.. currentmodule:: uarray
Documentation for backend providers
Backend providers can provide a back-end for a defined API within the :obj:`uarray` ecosystem. To find out how to define your own API with :obj:`uarray`, see :ref:`mmauthordocs`. To find out how your backend will be provided, use :ref:`userdocs`.
Backend providers need to be aware of three protocols: __ua_domain__,
__ua_function__ and __ua_convert__. The first two are mandatory and
the last is optional.
__ua_domain__
__ua_domain__ is a string containing the domain of the backend. This is,
by convention, the name of the module (or one of its dependencies or parents)
that contains the multimethods. For example, scipy and numpy.fft could
both be in the numpy domain or one of its subdomains.
Additionally, __ua_domain__ can be a sequence of domains, such as a tuple or
list of strings. This allows a single backend to implement functions from more
than one domain.
__ua_function__
This is the most important protocol, one that defines the implementation of a
multimethod. It has the signature (method, args, kwargs).
Note that it is called in this form, so if your backend is an object instead of
a module, you should add self. method is the multimethod being called,
and it is guaranteed that it is in the same domain as the backend. args and
kwargs are the arguments to the function, possibly after conversion
(explained below)
Returning :obj:`NotImplemented` signals that the backend does not support this operation.
__ua_convert__
All dispatchable arguments are passed through __ua_convert__ before being
passed into __ua_function__. This protocol has the signature
(dispatchables, coerce), where dispatchables is iterable of
:obj:`Dispatchable` and coerce is whether or not to coerce forcefully.
dispatch_type is the mark of the object to be converted, and coerce
specifies whether or not to "force" the conversion. By convention, operations
larger than O(log n) (where n is the size of the object in memory)
should only be done if coerce is True. In addition, there are arguments
wrapped as non-coercible via the coercible attribute, if these must be
coerced, then one should return NotImplemented.
A convenience wrapper for converting a single object, :obj:`wrap_single_convertor` is provided.
Returning :obj:`NotImplemented` signals that the backend does not support the conversion of the given object.
:obj:`skip_backend`
If a backend consumes multimethods from a domain and provides multimethods for that same domain, it may wish to have the ability to use multimethods while excluding itself from the list of tried backends in order to avoid infinite recursion. This allows the backend to implement its functions in terms of functions provided by other backends. This is the purpose of the :obj:`skip_backend` decorator.
The process that takes place when the backend is tried
First of all, the backend's __ua_convert__ method is tried. If this returns
:obj:`NotImplemented`, then the backend is skipped, otherwise, its
__ua_function__ protocol is tried. If a value other than
:obj:`NotImplemented` is returned, it is assumed to be the final
return value. Any exceptions raised are propagated up the call stack, except a
:obj:`BackendNotImplementedError`, which signals a skip of the backend. If all
backends are exhausted, or if a backend with its only flag set to True
is encountered, a :obj:`BackendNotImplementedError` is raised.
Examples
Examples for library authors can be found in the source of unumpy.numpy_backend
and other *_backend.py files in this directory.