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nep-0031-uarray.html
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<section id="nep-31-context-local-and-global-overrides-of-the-numpy-api">
<span id="nep31"></span><h1>NEP 31 — Context-local and global overrides of the NumPy API<a class="headerlink" href="#nep-31-context-local-and-global-overrides-of-the-numpy-api" title="Link to this heading">#</a></h1>
<dl class="field-list simple">
<dt class="field-odd">Author<span class="colon">:</span></dt>
<dd class="field-odd"><p>Hameer Abbasi <<a class="reference external" href="mailto:habbasi%40quansight.com">habbasi<span>@</span>quansight<span>.</span>com</a>></p>
</dd>
<dt class="field-even">Author<span class="colon">:</span></dt>
<dd class="field-even"><p>Ralf Gommers <<a class="reference external" href="mailto:rgommers%40quansight.com">rgommers<span>@</span>quansight<span>.</span>com</a>></p>
</dd>
<dt class="field-odd">Author<span class="colon">:</span></dt>
<dd class="field-odd"><p>Peter Bell <<a class="reference external" href="mailto:pbell%40quansight.com">pbell<span>@</span>quansight<span>.</span>com</a>></p>
</dd>
<dt class="field-even">Status<span class="colon">:</span></dt>
<dd class="field-even"><p>Superseded</p>
</dd>
<dt class="field-odd">Replaced-By<span class="colon">:</span></dt>
<dd class="field-odd"><p><a class="reference internal" href="nep-0056-array-api-main-namespace.html#nep56"><span class="std std-ref">NEP 56 — Array API standard support in NumPy’s main namespace</span></a></p>
</dd>
<dt class="field-even">Type<span class="colon">:</span></dt>
<dd class="field-even"><p>Standards Track</p>
</dd>
<dt class="field-odd">Created<span class="colon">:</span></dt>
<dd class="field-odd"><p>2019-08-22</p>
</dd>
<dt class="field-even">Resolution<span class="colon">:</span></dt>
<dd class="field-even"><p><a class="reference external" href="https://mail.python.org/archives/list/numpy-discussion@python.org/message/Z6AA5CL47NHBNEPTFWYOTSUVSRDGHYPN/">https://mail.python.org/archives/list/numpy-discussion@python.org/message/Z6AA5CL47NHBNEPTFWYOTSUVSRDGHYPN/</a></p>
</dd>
</dl>
<section id="abstract">
<h2>Abstract<a class="headerlink" href="#abstract" title="Link to this heading">#</a></h2>
<p>This NEP proposes to make all of NumPy’s public API overridable via an
extensible backend mechanism.</p>
<p>Acceptance of this NEP means NumPy would provide global and context-local
overrides in a separate namespace, as well as a dispatch mechanism similar
to NEP-18 <a class="footnote-reference brackets" href="#id21" id="id1" role="doc-noteref"><span class="fn-bracket">[</span>2<span class="fn-bracket">]</span></a>. First experiences with <code class="docutils literal notranslate"><span class="pre">__array_function__</span></code> show that it
is necessary to be able to override NumPy functions that <em>do not take an
array-like argument</em>, and hence aren’t overridable via
<code class="docutils literal notranslate"><span class="pre">__array_function__</span></code>. The most pressing need is array creation and coercion
functions, such as <code class="docutils literal notranslate"><span class="pre">numpy.zeros</span></code> or <code class="docutils literal notranslate"><span class="pre">numpy.asarray</span></code>; see e.g. NEP-30 <a class="footnote-reference brackets" href="#id28" id="id2" role="doc-noteref"><span class="fn-bracket">[</span>9<span class="fn-bracket">]</span></a>.</p>
<p>This NEP proposes to allow, in an opt-in fashion, overriding any part of the
NumPy API. It is intended as a comprehensive resolution to NEP-22 <a class="footnote-reference brackets" href="#id22" id="id3" role="doc-noteref"><span class="fn-bracket">[</span>3<span class="fn-bracket">]</span></a>, and
