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:

    for a in myarr:
      if somecond(a):
          myarr.append(newObj())

I have tried this in my code and it seems to works fine, however i dont know if its because i am just lucky and that it will break at some point in the future?

EDIT: i prefer not to copy the list since "myarr" is huge, and therefore it would be too slow. Also i need to check the appended objects with "somecond()".

EDIT: At some point "somecond(a)" will be false, so there can not be an infinite loop.

EDIT: Someone asked about the "somecond()" function. Each object in myarr has a size, and each time "somecond(a)" is true and a new object is appended to the list, the new object will have a size smaller than a. "somecond()" has an epsilon for how small objects can be and if they are too small it will return "false"

Answer 1

make copy of your original list, iterate over it, see the modified code below

for a in myarr[:]:
      if somecond(a):
          myarr.append(newObj())

Answer 2

well, according to http://docs.python.org/tutorial/controlflow.html

It is not safe to modify the sequence being iterated over in the loop (this can only happen for mutable sequence types, such as lists). If you need to modify the list you are iterating over (for example, to duplicate selected items) you must iterate over a copy.

Answer 3

The danger with doing this is that the elements added at the end will also be iterated through. This means that if those objects also satisfy somecond(), then more elements will be added on. This can create an infinite loop. For instance, here is a very simple infinite loop:

x = [1]
for a in x:
    x.append(a+1)

If you want to avoid copying the array, then you could use an index to loop through the items:

L = len(myarr)
for i in xrange(L):
    if somecond(a[i]):
        myarr.append(newObj())

This isn't Pythonic, but it will avoid copying the array.

Answer 4

Access your list elements directly by i. Then you can append to your list:

for i in xrange(len(myarr)):
    if somecond(a[i]):
        myarr.append(newObj())

Answer 5

You can do this.

bonus_rows = []
for a in myarr:
  if somecond(a):
      bonus_rows.append(newObj())
myarr.extend( bonus_rows )

Answer 6

You could use the islice from itertools to create an iterator over a smaller portion of the list. Then you can append entries to the list without impacting the items you're iterating over:

islice( myarr, 0, len(myarr)-1 )

Even better, you don't even have to iterate over all the elements. You can increment a step size.

Answer 7

Expanding S.Lott's answer so that new items are processed as well:

todo = myarr
done = []
while todo:
    added = []
    for a in todo:
        if somecond(a):
            added.append(newObj())
    done.extend(todo)
    todo = added

The final list is in done.