CooDict

This (Python 3) module provides a simple 'copy-on-write' dictionary-like class to allow you to treat an existing dictionary as a 'constant' and only make modifications to an 'overlay' dictionary, the contents of which are merged over the top of the original when read.

Features

This class behaves almost exactly like a standard dictionary, with a few important differences:

• The class takes 2 arguments - the base dictionary, and an overlay dictionary (both default to {})
• All modifications (writes, deletes etc) happen to an overlay dictionary.
• All reads (get, keys, values etc) come from the results of merging the overlay and base dictionaries:
  • If a key is in overlay, return it.
  • If not, try to return the key in base.
  • If the key is marked as deleted in overlay, or absent in base, KeyError is raised.
• clear() only empties the overlay dictionary. Note this will also undo deletions!
• copy() returns a copy of the CooDict (in the form of a CooDict instance).

Example

from coodict import CooDict

original_dict = {'animal': 'cat', 'vegetable': 'carrot'}
cd = CooDict(original_dict)

# Make some modifications
cd['animal'] = 'dog'
cd.pop('vegetable')
cd['mineral'] = 'salt'
cd.items()
# results => {'animal': 'dog', 'mineral': 'salt'}

# check the original
original_dict.items()
# results => {'animal': 'cat', 'vegetable': 'carrot'}

# Clear the overlay (note that popped item 'vegetable' reappears).
cd.clear()
cd.items()
# results => {'animal': 'cat', 'vegetable': 'carrot'}