On Sat, 03 Mar 2007 15:36:14 -0800, Paul Rubin wrote: > James Stroud <jstroud at mbi.ucla.edu> writes: >> for akey in dict1: >> if some_condition(akey): >> dict2[akey] = dict2.pop(akey) >> >> Which necessitates a key is a little cleaner than your latter example. > > Yeah, I also think removing keys from a dict while iterating over it > (like in Steven's examples) looks a bit dangerous dangerous. It is dangerous. That's why I didn't do it. I very carefully iterated over a list, not the dictionary, and in fact put in a comment explicitly saying that you can't iterate over the dictionary: for key in some_dict.keys(): # can't iterate over the dictionary directly! do_something_with(some_dict.pop(key)) If you try to iterate over the dictionary directly, you get a RuntimeError exception when the dictionary changes size. Unfortunately, the exception isn't raised until AFTER the dictionary has changed size. >>> D = {1:1, 2:2} >>> for key in D: ... D.pop(key) ... 1 Traceback (most recent call last): File "<stdin>", line 1, in <module> RuntimeError: dictionary changed size during iteration >>> D {2: 2} That's a gotcha to watch out for: the exception isn't raised until the damage is done. > Assuming you meant "dict1.pop" instead ot dict2.pop above, your > example might be written > > dict2 = dict((k, dict1.pop(k)) for k in dict1 if some_condition(k)) > > avoiding some namespace pollution etc. You get a RuntimeError exception when dict1 changes size. You know, if I were easily offended, I'd be offended that you accused _me_ of writing dangerous code when my code both worked and worked safely, while your code failed and did damage when it did so (dict1 irretrievably loses an item). *wink* -- Steven.