cowdict 0.2

Copy-on-write dictionary

cowdict is a python module implementing copy-on-write pattern on dictionaries.

This means that if someone change the dict-like object, the original object won’t be changed but the differences will be tracked instead.

Nutshell

Here’s a small example:

base_dict = {"foo": "bar"}
cd = CowDict(base_dict)
cd["foo"] = "baz"

print(base_dict["foo"]) # will still print 'bar'
print(cd["foo"]) # will print 'baz'

CowDict object (cd in the code above) implements the MutableMapping interface, simply speaking it has the same interface as the dict object.

This means that it is guaranteed that changing keys/values on this object will never cause the change of the underlying dictionary (base_dict in the example).

Philosophy

The idea behind this module is to avoid copying dictionary where it is possible and use this wrapper class instead. While it has some penalties on the performance on key lookup and length calculation, the memory footprint can be kept at minimum (compared to a full copy of the dictionary).

Behind the scenes

Behind the scenes, new items are added to a separate directory. Keys which exist on the base dictionary are ‘deleted’ by keeping a separate set object about the the keys deleted. Every time when a key is accessed either by getitem or items or other methods, these additional structures are involved to produce the correct result.

While having an assumption of having the base dictionary read-only would make the world easier, especially when calculating the length of the object, the library handles the situation when the base dictionary changes.