Python: Adding element to list while iterating

python iteration
WesDec picture WesDec · Sep 20, 2010 · Viewed 112.4k times · Source

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

Johnny picture Johnny · Apr 15, 2011

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

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