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 work fine, however I don't know if it's 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

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.

Answer 2

Alternate solution in one line :

reduce(lambda x,y : x +[newObj] if somecond else x,myarr,myarr)

Answer 3

Why don't you just do it the idiomatic C way? This ought to be bullet-proof, but it won't be fast. I'm pretty sure indexing into a list in Python walks the linked list, so this is a "Shlemiel the Painter" algorithm. But I tend not to worry about optimization until it becomes clear that a particular section of code is really a problem. First make it work; then worry about making it fast, if necessary.

If you want to iterate over all the elements:

i = 0  
while i < len(some_list):  
  more_elements = do_something_with(some_list[i])  
  some_list.extend(more_elements)  
  i += 1  

If you only want to iterate over the elements that were originally in the list:

i = 0  
original_len = len(some_list)  
while i < original_len:  
  more_elements = do_something_with(some_list[i])  
  some_list.extend(more_elements)  
  i += 1

Answer 4

You can use an index and a while loop instead of a for loop if you want the loop to also loop over the elements that is added to the list during the loop:

i = 0
while i < len(myarr):
    a = myarr[i];
    i = i + 1;
    if somecond(a):
        myarr.append(newObj())

Answer 5

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())