import warnings

from copy import deepcopy
from operator import setitem

UNDEFINED = object()

constraints = [
    ('age must be atleast 18', lambda new_state: new_state.get('age', UNDEFINED) is UNDEFINED or new_state['age'] >= 18),
]
state = {}
changes = [lambda state: setitem(state, 'age', 16)]

new_state = deepcopy(state)
for change in changes:
    change(new_state)


commit = True
for description, check in constraints:
    if not check(new_state):
        commit = False
        warnings.warn('constraint "{description}" broken'.format(description=description))


if commit:
    for change in changes:
        change(state)