obviates the need to add an ever-growing list of new protocols for each new
type of function or object that needs to become overridable.</p>
</section>
<section id="motivation-and-scope">
<h2>Motivation and scope<a class="headerlink" href="#motivation-and-scope" title="Link to this heading">#</a></h2>
<p>The primary end-goal of this NEP is to make the following possible:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># On the library side</span>
<span class="kn">import</span> <span class="nn">numpy.overridable</span> <span class="k">as</span> <span class="nn">unp</span>
<span class="k">def</span> <span class="nf">library_function</span><span class="p">(</span><span class="n">array</span><span class="p">):</span>
<span class="n">array</span> <span class="o">=</span> <span class="n">unp</span><span class="o">.</span><span class="n">asarray</span><span class="p">(</span><span class="n">array</span><span class="p">)</span>
<span class="c1"># Code using unumpy as usual</span>
<span class="k">return</span> <span class="n">array</span>
<span class="c1"># On the user side:</span>
<span class="kn">import</span> <span class="nn">numpy.overridable</span> <span class="k">as</span> <span class="nn">unp</span>
<span class="kn">import</span> <span class="nn">uarray</span> <span class="k">as</span> <span class="nn">ua</span>
<span class="kn">import</span> <span class="nn">dask.array</span> <span class="k">as</span> <span class="nn">da</span>
<span class="n">ua</span><span class="o">.</span><span class="n">register_backend</span><span class="p">(</span><span class="n">da</span><span class="p">)</span> <span class="c1"># Can be done within Dask itself</span>
<span class="n">library_function</span><span class="p">(</span><span class="n">dask_array</span><span class="p">)</span> <span class="c1"># works and returns dask_array</span>
<span class="k">with</span> <span class="n">unp</span><span class="o">.</span><span class="n">set_backend</span><span class="p">(</span><span class="n">da</span><span class="p">):</span>
<span class="n">library_function</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">])</span> <span class="c1"># actually returns a Dask array.</span>
</pre></div>
</div>
<p>Here, <code class="docutils literal notranslate"><span class="pre">backend</span></code> can be any compatible object defined either by NumPy or an
external library, such as Dask or CuPy. Ideally, it should be the module
<code class="docutils literal notranslate"><span class="pre">dask.array</span></code> or <code class="docutils literal notranslate"><span class="pre">cupy</span></code> itself.</p>
<p>These kinds of overrides are useful for both the end-user as well as library
authors. End-users may have written or wish to write code that they then later
speed up or move to a different implementation, say PyData/Sparse. They can do
this simply by setting a backend. Library authors may also wish to write code
that is portable across array implementations, for example <code class="docutils literal notranslate"><span class="pre">sklearn</span></code> may wish
to write code for a machine learning algorithm that is portable across array
implementations while also using array creation functions.</p>
<p>This NEP takes a holistic approach: It assumes that there are parts of
the API that need to be overridable, and that these will grow over time. It
provides a general framework and a mechanism to avoid a design of a new
protocol each time this is required. This was the goal of <code class="docutils literal notranslate"><span class="pre">uarray</span></code>: to
allow for overrides in an API without needing the design of a new protocol.</p>
<p>This NEP proposes the following: That <code class="docutils literal notranslate"><span class="pre">unumpy</span></code> <a class="footnote-reference brackets" href="#id27" id="id4" role="doc-noteref"><span class="fn-bracket">[</span>8<span class="fn-bracket">]</span></a> becomes the
