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# Persistent Data

Ren'Py supports persistent data, saved data that is not associated with a particular point in a game. Persistent data is accessed through fields of the persistent object, which is bound to the variable `persistent`.

All data reachable through fields on `persistent` is saved when Ren'Py terminates, or when `renpy.save_persistent()` is called. Persistent data is loaded when Ren'Py starts, and when Ren'Py detects that the persistent data has been updated on disk.

The persistent object is special in that an access to an undefined field will have a None value, rather than causing an exception.

An example use of persistent is the creation of an unlockable image gallery. This is done by storing a flag in persistent that determines if the gallery has been unlocked, as in

    label gallery:

        if not persistent.gallery_unlocked:
            show background
            centered "You haven't unlocked this gallery yet."
            $ renpy.full_restart()

        # Actually show the gallery here.

When the user gets an ending that causes the gallery to be unlocked, the flag must be set to True.

    $ persistent.gallery_unlocked = True

As persistent data is loaded before the init code is run, persistent data should only contain types that are native to python or Ren'Py. Alternatively, classes that are defined in `python early` blocks can be used, provided those classes can be pickled and implement equality.

## Merging Persistent Data

There are cases where Ren'Py has to merge persistent data from two sources. For example, Ren'Py may need to merge persistent data stored on a USB drive with persistent data from the local machine.

Ren'Py does this merging on a field-by-field basis, taking the value of the field that was updated more recently. In some cases, this is not the desired behavior. In that case, the `renpy.register_persistent()` function can be used.

For example, if we have a set of seen endings, we'd like to take the union of that set when merging data.

    init python:
        if persistent.endings is None:
            persistent.endings = set()

        def merge_endings(old, new, current):
            current.update(old)
            current.update(new)

        renpy.register_persistent('endings', merge_endings)

## Persistent Functions

`renpy.``register_persistent`(field, func)

  Registers a function that is used to merge values of a persistent field loaded from disk with values of current persistent object.

  field
    The name of a field on the persistent object.
  function

    A function that is called with three parameters, old, new, and current:

    old
      The value of the field in the older object.
    new
      The value of the field in the newer object.
    current
      The value of the field in the current persistent object. This is provided for cases where the identity of the object referred to by the field can't change.

    The function is expected to return the new value of the field in the persistent object.

`renpy.``save_persistent`()

  Saves the persistent data to disk.

## Multi-Game Persistence

Multi-Game persistence is a feature that lets you share information between Ren'Py games. This may be useful if you plan to make a series of games, and want to have them share information.

To use multipersistent data, a MultiPersistent object must be created inside an init block. The user can then update this object, and save it to disk by calling its save method. Undefined fields default to None. To ensure the object can be loaded again, we suggest not assigning the object instances of user-defined types.

class `MultiPersistent`(key)

  Creates a new MultiPersistent object. This should only be called inside an init block, and it returns a new MultiPersistent with the given key.

  key
    The key used to to access the multipersistent data. Games using the same key will access the same multipersistent data.
  `save`()

    Saves the multipersistent data to disk. This must be called after the data is modified.

As an example, take the first part of a two-part game:

    init python:
        mp = MultiPersistent("demo.renpy.org")

    label start:

        # ...

        # Record the fact that the user beat part 1.

        $ mp.beat_part_1 = True
        $ mp.save()

        e "You beat part 1. See you in part 2!"

And the second part:

    init python:
        mp = MultiPersistent("demo.renpy.org")

    label start:

        if mp.beat_part_1:
             e "I see you've beaten part 1, so welcome back!"
        else:
             e "Hmm, you haven't played part 1, why not try it first?"

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