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memento.py
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memento.py
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#!/usr/bin/env python
# -*- coding: utf-8 -*-
"""http://code.activestate.com/recipes/413838-memento-closure/"""
from copy import copy, deepcopy
def memento(obj, deep=False):
state = copy(obj.__dict__) if deep else deepcopy(obj.__dict__)
def restore():
obj.__dict__.clear()
obj.__dict__.update(state)
return restore
class Transaction:
"""A transaction guard.
This is, in fact, just syntactic sugar around a memento closure.
"""
deep = False
states = []
def __init__(self, *targets):
self.targets = targets
self.commit()
def commit(self):
self.states = [memento(target, self.deep) for target in self.targets]
def rollback(self):
for a_state in self.states:
a_state()
class Transactional(object):
"""Adds transactional semantics to methods. Methods decorated with
@Transactional will rollback to entry-state upon exceptions.
"""
def __init__(self, method):
self.method = method
def __get__(self, obj, T):
def transaction(*args, **kwargs):
state = memento(obj)
try:
return self.method(obj, *args, **kwargs)
except Exception as e:
state()
raise e
return transaction
class NumObj(object):
def __init__(self, value):
self.value = value
def __repr__(self):
return '<%s: %r>' % (self.__class__.__name__, self.value)
def increment(self):
self.value += 1
@Transactional
def do_stuff(self):
self.value = '1111' # <- invalid value
self.increment() # <- will fail and rollback
if __name__ == '__main__':
num_obj = NumObj(-1)
print(num_obj)
a_transaction = Transaction(num_obj)
try:
for i in range(3):
num_obj.increment()
print(num_obj)
a_transaction.commit()
print('-- committed')
for i in range(3):
num_obj.increment()
print(num_obj)
num_obj.value += 'x' # will fail
print(num_obj)
except Exception as e:
a_transaction.rollback()
print('-- rolled back')
print(num_obj)
print('-- now doing stuff ...')
try:
num_obj.do_stuff()
except Exception as e:
print('-> doing stuff failed!')
import sys
import traceback
traceback.print_exc(file=sys.stdout)
print(num_obj)
### OUTPUT ###
# <NumObj: -1>
# <NumObj: 0>
# <NumObj: 1>
# <NumObj: 2>
# -- committed
# <NumObj: 3>
# <NumObj: 4>
# <NumObj: 5>
# -- rolled back
# <NumObj: 2>
# -- now doing stuff ...
# -> doing stuff failed!
# Traceback (most recent call last):
# File "memento.py", line 97, in <module>
# num_obj.do_stuff()
# File "memento.py", line 52, in transaction
# raise e
# File "memento.py", line 49, in transaction
# return self.method(obj, *args, **kwargs)
# File "memento.py", line 70, in do_stuff
# self.increment() # <- will fail and rollback
# File "memento.py", line 65, in increment
# self.value += 1
# TypeError: Can't convert 'int' object to str implicitly
# <NumObj: 2>