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's an answer that's a lot like @unutbu's, but more general. It gives you a function you can call to sync your object to disk, and it works with other pickle-able classes besides list.

with pickle_wrap(os.path.expanduser("~/Desktop/simple_list"), list) as (lst, lst_sync):
    lst.append("spam")
    lst_sync()
    lst.append("ham")
    print(str(lst))
    # lst is synced one last time by __exit__

Here's the code that makes that possible:

import contextlib, pickle, os, warnings

def touch_new(filepath):
    "Will fail if file already exists, or if relevant directories don't already exist"
    # http://stackoverflow.com/a/1348073/2829764
    os.close(os.open(filepath, os.O_WRONLY | os.O_CREAT | os.O_EXCL))

@contextlib.contextmanager
def pickle_wrap(filepath, make_new, check_type=True):
    "Context manager that loads a file using pickle and then dumps it back out in __exit__"
    try:
        with open(filepath, "rb") as ifile:
            result = pickle.load(ifile)
        if check_type:
            new_instance = make_new()
            if new_instance.__class__ != result.__class__:
                # We don't even allow one class to be a subclass of the other
                raise TypeError(("Class {} of loaded file does not match class {} of "
                    + "value returned by make_new()")
                    .format(result.__class__, new_instance.__class__))
    except IOError:
        touch_new(filepath)
        result = make_new()
    try:
        hash(result)
    except TypeError:
        pass
    else:
        warnings.warn("You probably don't want to use pickle_wrap on a hashable (and therefore likely immutable) type")

    def sync():
        print("pickle_wrap syncing")
        with open(filepath, "wb") as ofile:
            pickle.dump(result, ofile)

    yield result, sync
    sync()