Question

I’ve done my reading trust me. And python still acts up when removing an item from the list.
I have a graph implementation.

Now there is this one function that is supposed to delete some edges that the node has attached. I have a list of Edge objects and want to remove them by value…

Code is something like this:

for edge in node.edgeList:
...
   edgeToRemove = edge # edgeToRemove now holds something like <edge.Edge object at 0x107dcaf90>
   node.edgeList.remove(edgeToRemove) #KINDA WORKS - just doesnt behave consistently...It removes some edges but not others

What is the best way to remove them?

Answer 1

Don’t change the length of a list while iterating over it. It won’t work.

>>> l = range(10)
>>> for i in l:
...     l.remove(i)
... 
>>> l
[1, 3, 5, 7, 9]

See? The problem is that when you remove an item, the following items are all shifted back by one, but the location of the index remains the same. The effect is that the item after the removed item gets skipped. Depending on what you’re doing, a list comprehension is preferable.

>>> l = range(10)
>>> for i in l:
...     if i in [2, 3, 5, 6, 8, 9]:
...         l.remove(i)
... 
>>> l
[0, 1, 3, 4, 6, 7, 9]
>>> [i for i in range(10) if not i in [2, 3, 5, 6, 8, 9]]
[0, 1, 4, 7]