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apps/labs/posts/uarray-intro.md

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+---
+title: "`uarray`: A Generic Override Framework for Methods"
+author: hameer-abbasi
+published: April 30, 2019
+description: 'The problem is, stated simply: How do we use all of the PyData libraries in tandem, moving seamlessly from one to the other, without actually changing the API, or even the imports?'
+category: [PyData ecosystem]
+featuredImage:
+  src: /posts/hello-world-post/blog_hero_var2.svg
+  alt: 'An illustration of a brown and a white hand coming towards each other to pass a business card with the logo of Quansight Labs.'
+hero:
+  imageSrc: /posts/hello-world-post/blog_feature_org/svg
+  imageAlt: 'An illustration of a brown hand holding up a microphone, with some graphical elements highlighting the top of the microphone.'
+---
+
+`uarray` is an override framework for methods in Python. In the
+scientific Python ecosystem, and in other similar places, there has been
+one recurring problem: That similar tools to do a job have existed, but
+don't conform to a single, well-defined API. `uarray` tries to solve
+this problem in general, but also for the scientific Python ecosystem in
+particular, by defining APIs independent of their implementations.
+
+## Array Libraries in the Scientific Python Ecosystem
+
+When SciPy was created, and Numeric and Numarray unified into NumPy, it
+jump-started Python's data science community. The ecosystem grew
+quickly: Academics started moving to SciPy, and the Scikits that popped
+up made the transition all the more smooth.
+
+However, the scientific Python community also shifted during that time:
+GPUs and distributed computing emerged. Also, there were old ideas that
+couldn't really be used with NumPy's API, such as sparse arrays. To
+solve these problems, various libraries emerged:
+
+-   Dask, for distributed NumPy
+-   CuPy, for NumPy on Nvidia-branded GPUs.
+-   PyData/Sparse, a project started to make sparse arrays conform to
+    the NumPy API
+-   Xnd, which extends the type system and the universal function
+    concept found in NumPy
+
+There were yet other libraries that emerged: PyTorch, which mimics NumPy
+to a certain degree; TensorFlow, which defines its own API; and MXNet,
+which is another deep learning framework that mimics NumPy.
+
+## The Problem
+
+The problem is, stated simply: How do we use all of these libraries in
+tandem, moving seamlessly from one to the other, without actually
+changing the API, or even the imports? How do we take functions written
+for one library and allow it to be used by another, without, as Travis
+Oliphant so eloquently puts it, \"re-writing the world\"?
+
+In my mind, the goals are (stated abstractly):
+
+1.  Methods that are not tied to a specific implementation.
+
+-   For example `np.arange`
+
+1.  Backends that implement these methods.
+
+-   NumPy, Dask, PyTorch are all examples of this.
+
+1.  Coercion of objects to other forms to move between backends.
+
+-   This means converting a NumPy array to a Dask array, and vice versa.
+
+In addition, we wanted to be able to do this for arbitrary objects. So
+`dtype`s, `ufunc`s etc. should also be dispatchable and coercible.
+
+## The Solution?
+
+With that said, let's dive into `uarray`. If you're not interested in
+the gory details, you can jump down to
+`<a href="#how-to-use-it">`{=html}this section`</a>`{=html}.
+
+``` python
+import uarray as ua
+
+# Let's ignore this for now
+def myfunc_rd(a, kw, d):
+    return a, kw
+
+# We define a multimethod
+@ua.create_multimethod(myfunc_rd)
+def myfunc():
+    return () # Let's also ignore this for now
+
+# Now let's define two backends!
+be1 = ua.Backend()
+be2 = ua.Backend()
+
+# And register their implementations for the method!
+@ua.register_implementation(myfunc, backend=be1)
+def myfunc_be1(): # Note that it has exactly the same signature
+    return "Potato"
+
+@ua.register_implementation(myfunc, backend=be2)
+def myfunc_be2(): # Note that it has exactly the same signature
+    return "Strawberry"
+```
+
+``` python
+with ua.set_backend(be1):
+    print(myfunc())
+```
+
+    Potato
+
+``` python
+with ua.set_backend(be2):
+    print(myfunc())
+```
+
+    Strawberry
+
+As we can clearly see: We have already provided a way to do (1) and (2)
+above. But then we run across the problem: How do we decide between
+these backends? How do we move between them? Let's go ahead and
+register both of these backends for permanent use. And see what happens
+when we want to implement both of their methods!
+
+``` python
+ua.register_backend(be1)
+ua.register_backend(be2)
+```
+
+``` python
+print(myfunc())
+```
+
+    Potato
+
+As we see, we get only the first backend's answer. In general, it's
+indeterminate what backend will be selected. But, this is a special
+case: We're not passing arguments in! What if we change one of these to
+return `NotImplemented`?
+
+``` python
+# We redefine the multimethod so it's new again
+@ua.create_multimethod(myfunc_rd)
+def myfunc():
+    return ()
+
+# Now let's redefine the two backends!
+be1 = ua.Backend()
+be2 = ua.Backend()
+
+# And register their implementations for the method!
+@ua.register_implementation(myfunc, backend=be1)
+def myfunc_be1(): # Note that it has exactly the same signature
+    return NotImplemented
+
+@ua.register_implementation(myfunc, backend=be2)
+def myfunc_be2(): # Note that it has exactly the same signature
+    return "Strawberry"
+
+ua.register_backend(be1)
+ua.register_backend(be2)
+```
+
+``` python
+with ua.set_backend(be1):
+    print(myfunc())
+```
+
+    Strawberry
+
+Wait\... What? Didn't we just set the first `Backend`? Ahh, but, you
+see\... It's signalling that it has *no* implementation for `myfunc`.