recommended override mechanism for the parts of the NumPy API not yet covered
by <code class="docutils literal notranslate"><span class="pre">__array_function__</span></code> or <code class="docutils literal notranslate"><span class="pre">__array_ufunc__</span></code>, and that <code class="docutils literal notranslate"><span class="pre">uarray</span></code> is
vendored into a new namespace within NumPy to give users and downstream
dependencies access to these overrides. This vendoring mechanism is similar
to what SciPy decided to do for making <code class="docutils literal notranslate"><span class="pre">scipy.fft</span></code> overridable (see <a class="footnote-reference brackets" href="#id29" id="id5" role="doc-noteref"><span class="fn-bracket">[</span>10<span class="fn-bracket">]</span></a>).</p>
<p>The motivation behind <code class="docutils literal notranslate"><span class="pre">uarray</span></code> is manyfold: First, there have been several
attempts to allow dispatch of parts of the NumPy API, including (most
prominently), the <code class="docutils literal notranslate"><span class="pre">__array_ufunc__</span></code> protocol in NEP-13 <a class="footnote-reference brackets" href="#id23" id="id6" role="doc-noteref"><span class="fn-bracket">[</span>4<span class="fn-bracket">]</span></a>, and the
<code class="docutils literal notranslate"><span class="pre">__array_function__</span></code> protocol in NEP-18 <a class="footnote-reference brackets" href="#id21" id="id7" role="doc-noteref"><span class="fn-bracket">[</span>2<span class="fn-bracket">]</span></a>, but this has shown the need
for further protocols to be developed, including a protocol for coercion (see
<a class="footnote-reference brackets" href="#id24" id="id8" role="doc-noteref"><span class="fn-bracket">[</span>5<span class="fn-bracket">]</span></a>, <a class="footnote-reference brackets" href="#id28" id="id9" role="doc-noteref"><span class="fn-bracket">[</span>9<span class="fn-bracket">]</span></a>). The reasons these overrides are needed have been extensively
discussed in the references, and this NEP will not attempt to go into the
details of why these are needed; but in short: It is necessary for library
authors to be able to coerce arbitrary objects into arrays of their own types,
such as CuPy needing to coerce to a CuPy array, for example, instead of
a NumPy array. In simpler words, one needs things like <code class="docutils literal notranslate"><span class="pre">np.asarray(...)</span></code> or
an alternative to “just work” and return duck-arrays.</p>
</section>
<section id="usage-and-impact">
<h2>Usage and impact<a class="headerlink" href="#usage-and-impact" title="Link to this heading">#</a></h2>
<p>This NEP allows for global and context-local overrides, as well as
automatic overrides a-la <code class="docutils literal notranslate"><span class="pre">__array_function__</span></code>.</p>
<p>Here are some use-cases this NEP would enable, besides the
first one stated in the motivation section:</p>
<p>The first is allowing alternate dtypes to return their
respective arrays.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># Returns an XND array</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">unp</span><span class="o">.</span><span class="n">ones</span><span class="p">((</span><span class="mi">5</span><span class="p">,</span> <span class="mi">5</span><span class="p">),</span> <span class="n">dtype</span><span class="o">=</span><span class="n">xnd_dtype</span><span class="p">)</span> <span class="c1"># Or torch dtype</span>
</pre></div>
</div>
<p>The second is allowing overrides for parts of the API.
