Conversion Manager

The ConversionManager is a simple utility class that acts as a repository of functions that can upgrade a objects state from a previous version to a more current one. The VersionedSerializable provides the interface for objects that are “versioned” in the eyes of the default formatters.

Note

Subclassing VersionedSerializable is not necessary for the default formatters to understand that a class is versioned. Just the method names are enough. ConversionManager is not a requirement either. It can be switched out for a different class or implemented directly in the user object

The ConversionManager is also responsible for creating a “version object” from a versioned user object. See The interface for VersionedSerializable for more info about this. Essentially the conversion manager needs to create a serializable object that can differentiate version effectively.

The default ConversionManager, being simplistic, can only convert from one version to another. This means if you have a file that is 4 versions behind and only write 4 converters each for converting from the subsequent version to it’s next. The ConversionManager will have to run all 4 converters in sequence to update the state object to the correct version.

Here is what a version object looks like for the following class Graph3D

class Graph(VersionedSerializable):
    VERSION = '1'


class Graph3D(Graph):
    VERSION = '2'

{
    "__main__.Graph": "1",
    "__main__.Graph3D": "2"
}

Note that the conversion manager saves the version of the super classes also, as they may change independently.

This is what a simple converter might look like:

def convert_1_to_2(state_obj: dict):
    theta = state_obj.pop('thetha')
    state_obj['theta'] = theta
    return state_obj

This function just fixes a spelling error. To add it to the conversion manager it would look like this:

cm = ConversionManager()
cm.add_converter('1', Graph3D, convert_1_to_2, '2')

This is for the class Graph3D and converts version 1 to version 2.