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a list-like type with better asymptotic performance

Project description

The BList is a type that looks, acts, and quacks like a Python list(), but has better performance for many (but not all) use cases. Below are some of the unique features of the BList:

  • just as fast as a Python list() when the list is small
  • insertion or removal from the list takes O(log n) time
  • getslice runs in O(log n) time and uses O(log n) memory, regardless of slice size
  • making a shallow copy runs in O(1) time and uses O(1) memory
  • setslice runs in O(log n + log k) time if the inserted slice is a BList of length k
  • multipling a BList by k takes O(log k) time and O(log k) memory


>>> from blist import *
>>> x = blist([0])
>>> x *= 2**29
>>> x.append(5)
>>> y = x[4:-234234]
>>> del x[3:1024]

None of the above operations have a noticeable delay, even though the lists have over 500 million elements due to line 3. The BList has two key features that allow it to pull this off this performance:

  1. Internally, a B+Tree is a wide, squat tree. Each node has a maximum of 128 children. If the entire list contains 128 or fewer objects, then there is only one node, which simply contains an array of the objects. In other words, for short lists, a BList works just like Python’s array-based list() type. Thus, it has the same good performance on small lists.
  2. The BList type features transparent copy-on-write. If a non-root node needs to be copied (as part of a getslice, copy, setslice, etc.), the node is shared between multiple parents instead of being copied. If it needs to be modified later, it will be copied at that time. This is completely behind-the-scenes; from the user’s point of view, the BList works just like a regular Python list.

So you can see the performance of the BList in more detail, several performance graphs available at the following link:

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