This is to allow alternate and/or optimized implementations
for <code class="docutils literal notranslate"><span class="pre">np.linalg</span></code>, BLAS, and <code class="docutils literal notranslate"><span class="pre">np.random</span></code>.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="kn">import</span> <span class="nn">pyfftw</span> <span class="c1"># Or mkl_fft</span>
<span class="c1"># Makes pyfftw the default for FFT</span>
<span class="n">np</span><span class="o">.</span><span class="n">set_global_backend</span><span class="p">(</span><span class="n">pyfftw</span><span class="p">)</span>
<span class="c1"># Uses pyfftw without monkeypatching</span>
<span class="n">np</span><span class="o">.</span><span class="n">fft</span><span class="o">.</span><span class="n">fft</span><span class="p">(</span><span class="n">numpy_array</span><span class="p">)</span>
<span class="k">with</span> <span class="n">np</span><span class="o">.</span><span class="n">set_backend</span><span class="p">(</span><span class="n">pyfftw</span><span class="p">)</span> <span class="c1"># Or mkl_fft, or numpy</span>
<span class="c1"># Uses the backend you specified</span>
<span class="n">np</span><span class="o">.</span><span class="n">fft</span><span class="o">.</span><span class="n">fft</span><span class="p">(</span><span class="n">numpy_array</span><span class="p">)</span>
</pre></div>
</div>
<p>This will allow an official way for overrides to work with NumPy without
monkeypatching or distributing a modified version of NumPy.</p>
<p>Here are a few other use-cases, implied but not already
stated:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">data</span> <span class="o">=</span> <span class="n">da</span><span class="o">.</span><span class="n">from_zarr</span><span class="p">(</span><span class="s1">'myfile.zarr'</span><span class="p">)</span>
<span class="c1"># result should still be dask, all things being equal</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">library_function</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="n">result</span><span class="o">.</span><span class="n">to_zarr</span><span class="p">(</span><span class="s1">'output.zarr'</span><span class="p">)</span>
</pre></div>
</div>
<p>This second one would work if <code class="docutils literal notranslate"><span class="pre">magic_library</span></code> was built
on top of <code class="docutils literal notranslate"><span class="pre">unumpy</span></code>.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">dask</span> <span class="kn">import</span> <span class="n">array</span> <span class="k">as</span> <span class="n">da</span>
<span class="kn">from</span> <span class="nn">magic_library</span> <span class="kn">import</span> <span class="n">pytorch_predict</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">da</span><span class="o">.</span><span class="n">from_zarr</span><span class="p">(</span><span class="s1">'myfile.zarr'</span><span class="p">)</span>
<span class="c1"># normally here one would use e.g. data.map_overlap</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">pytorch_predict</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="n">result</span><span class="o">.</span><span class="n">to_zarr</span><span class="p">(</span><span class="s1">'output.zarr'</span><span class="p">)</span>
</pre></div>
</div>
<p>There are some backends which may depend on other backends, for example xarray
depending on <cite>numpy.fft</cite>, and transforming a time axis into a frequency axis,
or Dask/xarray holding an array other than a NumPy array inside it. This would
be handled in the following manner inside code:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">ua</span><span class="o">.</span><span class="n">set_backend</span><span class="p">(</span><span class="n">cupy</span><span class="p">),</span> <span class="n">ua</span><span class="o">.</span><span class="n">set_backend</span><span class="p">(</span><span class="n">dask</span><span class="o">.</span><span class="n">array</span><span class="p">):</span>
<span class="c1"># Code that has distributed GPU arrays here</span>
</pre></div>
</div>
</section>
<section id="backward-compatibility">
<h2>Backward compatibility<a class="headerlink" href="#backward-compatibility" title="Link to this heading">#</a></h2>
<p>There are no backward incompatible changes proposed in this NEP.</p>
</section>
<section id="detailed-description">
<h2>Detailed description<a class="headerlink" href="#detailed-description" title="Link to this heading">#</a></h2>
<section id="proposals">
<h3>Proposals<a class="headerlink" href="#proposals" title="Link to this heading">#</a></h3>
<p>The only change this NEP proposes at its acceptance, is to make <code class="docutils literal notranslate"><span class="pre">unumpy</span></code> the
officially recommended way to override NumPy, along with making some submodules
overridable by default via <code class="docutils literal notranslate"><span class="pre">uarray</span></code>. <code class="docutils literal notranslate"><span class="pre">unumpy</span></code> will remain a separate
repository/package (which we propose to vendor to avoid a hard dependency, and
use the separate <code class="docutils literal notranslate"><span class="pre">unumpy</span></code> package only if it is installed, rather than depend
on for the time being). In concrete terms, <code class="docutils literal notranslate"><span class="pre">numpy.overridable</span></code> becomes an
alias for <code class="docutils literal notranslate"><span class="pre">unumpy</span></code>, if available with a fallback to the a vendored version if
not. <code class="docutils literal notranslate"><span class="pre">uarray</span></code> and <code class="docutils literal notranslate"><span class="pre">unumpy</span></code> and will be developed primarily with the input
of duck-array authors and secondarily, custom dtype authors, via the usual
GitHub workflow. There are a few reasons for this:</p>
<ul class="simple">
<li><p>Faster iteration in the case of bugs or issues.</p></li>
<li><p>Faster design changes, in the case of needed functionality.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">unumpy</span></code> will work with older versions of NumPy as well.</p></li>
<li><p>The user and library author opt-in to the override process,
rather than breakages happening when it is least expected.
