Python: Adding element to list while iterating

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

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.

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.

You can do this.

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