Add support for custom serializers

[msiemens]

As discussed in #48, it would be nice to have support for custom serialization of objects unknown to TinyDB (= not supported by the JSON backend). This implements this interface, but I'm unsure how useful it will be.

cc @eugene-eeo @EmilStenstrom

[EmilStenstrom]

This looks excellent to me. Will use for all projects where I want to store dates in the database.

[eugene-eeo]

I remain skeptical but I think there must be a better way of doing this... Currently the code looks quite complex and no offence, "hack-ish" but I'm sure you'd clean it up. Coming from a performance point of view, I think the registered serializers should be stored in a hash-table, i.e.

self._serializers = {}

Then prefix lookup will be very efficient and straightforward. And it also forces developers to choose a unique name ;)

UPDATE: Also better to move the serializing and deserializing parts into a separate class, then add hooks in order to preserve current performance since not every use case demands custom serialization.

[msiemens]

First, sorry for not moving forward with this PR for so long! I hope we're now ready to go.

@eugene-eeo I've now refactored the serialization/deserialization into methods. Now I also use a dict for storing the serializers.

Also better to move the serializing and deserializing parts into a separate class, then add hooks in order to preserve current performance since not every use case demands custom serialization.

I don't think creating a hooking infracstructure will perform better than the current code because when no serializers are registered as the outer for-loop of the (de)serialization will not run at all.

But on the other hand I'm currently toying with the idea of using a middleware for serialization. This could be used like this:

serializer = Serializer(JSONStorage)
serializer.register(DateTimeSerializer(), 'TinyDate')
db = TinyDB(storage=serializer)

# ...

The advantage of this approach is that it doesn't modify the core at all and is totally opt-in. What do you think, @EmilStenstrom, @eugene-eeo?

[eugene-eeo]

👍 for not modifying core. By separating hooking infrastructure I meant the same thing as keeping it out of core and attaching the hooking class as a middleware, sorry for not explaining myself clearly.

[msiemens]

@eugene-eeo I've updated this PR again, now with a Middleware that does the serialization. The only downside is that its usage is now less ergonomic (see docs/extend.rst).

[eugene-eeo]

@msiemens Just a suggestion, I think I must be frustrating you but why not make it an extension? Custom serialisation is not something that we'd expect to be in core.

[EmilStenstrom]

As I've said before, I'm in favor of having this in core. Having dates in JSON is very common in my experience, and being able to point to the serialization docs is a big advantage over having to install an extension in my opinion.

[msiemens]

I agree with @EmilStenstrom about the extension question. I think it's okay to ship the middleware with tinydb as it's totally opt-in and we already provide a middleware that could be moved to an extension (CachingMiddleware).

[msiemens]

For the record: This feature has been moved to tinydb-serialization in TinyDB 3.0