In simple terms, bugs in <code class="docutils literal notranslate"><span class="pre">unumpy</span></code> mean that <code class="docutils literal notranslate"><span class="pre">numpy</span></code> remains
unaffected.</p></li>
<li><p>For <code class="docutils literal notranslate"><span class="pre">numpy.fft</span></code>, <code class="docutils literal notranslate"><span class="pre">numpy.linalg</span></code> and <code class="docutils literal notranslate"><span class="pre">numpy.random</span></code>, the functions in
the main namespace will mirror those in the <code class="docutils literal notranslate"><span class="pre">numpy.overridable</span></code> namespace.
The reason for this is that there may exist functions in the in these
submodules that need backends, even for <code class="docutils literal notranslate"><span class="pre">numpy.ndarray</span></code> inputs.</p></li>
</ul>
<section id="advantages-of-unumpy-over-other-solutions">
<h4>Advantages of <code class="docutils literal notranslate"><span class="pre">unumpy</span></code> over other solutions<a class="headerlink" href="#advantages-of-unumpy-over-other-solutions" title="Link to this heading">#</a></h4>
<p><code class="docutils literal notranslate"><span class="pre">unumpy</span></code> offers a number of advantages over the approach of defining a new
protocol for every problem encountered: Whenever there is something requiring
an override, <code class="docutils literal notranslate"><span class="pre">unumpy</span></code> will be able to offer a unified API with very minor
changes. For example:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">ufunc</span></code> objects can be overridden via their <code class="docutils literal notranslate"><span class="pre">__call__</span></code>, <code class="docutils literal notranslate"><span class="pre">reduce</span></code> and
other methods.</p></li>
<li><p>Other functions can be overridden in a similar fashion.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">np.asduckarray</span></code> goes away, and becomes <code class="docutils literal notranslate"><span class="pre">np.overridable.asarray</span></code> with a
backend set.</p></li>
<li><p>The same holds for array creation functions such as <code class="docutils literal notranslate"><span class="pre">np.zeros</span></code>,
<code class="docutils literal notranslate"><span class="pre">np.empty</span></code> and so on.</p></li>
</ul>
<p>This also holds for the future: Making something overridable would require only
minor changes to <code class="docutils literal notranslate"><span class="pre">unumpy</span></code>.</p>
<p>Another promise <code class="docutils literal notranslate"><span class="pre">unumpy</span></code> holds is one of default implementations. Default
implementations can be provided for any multimethod, in terms of others. This
allows one to override a large part of the NumPy API by defining only a small
part of it. This is to ease the creation of new duck-arrays, by providing
default implementations of many functions that can be easily expressed in
terms of others, as well as a repository of utility functions that help in the
implementation of duck-arrays that most duck-arrays would require. This would
allow us to avoid designing entire protocols, e.g., a protocol for stacking
and concatenating would be replaced by simply implementing <code class="docutils literal notranslate"><span class="pre">stack</span></code> and/or
<code class="docutils literal notranslate"><span class="pre">concatenate</span></code> and then providing default implementations for everything else
in that class. The same applies for transposing, and many other functions for
which protocols haven’t been proposed, such as <code class="docutils literal notranslate"><span class="pre">isin</span></code> in terms of <code class="docutils literal notranslate"><span class="pre">in1d</span></code>,
<code class="docutils literal notranslate"><span class="pre">setdiff1d</span></code> in terms of <code class="docutils literal notranslate"><span class="pre">unique</span></code>, and so on.</p>
