Change task types to class hierarchy

[gavento]

In the Python API, all the tasks are currently an instance of the Task class with different types indicated by task_type. It would be more pythonic to implement them as a hierarchy of classes, one for each task_type (or even one for every Program or python Remote func. - TBD). This would have the advantages to allow using isinstance and better introspection (__repr__), access to parameters specific to the task type and allow subclassing.

Independently of this change, we can keep the task-creation operations as functions (and lower-cased), e.g. keep tasks.concat in additon to having tasks.ConcatTask.

This should be mostly non-disruptive change and needs a more detailed spec of Task interface.

[spirali]

I agree with this change. For me, the main point is better __repr__ than what we have now.

The only potential problem that I see is now is persistent sessions; reconstructing a graph in different Python client will be more complicated than now.

[gavento]

Good point about reconstruction from in-server data. We can map known types to their classes and then fallback with CustomTask (or such) - for those it is no worse than just a Task instance now.

[gavento]

We need a representation for the task types. We have the following task kinds:

• Built-in task, names prefixed with !, finite and relatively stable set
  • Python @remote tasks
    • Task type !py
    • Class: Either just PyTask, or one PyTask subclass per @remote. Possible drawback:
  • External programs
    • Task type !exec
    • Class: ExecTask
• User-provided subworkers (possibly including Python)
  • Propose subworker_name/provided_task (where subworker_name is the name subworker was registered with at the worker, preferably the binary name, provided_task is the name the subworker function was registered with, preferably based on the function name)
  • Class: SubworkerTask and subclasses
  • Possibly let the client declare MyownSubworker classes?

One TODO is to bikeshed task class names

[gavento]

Done on v0.3: All builtin and user tasks derive from the Task class.