Python: Adding element to list while iterating

Question

I know that it is not allowed to remove elements while iterating a list, but is it allowed to add elements to a python list while iterating. Here is an example:

Answer 1

In short: If you'are absolutely sure all new objects fail somecond() check, then your code works fine, it just wastes some time iterating the newly added objects.

Before giving a proper answer, you have to understand why it considers a bad idea to change list/dict while iterating. When using for statement, Python tries to be clever, and returns a dynamically calculated item each time. Take list as example, python remembers a index, and each time it returns l[index] to you. If you are changing l, the result l[index] can be messy.

NOTE: Here is a stackoverflow question to demonstrate this.

The worst case for adding element while iterating is infinite loop, try(or not if you can read a bug) the following in a python REPL:

import random

l = [0]
for item in l:
    l.append(random.randint(1, 1000))
    print item

It will print numbers non-stop until memory is used up, or killed by system/user.

Understand the internal reason, let's discuss the solutions. Here are a few:

1. make a copy of origin list

Iterating the origin list, and modify the copied one.

result = l[:]
for item in l:
    if somecond(item):
        result.append(Obj())

2. control when the loop ends

Instead of handling control to python, you decides how to iterate the list:

length = len(l)
for index in range(length):
    if somecond(l[index]):
        l.append(Obj())

Before iterating, calculate the list length, and only loop length times.

3. store added objects in a new list

Instead of modifying the origin list, store new object in a new list and concatenate them afterward.

added = [Obj() for item in l if somecond(item)]
l.extend(added)