Python: Removing Items From Inner And Outer For Loop Iterators

Is this piece of code potentially dangerous? Will it mess up with the inner and outer iterations? for a in listA: for b in listB: if [... something...]

Solution 1:

Is this piece of code potentially dangerous? Depends. Reducing the size of a sequence while iterating over it would give unexpected behavior.

Consider this Example

listA = [1,2,3,4]

>>> for a in listA:
    listA.remove(a)
    print a

Because, on removing the items, all the items beyond it, are pushed towards the left, the item that you are supposing to iterate would automatically move to the next element

First Iteration:

    listA = [1,2,3,4]
             ^
             |
_____________|
    listA.remove(a)
    listA = [2,3,4]
             ^
             |
_____________|
    print a
    (outputs) 1

Second Iteration:

    listA = [2,3,4]
               ^
               |
_______________|
    listA.remove(a)
    listA = [2,4]
               ^
               |
_______________|
    print a
    (outputs) 3

Third Iteration:

    listA = [2,4]
                 ^
                 |
_________________|

(Exits the Loop)

Solution 2:

Changing a sequence being iterated over is generally an anti-pattern in Python. While you can dance around it in specific cases, it's best to see if you can construct a new list (or dict) containing only the items you need.

Solution 3:

I agree with jknupp - removing items in a list can be more expensive than creating a new one. However, another trick is to do it backwards:

>>>l = range(5)>>>for a inreversed(l):...print a...    l.remove(a)... 
4
3
2
1
0