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 OrderedDictclass 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.

Here's a simple, no-LRU Python 2.6+ solution (in older Pythons you could do something similar with UserDict.DictMixin, but in 2.6 and better that's not recommended, and the ABCs from collections are preferable anyway...):

import collectionsclass MyDict(collections.MutableMapping):    def __init__(self, maxlen, *a, **k):        self.maxlen = maxlen        self.d = dict(*a, **k)        while len(self) > maxlen:            self.popitem()    def __iter__(self):        return iter(self.d)    def __len__(self):        return len(self.d)    def __getitem__(self, k):        return self.d[k]    def __delitem__(self, k):        del self.d[k]    def __setitem__(self, k, v):        if k not in self and len(self) == self.maxlen:            self.popitem()        self.d[k] = vd = MyDict(5)for i in range(10):    d[i] = i    print(sorted(d))

As other answers mentioned, you probably don't want to subclass dict -- the explicit delegation to self.d is unfortunately boilerplatey but it does guarantee that every other method is properly supplied by collections.MutableMapping.