Persistent data structures
Pyrsistent is a number of persistent collections. Persistent in the sense that they are immutable (if only accessing the public API). The collection types currently implemented are PVector (similar to a python list), PMap (similar to a python dict) and PSet (similar to a python set).
All methods on a data structure that manipulates it returns a new copy of the object with the containing the requested updates rather than manipulating the original.
This will simplify the reasoning about what a program does since no hidden side effects ever can take place to these data structures. You can rest assured that the object you hold a reference to will remain the same throughout its lifetime and need not worry that somewhere five stack levels below you in the darkest corner of your application someone has decided to remove that element that you expected to be there.
The following code snippet illustrated the difference between the built in, regular, list and the vector which is part of this library
>>> from pyrsistent import v >>> l = [1, 2, 3] >>> l.append(4) >>> print l [1, 2, 3, 4] >>> p1 = v(1, 2, 3) >>> p2 = p1.append(4) >>> print p1 (1, 2, 3) >>> print p2 (1, 2, 3, 4)
Performance is generally in the range of 1 - 100 times slower than using the corresponding built in types in Python. In the cases where attempts at optimizations have been done, speed has generally been valued over space.
Pyrsistent comes with two API compatible flavors of PVector (on which PMap and PSet are based), one pure Python implementation and one implemented as a C extension. The latter generally beeing 2 - 10 times faster then the former. The C extension will be used automatically when possible. It is currently only available for CPython 2.7 (and possibly 2.5 and 2.6, but this has not been tested).
The pure python implementation is fully PyPy compatible. Running it under PyPy speeds operations up considerably if the structures are used heavily (if JITed), for some cases the performance is almost on par with the built in counterparts.
Pyrsistent has been developed on Python 2.7 and is not yet tested on Python 3.x.
Pyrsistent is greatly influenced by the persistent data structures that are part Clojures standard library.
pip install pyrsistent
Available at http://pyrsistent.readthedocs.org/
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