python lens

lens.py

from pprint import pprint
from copy import copy


class KeyLens(object):
    def __init__(self, key):
        self.key = key

    def lget(self, data):
        return data[self.key]

    def __str__(self):
        return 'KeyLens("%s")' % self.key

    def lset(self, data, v):
        data = copy(data)        
        data[self.key] = v

        return data

class IndexLens(KeyLens):
    def __str__(self):
        return 'IndexLens("%s")' % self.key

class ComposeLens(object):
    def __init__(self, *lenses):
        self.lenses = lenses

    def lget(self, data):
        for lens in self.lenses:
            data = lens.lget(data)

        return data

    def lset(self, data, v):
        original_data = []

        outer_lenses = self.lenses[:-1]

        for lens in outer_lenses:
            original_data.append((lens, data))
            data = lens.lget(data)

        data = self.lenses[-1].lset( data, v)
        for lens, original_datam in reversed(original_data):
            data = lens.lset(original_datam, data)

            
        return data

todos = {"home": [{"description": "cook",
                   "completed" : False},
                  {"description": "clean",
                   "completed" : False}
                  ],
         "work": [{"description": "program",
                   "completed" : False},
                  {"description": "send emails",
                   "completed" : False}
                  ]}

work_todos_lens = KeyLens("work")
print work_todos_lens.lget(todos)
# [{'completed': False, 'description': 'program'}, {'completed': False, 'description': 'send emails'}]

first_work_todo_complete_lens = ComposeLens(KeyLens("work"),
                                            IndexLens(0),
                                            KeyLens("completed"))

print first_work_todo_complete_lens.lget(todos)
# False
                                            
pprint(first_work_todo_complete_lens.lset(todos, True))
# {'home': [{'completed': False, 'description': 'cook'},
#           {'completed': False, 'description': 'clean'}],
#  'work': [{'completed': True, 'description': 'program'},
#           {'completed': False, 'description': 'send emails'}]}