3 ms·

So, uh, why are people doing so much list copying? It's not something that occurs often in idiomatic code. I understand that it's something to be aware of, but

• xxbondsxx 15y ago
  Calling methods like pop(), insert(), and remove() on a list actually affects the contents of a list rather than returning a _copy_ of the list with everything removed. For example: all = range(10) allbut2 = all.remove(2) Actually removes 2 from all as well. Hence, you have to copy lists a lot if you are doing a lot of list creation or change from a master list.
  • d0mine 15y ago
    list.remove() returns None. So it is a mistake to bind its return value to allbut2. It follows the convention that methods that modify their object inplace should return None. list.pop() is an obvious exception. You create a new list via list comprehension instead of copying and then removing: even = [i for i in L if i % 2 == 0] # remove odd numbers
    • tedunangst 15y ago
      I believe he meant: all = range() allbut2 = all allbut2.remove(2) which is something that catches people all the time.
  • MostAwesomeDude 15y ago
    I'm totally aware of that. I'm just confused because in idiomatic Python, list copies without a map, filter, or reduce operation are very rare, so a list copy is usually replaced with a list comprehension or iterative loop of some sort. For example, in Bravo, there are exactly eight list copies, all in contributed code which I didn't write, and another six in Exocet, which I also didn't write. The ones in Exocet are probably required, but the others are from code that was written without forethought.