Documentation for backend providers

Backend providers can provide a back-end for a defined API within the uarray ecosystem. To find out how to define your own API with uarray, see Documentation for API authors. To find out how your backend will be provided, use End-user quickstart.

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__ 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.


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 NotImplemented signals that the backend does not support this operation.


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 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, wrap_single_convertor is provided.

Returning NotImplemented signals that the backend does not support the conversion of the given object.


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 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 NotImplemented, then the backend is skipped, otherwise, its __ua_function__ protocol is tried. If a value other than NotImplemented is returned, it is assumed to be the final return value. Any exceptions raised are propagated up the call stack, except a 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 BackendNotImplementedError is raised.


Examples for library authors can be found in the source of unumpy.numpy_backend and other * files in this directory.