LazyDictionary

Lazily-evaluated dictionaries.

Overview

from lazydict import LazyDictionary
lazy = LazyDictionary()
lazy['sum'] = lambda ld: ld['a'] + ld['b']
lazy['a'] = 1
lazy['b'] = 2
print lazy['sum']
# 3

A LazyDicitonary behaves mostly like an ordinary dict, except:

• item values are frozen upon reading, and

• values that are callable and take 1 or 0 arguments are called once before freezing

  • if the value takes 1 argument, the LazyDictionary instance will be supplied as the argument.

  • if calling the value raises an error, that error is frozen, and will be raised upon each subsequent read.

These features allow values in the dictionary to be dependent on other values in the dictionary without regard to order of assignment. It also allows lazily not executing unused code:

import tempfile
lazy = LazyDictionary()
lazy['temp'] = lambda: tempfile.mkdtemp()

If lazy['temp'] is never an R-value, mkdtemp() will never be called.

Callable 1-argument values are nodes in directed graphs. Edges are instances of indexing the argument within the value's body. In the first example, sum is the root and a and b are leaves. If a cycle exists in such a graph and a node within the cycle is evaluated, a CircularReferenceError will be thrown when the node is evaluated a second time.

If a frozen value is updated, a ConstantRedefinitionError will be thrown.

Install

pip install --pre lazydict

Testing

python setup.py test