<p>It also allows one to override functions in a manner which
<code class="docutils literal notranslate"><span class="pre">__array_function__</span></code> simply cannot, such as overriding <code class="docutils literal notranslate"><span class="pre">np.einsum</span></code> with the
version from the <code class="docutils literal notranslate"><span class="pre">opt_einsum</span></code> package, or Intel MKL overriding FFT, BLAS
or <code class="docutils literal notranslate"><span class="pre">ufunc</span></code> objects. They would define a backend with the appropriate
multimethods, and the user would select them via a <code class="docutils literal notranslate"><span class="pre">with</span></code> statement, or
registering them as a backend.</p>
<p>The last benefit is a clear way to coerce to a given backend (via the
<code class="docutils literal notranslate"><span class="pre">coerce</span></code> keyword in <code class="docutils literal notranslate"><span class="pre">ua.set_backend</span></code>), and a protocol
for coercing not only arrays, but also <code class="docutils literal notranslate"><span class="pre">dtype</span></code> objects and <code class="docutils literal notranslate"><span class="pre">ufunc</span></code> objects
with similar ones from other libraries. This is due to the existence of actual,
third party dtype packages, and their desire to blend into the NumPy ecosystem
(see <a class="footnote-reference brackets" href="#id25" id="id10" role="doc-noteref"><span class="fn-bracket">[</span>6<span class="fn-bracket">]</span></a>). This is a separate issue compared to the C-level dtype redesign
proposed in <a class="footnote-reference brackets" href="#id26" id="id11" role="doc-noteref"><span class="fn-bracket">[</span>7<span class="fn-bracket">]</span></a>, it’s about allowing third-party dtype implementations to
work with NumPy, much like third-party array implementations. These can provide
features such as, for example, units, jagged arrays or other such features that
are outside the scope of NumPy.</p>
</section>
<section id="mixing-numpy-and-unumpy-in-the-same-file">
<h4>Mixing NumPy and <code class="docutils literal notranslate"><span class="pre">unumpy</span></code> in the same file<a class="headerlink" href="#mixing-numpy-and-unumpy-in-the-same-file" title="Link to this heading">#</a></h4>
<p>Normally, one would only want to import only one of <code class="docutils literal notranslate"><span class="pre">unumpy</span></code> or <code class="docutils literal notranslate"><span class="pre">numpy</span></code>,
you would import it as <code class="docutils literal notranslate"><span class="pre">np</span></code> for familiarity. However, there may be situations
where one wishes to mix NumPy and the overrides, and there are a few ways to do
this, depending on the user’s style:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">numpy</span> <span class="kn">import</span> <span class="n">overridable</span> <span class="k">as</span> <span class="n">unp</span>
<span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
</pre></div>
</div>
<p>or:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="c1"># Use unumpy via np.overridable</span>
</pre></div>
</div>
</section>
</section>
<section id="duck-array-coercion">
<h3>Duck-array coercion<a class="headerlink" href="#duck-array-coercion" title="Link to this heading">#</a></h3>
<p>There are inherent problems about returning objects that are not NumPy arrays
from <code class="docutils literal notranslate"><span class="pre">numpy.array</span></code> or <code class="docutils literal notranslate"><span class="pre">numpy.asarray</span></code>, particularly in the context of C/C++
or Cython code that may get an object with a different memory layout than the
one it expects. However, we believe this problem may apply not only to these
two functions but all functions that return NumPy arrays. For this reason,
overrides are opt-in for the user, by using the submodule <code class="docutils literal notranslate"><span class="pre">numpy.overridable</span></code>
