numbers = list(range(1, 50)) for i in numbers: if i < 20: numbers.remove(i) print(numbers) This code should go through all the elements of the list and remove those elements that are less than 20. But, the result I'm getting is: [2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49] As you can see, the code removed all even numbers less than 20, but left the odd ones. Looks like I'm doing something wrong with the remove.
You're modifying the list while you iterate over it. That means that the first time through the loop, i == 1, so 1 is removed from the list. Then the for loop goes to the second item in the list, which is not 2, but 3! Then that's removed from the list, and then the for loop goes on to the third item in the list, which is now 5. And so on. Perhaps it's easier to visualize like so, with a ^ pointing to the value of i: [1, 2, 3, 4, 5, 6...] ^ That's the state of the list initially; then 1 is removed and the loop goes to the second item in the list: [2, 3, 4, 5, 6...] ^ [2, 4, 5, 6...] ^ And so on. There's no good way to alter a list's length while iterating over it. The best you can do is something like this: numbers = [n for n in numbers if n >= 20] or this, for in-place alteration (the thing in parens is a generator expression, which is implicitly converted into a tuple before slice-assignment): numbers[:] = (n for n in numbers if n >= 20) If you want to perform an operation on n before removing it, one trick you could try is this: for i, n in enumerate(numbers): if n < 20: print("do something") numbers[i] = None numbers = [n for n in numbers if n is not None]