How to remove an element from a list while traversing it?

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?

senderle · Accepted Answer · 2012-02-28 18:13:26Z

6

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]

answered Feb 28, 2012 at 18:13

senderle

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5 Comments

kindall Over a year ago

Another approach is to iterate over the list in reverse order using reversed(). That way, deleting an item only shifts the items you've already seen.

senderle Over a year ago

@user965847, yes.

kosta5 Over a year ago

thanks :) I will figure out a solution myself no problem I just didnt realize you shouldnt do this...

GoingTharn Over a year ago

or you iterate over a copy of the list, and only remove from the parent list. for i in l[:]:

senderle Over a year ago

@kindall, @GoingTharn, both good points. In general, though, I prefer a list comprehension most of the time (with slice-assignment for in-place operations, i.e. x[:] = [i for i in x if foo]), because every time you call x.remove(v), x.pop(i), or del x[i], all the following items in the list must be shifted by one. So when you iterate over a list, removing many items, you're using the Shlemiel the painter's algorithm. :)

Ignacio Vazquez-Abrams · Accepted Answer · 2012-02-28 18:13:32Z

1

Get the index of the element and del it.

del somelist[n]

answered Feb 28, 2012 at 18:13

Ignacio Vazquez-Abrams

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Comments

user130076 · Accepted Answer · 2012-02-28 18:15:01Z

You could use lit comprehension - although your problem may be in your __eq__/__ne__ methods for edges (you should post that). Try this, though:

node.edgeList = [edge for edge in node.edgeList if edge != edgeToRemove]

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