rather than <code class="docutils literal notranslate"><span class="pre">numpy</span></code>. NumPy will continue to work unaffected by anything in
<code class="docutils literal notranslate"><span class="pre">numpy.overridable</span></code>.</p>
<p>If the user wishes to obtain a NumPy array, there are two ways of doing it:</p>
<ol class="arabic simple">
<li><p>Use <code class="docutils literal notranslate"><span class="pre">numpy.asarray</span></code> (the non-overridable version).</p></li>
<li><p>Use <code class="docutils literal notranslate"><span class="pre">numpy.overridable.asarray</span></code> with the NumPy backend set and coercion
enabled</p></li>
</ol>
</section>
<section id="aliases-outside-of-the-numpy-overridable-namespace">
<h3>Aliases outside of the <code class="docutils literal notranslate"><span class="pre">numpy.overridable</span></code> namespace<a class="headerlink" href="#aliases-outside-of-the-numpy-overridable-namespace" title="Link to this heading">#</a></h3>
<p>All functionality in <code class="docutils literal notranslate"><span class="pre">numpy.random</span></code>, <code class="docutils literal notranslate"><span class="pre">numpy.linalg</span></code> and <code class="docutils literal notranslate"><span class="pre">numpy.fft</span></code>
will be aliased to their respective overridable versions inside
<code class="docutils literal notranslate"><span class="pre">numpy.overridable</span></code>. The reason for this is that there are alternative
implementations of RNGs (<code class="docutils literal notranslate"><span class="pre">mkl-random</span></code>), linear algebra routines (<code class="docutils literal notranslate"><span class="pre">eigen</span></code>,
<code class="docutils literal notranslate"><span class="pre">blis</span></code>) and FFT routines (<code class="docutils literal notranslate"><span class="pre">mkl-fft</span></code>, <code class="docutils literal notranslate"><span class="pre">pyFFTW</span></code>) that need to operate on
<code class="docutils literal notranslate"><span class="pre">numpy.ndarray</span></code> inputs, but still need the ability to switch behaviour.</p>
<p>This is different from monkeypatching in a few different ways:</p>
<ul class="simple">
<li><p>The caller-facing signature of the function is always the same,
so there is at least the loose sense of an API contract. Monkeypatching
does not provide this ability.</p></li>
<li><p>There is the ability of locally switching the backend.</p></li>
<li><p>It has been <a class="reference external" href="https://mail.python.org/archives/list/numpy-discussion@python.org/message/PS7EN3CRT6XERNTCN56MAYOXFFFEC55G/">suggested</a>
that the reason that 1.17 hasn’t landed in the Anaconda defaults channel is
due to the incompatibility between monkeypatching and <code class="docutils literal notranslate"><span class="pre">__array_function__</span></code>,
as monkeypatching would bypass the protocol completely.</p></li>
<li><p>Statements of the form <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">numpy</span> <span class="pre">import</span> <span class="pre">x;</span> <span class="pre">x</span></code> and <code class="docutils literal notranslate"><span class="pre">np.x</span></code> would have
different results depending on whether the import was made before or
after monkeypatching happened.</p></li>
</ul>
<p>All this isn’t possible at all with <code class="docutils literal notranslate"><span class="pre">__array_function__</span></code> or
<code class="docutils literal notranslate"><span class="pre">__array_ufunc__</span></code>.</p>
<p>It has been formally realized (at least in part) that a backend system is
needed for this, in the <a class="reference external" href="https://numpy.org/neps/roadmap.html#other-functionality">NumPy roadmap</a>.</p>
<p>For <code class="docutils literal notranslate"><span class="pre">numpy.random</span></code>, it’s still necessary to make the C-API fit the one
proposed in <a class="reference external" href="https://numpy.org/neps/nep-0019-rng-policy.html">NEP-19</a>.
