How to implement a persistent Python `list`?

Question

I\'m trying to make an object act like a built-in list, except that its value be saved once modified.

The implementation I come up with is wrapping a

Answer 1

Here is a way to avoid having to decorate every list method. It makes PersistentList a context manager, so you can use the

with PersistentList('key', db) as persistent:
    do_stuff()

syntax. Admittedly, this does not cause the _save method to be called after each list operation, only when you exit the with-block. But I think it gives you enough control to save when you want to save, especially since the __exit__ method is guaranteed to be executed no matter how you leave the with-block, including if it happens because of an exception.

You might be an advantage that _save is not called after every list operation. Imagine appending to the list 10,000 times. So many individual calls to db.set (a database?) could be quite time-consuming. I would be better, at least from a performance point of view, to make all the appends and the save once.

---

class PersistentList(list):
    def __init__(self, key, db):
        self.key = key
        self.extend(db.get(key, []))
    def _save(self):
        # db.set(self.key, self)
        print('saving {x}'.format(x = self))
    def __enter__(self):
        return self
    def __exit__(self,ext_type,exc_value,traceback):
        self._save()

db = {}
p = PersistentList('key', db)

with p:
    p.append(1)
    p.append(2)

with p:
    p.pop()
    p += [1,2,3]

# saving [1, 2]
# saving [1, 1, 2, 3]