I've found some counterintuitive behavior in collections.Counter while hacking on the scikit-learn project [1]. I wanted to use a bunch of Counters to do some simple term counting in a set of documents, roughly as follows: count_total = Counter() for doc in documents: count_current = Counter(analyze(doc)) count_total += count_current count_per_doc.append(count_current) Performance was horrible. After some digging, I found out that Counter [2] does not have __iadd__ and += copies the entire left-hand side in __add__. I've attached a patch that fixes the issue (for += only, and I've not patched the testsuite.) [1] https://github.com/scikit-learn/scikit-learn/commit/de6e93094499e4d81b8e3b15fc66b6b9252945af
If this is not implemented because it is backwards incompat, then it might be useful to add a note to update's docstring explaining that it is much more efficient than +=. I was very surprised that it took *minutes* to add a few thousand moderate-sized Counters.
I'll add the in-place methods including __iadd__, __isub__, __iand__, and __ior__. If speed is your issue, you should continue to use the update() method which will always be faster because it doesn't have a step to strip zeros and negative values from the existing Counter. Also, update() has a fast-path written in C.
