Sets

The basic data structure Rediset operates on is the set. Here's an extract from the Redis documentation on the set data type:

Redis Sets are an unordered collection of Strings. It is possible to add, remove, and test for existence of members in O(1) (constant time regardless of the number of elements contained inside the Set).

Redis Sets have the desirable property of not allowing repeated members. Adding the same element multiple times will result in a set having a single copy of this element. Practically speaking this means that adding a member does not require a check if exists then add operation.

A very interesting thing about Redis Sets is that they support a number of server side commands to compute sets starting from existing sets, so you can do unions, intersections, differences of sets in very short time.

Sets are instantiated as follows:

>>> from rediset import Rediset
>>> rs = Rediset()
>>> s = rs.Set('somekey')

Note that you can't pass an initial iterable of items when instantiating a new Set (like you can for native Python sets). This is because the set might already exist in Redis. You are instantiating a wrapper around a Redis set containing zero or more elements.

You can add and remove items from the set:

>>> s.add('a', 'b', 'c')
>>> s.remove('b')

You can get its size, check if an item is in the set, and get all of its members by converting it to a native Python set (or by iterating over it):

>>> len(s)
2
>>> 'a' in s
True
>>> 'b' in s
False
>>> set(s)
set(['a', 'c'])
>>>
>>> [item for item in s]
['a', 'c']

All of these operations are performed in Redis:

>>> s.add('b')     # "SADD" "somekey" "b"
>>> len(s)         # "SCARD" "somekey"
>>> 'a' in s       # "SISMEMBER" "somekey" "a"
>>> set(s)         # "SMEMBERS" "somekey"