How to limit the size of a dictionary?

Python 2.7 and 3.1 have OrderedDict and there are pure-Python implementations for earlier Pythons.

from collections import OrderedDict

class LimitedSizeDict(OrderedDict):
    def __init__(self, *args, **kwds):
        self.size_limit = kwds.pop("size_limit", None)
        OrderedDict.__init__(self, *args, **kwds)
        self._check_size_limit()

    def __setitem__(self, key, value):
        OrderedDict.__setitem__(self, key, value)
        self._check_size_limit()

    def _check_size_limit(self):
        if self.size_limit is not None:
            while len(self) > self.size_limit:
                self.popitem(last=False)

You would also have to override other methods that can insert items, such as update. The primary use of OrderedDict is so you can control what gets popped easily, otherwise a normal dict would work.

cachetools will provide you nice implementation of Mapping Hashes that does this (and it works on python 2 and 3).

Excerpt of the documentation:

For the purpose of this module, a cache is a mutable mapping of a fixed maximum size. When the cache is full, i.e. by adding another item the cache would exceed its maximum size, the cache must choose which item(s) to discard based on a suitable cache algorithm.