Question

I’m porting a C++ program to Python. There are some places where it uses std::set to store objects that define their own comparison operators. Since the Python standard library has no equivalent of std::set (a sorted key-value mapping data structure) I tried using a normal dictionary and then sorting it when iterating, like this:

def __iter__(self):
    items = self._data.items()
    items.sort()
    return iter(items)

However, profiling has shown that all the calls from .sort() to __cmp__ are a serious bottleneck. I need a better data structure – essentially a sorted dictionary. Does anyone know of an existing implementation? Failing that, any recommendations on how I should implement this? Read performance is more important than write performance and time is more important than memory.

Bonus points if it supports multiple values per key, like the C++ std::multimap.

Note that the OrderedDict class doesn’t fit my needs, because it returns items in the order of insertion, whereas I need them sorted using their __cmp__ methods.

Answer 1

For the sorted dictionary, you can (ab)use the stable nature of python’s timsort: basically, keep the items partially sorted, append items at the end when needed, switching a “dirty” flag, and sort the remaining before iterating. See this entry for details and implementation (A Martelli’s answer):
Key-ordered dict in Python