Save Game Incompatibility in modified Python class structures

Direct Answer

Ren'Py does not provide a native declarative mechanism to define default values for attributes that are missing from a loaded save. When a save is loaded, the engine reconstructs the object by restoring its instance dictionary (__dict__) from the serialized state; it does not call __init__. Therefore, if you added a new attribute after the save was created, the loaded object will lack it, and accessing it will raise AttributeError unless you patch it manually.

Conversely, if you removed an attribute from the class definition, the loaded object will still carry the old attribute in memory. Ren'Py does not purge stale keys during deserialization. That stale data remains accessible and will be written back on the next save.

Likely Explanation vs. Confirmed Behavior

The underlying cause is that Ren'Py's save system extends Python's pickle protocol, which serializes instance state rather than class structure. Because __init__ is bypassed, any defaults or initialization logic you placed there are not applied to unpickled objects. This is well-established Python behavior that Ren'Py inherits.

What is confirmed by the engine's design: missing attributes are not backfilled, and removed attributes are not stripped. There is no built-in schema-migration layer that reconciles class changes automatically.

Minimal Migration Steps

1. Add a version tag to your class or a global save-version variable so you can detect outdated state.
2. Write a migration function that checks for missing attributes and assigns defaults, and optionally deletes attributes that no longer exist.
3. Invoke the migration immediately after load using Ren'Py's after_load hook or a wrapper around your load logic, before gameplay resumes.
4. Verify the result in the developer console (Shift+O) by inspecting the object's __dict__.

Scoped Example

def migrate_player(p):
    if not hasattr(p, "new_attribute"):
        p.new_attribute = 0
    if hasattr(p, "removed_attribute"):
        del p.removed_attribute

label after_load:
    python:
        migrate_player(player)
    return

Assumptions and Verification

This behavior has been consistent across modern Ren'Py 7.x and 8.x releases, but exact hook names and console commands should be checked against your project's version. Because serializer internals can evolve, treat the migration pattern above as a conservative baseline and test with a save created before your class change.