+The same would happen if you simply didn't register one. To force a
+`Backend`, we must use `only=True` or `coerce=True`, the difference will
+be explained in just a moment.
+
+``` python
+with ua.set_backend(be1, only=True):
+    print(myfunc())
+```
+
+    ---------------------------------------------------------------------------
+    BackendNotImplementedError                Traceback (most recent call last)
+    <ipython-input-8-ec856cf7c88b> in <module>
+          1 with ua.set_backend(be1, only=True):
+    ----> 2     print(myfunc())
+
+    ~/Quansight/uarray/uarray/backend.py in __call__(self, *args, **kwargs)
+        108 
+        109         if result is NotImplemented:
+    --> 110             raise BackendNotImplementedError('No selected backends had an implementation for this method.')
+        111 
+        112         return result
+
+    BackendNotImplementedError: No selected backends had an implementation for this method.
+
+Now we are told that no backends had an implementation for this function
+(which is nice, good error messages are nice!)
+
+## Coercion and passing between backends
+
+Let's say we had two `Backend`s. Let's choose the completely useless
+example of one storing a number as an `int` and one as a `float`.
+
+``` python
+class Number(ua.DispatchableInstance):
+    pass
+
+def myfunc_rd(args, kwargs, dispatchable_args):
+    # Here, we're "replacing" the dispatchable args with the ones supplied.
+    # In general, this may be more complex, like inserting them in between
+    # other args and kwargs.
+    return dispatchable_args, kwargs
+
+@ua.create_multimethod(myfunc_rd)
+def myfunc(a):
+    # Here, we're marking a as a Number, and saying that "we want to dispatch/convert over this"
+    # We return as a tuple as there may be more dispatchable arguments
+    return (Number(a),)
+
+Number.register_convertor(be1, lambda x: int(x))
+Number.register_convertor(be2, lambda x: str(x))
+```
+
+Let's also define a \"catch-all\" method. This catches all
+implementations of methods not already registered.
+
+``` python
+# This can be arbitrarily complex
+def gen_impl1(method, args, kwargs, dispatchable_args):
+    if not all(isinstance(a, Number) and isinstance(a.value, int) for a in dispatchable_args):
+        return NotImplemented
+    
+    return args[0]
+
+# This can be arbitrarily complex
+def gen_impl2(method, args, kwargs, dispatchable_args):
+    if not all(isinstance(a, Number) and isinstance(a.value, str) for a in dispatchable_args):
+        return NotImplemented
+    
+    return args[0]
+
+be1.register_implementation(None, gen_impl1)
+be2.register_implementation(None, gen_impl2)
+```
+
+``` python
+myfunc('1') # This calls the second implementation
+```
+
+    '1'
+
+``` python
+myfunc(1) # This calls the first implementation
+```
+
+    1
+
+``` python
+myfunc(1.0) # This fails
+```
+
+    ---------------------------------------------------------------------------
+    BackendNotImplementedError                Traceback (most recent call last)
+    <ipython-input-13-8431c1275db5> in <module>
+    ----> 1 myfunc(1.0) # This fails
+
+    ~/Quansight/uarray/uarray/backend.py in __call__(self, *args, **kwargs)
+        108 
+        109         if result is NotImplemented:
+    --> 110             raise BackendNotImplementedError('No selected backends had an implementation for this method.')
+        111 
+        112         return result
+
+    BackendNotImplementedError: No selected backends had an implementation for this method.
+
+``` python
+# But works if we do this:
+
+with ua.set_backend(be1, coerce=True):
+    print(type(myfunc(1.0)))
+
+with ua.set_backend(be2, coerce=True):
+    print(type(myfunc(1.0)))
+```
+
+    <class 'int'>
+    <class 'str'>
+
+This may seem like too much work, but remember that it's broken down
+into a lot of small steps:
+
+1.  Extract the dispatchable arguments.
+2.  Realise the types of the dispatchable arguments.
+3.  Convert them.
+4.  Place them back into args/kwargs
+5.  Call the right function.
+
+Note that `only=True` does not coerce, just enforces the backend
+strictly.
+
+With this, we have solved problem (3). Now remains the grunt-work of
+actually retrofitting the NumPy API into `unumpy` and extracting the
+right values from it.
+
+## How To Use It Today
+
+`unumpy` is a set of NumPy-related multimethods built on top of
+`uarray`. You can use them as follows:
+
+``` python
+import unumpy as np # Note the changed import statement
+from unumpy.xnd_backend import XndBackend
+
+with ua.set_backend(XndBackend):
+    print(type(np.arange(0, 100, 1)))
+```
+
+    <class 'xnd.array'>
+
+And, as you can see, we get back an Xnd array when using a NumPy-like
+API. Currently, there are three back-ends: NumPy, Xnd and PyTorch. The
+NumPy and Xnd backends have feature parity, while the PyTorch backend is
+still being worked on.
+
+We are also working on supporting more of the NumPy API, and dispatching
+over dtypes.
+
+Feel free to browse the source and open issues at:
+<https://github.com/Quansight-Labs/uarray> or shoot me an email at
+`<a href="mailto:[email protected]">`{=html}[email protected]`</a>`{=html}
+if you want to contact me directly. You can also find the full
+documentation at <https://uarray.readthedocs.io/en/latest/>.
+