How do I remove entries within a Counter object with a loop without invoking a RuntimeError?

Question

from collections import *
ignore = ['the','a','if','in','it','of','or']
ArtofWarCounter = Counter(ArtofWarLIST)
for word in ArtofWarCounter:
    if word in ignore:
        del ArtofWarCounter[word]

ArtofWarCounter is a Counter object containing all the words from the Art of War. I'm trying to have words in ignore deleted from the ArtofWarCounter.

Traceback:

  File "<pyshell#10>", line 1, in <module>
    for word in ArtofWarCounter:
RuntimeError: dictionary changed size during iteration

Answer 1

Don't loop over all words of a dict to find a entry, dicts are much better at lookups.

You loop over the ignore list and remove the entries that exist:

ignore = ['the','a','if','in','it','of','or']
for word in ignore:
    if word in ArtofWarCounter:
        del ArtofWarCounter[word]

Answer 2

For minimal code changes, use list, so that the object you are iterating over is decoupled from the Counter

ignore = ['the','a','if','in','it','of','or']
ArtofWarCounter = Counter(ArtofWarLIST)
for word in list(ArtofWarCounter):
    if word in ignore:
        del ArtofWarCounter[word]

In Python2, you can use ArtofWarCounter.keys() instead of list(ArtofWarCounter), but when it is so simple to write code that is futureproofed, why not do it?

It is a better idea to just not count the items you wish to ignore

ignore = {'the','a','if','in','it','of','or'}
ArtofWarCounter = Counter(x for x in ArtofWarLIST if x not in ignore)

note that I made ignore into a set which makes the test x not in ignore much more efficient