This is impossible for <cite>mkl-random</cite>, because then it would need to be
rewritten to fit that framework. The guarantees on stream
compatibility will be the same as before, but if there’s a backend that affects
<code class="docutils literal notranslate"><span class="pre">numpy.random</span></code> set, we make no guarantees about stream compatibility, and it
is up to the backend author to provide their own guarantees.</p>
</section>
<section id="providing-a-way-for-implicit-dispatch">
<h3>Providing a way for implicit dispatch<a class="headerlink" href="#providing-a-way-for-implicit-dispatch" title="Link to this heading">#</a></h3>
<p>It has been suggested that the ability to dispatch methods which do not take
a dispatchable is needed, while guessing that backend from another dispatchable.</p>
<p>As a concrete example, consider the following:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">unumpy</span><span class="o">.</span><span class="n">determine_backend</span><span class="p">(</span><span class="n">array_like</span><span class="p">,</span> <span class="n">np</span><span class="o">.</span><span class="n">ndarray</span><span class="p">):</span>
<span class="n">unumpy</span><span class="o">.</span><span class="n">arange</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">array_like</span><span class="p">))</span>
</pre></div>
</div>
<p>While this does not exist yet in <code class="docutils literal notranslate"><span class="pre">uarray</span></code>, it is trivial to add it. The need for
this kind of code exists because one might want to have an alternative for the
proposed <code class="docutils literal notranslate"><span class="pre">*_like</span></code> functions, or the <code class="docutils literal notranslate"><span class="pre">like=</span></code> keyword argument. The need for these
exists because there are functions in the NumPy API that do not take a dispatchable
argument, but there is still the need to select a backend based on a different
dispatchable.</p>
</section>
<section id="the-need-for-an-opt-in-module">
<h3>The need for an opt-in module<a class="headerlink" href="#the-need-for-an-opt-in-module" title="Link to this heading">#</a></h3>
<p>The need for an opt-in module is realized because of a few reasons:</p>
<ul class="simple">
<li><p>There are parts of the API (like <cite>numpy.asarray</cite>) that simply cannot be
overridden due to incompatibility concerns with C/Cython extensions, however,
one may want to coerce to a duck-array using <code class="docutils literal notranslate"><span class="pre">asarray</span></code> with a backend set.</p></li>
<li><p>There are possible issues around an implicit option and monkeypatching, such
as those mentioned above.</p></li>
</ul>
<p>NEP 18 notes that this may require maintenance of two separate APIs. However,
this burden may be lessened by, for example, parameterizing all tests over
<code class="docutils literal notranslate"><span class="pre">numpy.overridable</span></code> separately via a fixture. This also has the side-effect
of thoroughly testing it, unlike <code class="docutils literal notranslate"><span class="pre">__array_function__</span></code>. We also feel that it
provides an opportunity to separate the NumPy API contract properly from the
implementation.</p>
</section>
<section id="benefits-to-end-users-and-mixing-backends">
<h3>Benefits to end-users and mixing backends<a class="headerlink" href="#benefits-to-end-users-and-mixing-backends" title="Link to this heading">#</a></h3>
<p>Mixing backends is easy in <code class="docutils literal notranslate"><span class="pre">uarray</span></code>, one only has to do:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># Explicitly say which backends you want to mix</span>
<span class="n">ua</span><span class="o">.</span><span class="n">register_backend</span><span class="p">(</span><span class="n">backend1</span><span class="p">)</span>
<span class="n">ua</span><span class="o">.</span><span class="n">register_backend</span><span class="p">(</span><span class="n">backend2</span><span class="p">)</span>
<span class="n">ua</span><span class="o">.</span><span class="n">register_backend</span><span class="p">(</span><span class="n">backend3</span><span class="p">)</span>
<span class="c1"># Freely use code that mixes backends here.</span>
</pre></div>
</div>
<p>The benefits to end-users extend beyond just writing new code. Old code
(usually in the form of scripts) can be easily ported to different backends
by a simple import switch and a line adding the preferred backend. This way,
users may find it easier to port existing code to GPU or distributed computing.</p>
</section>
</section>
<section id="related-work">
<h2>Related work<a class="headerlink" href="#related-work" title="Link to this heading">#</a></h2>
<section id="other-override-mechanisms">
<h3>Other override mechanisms<a class="headerlink" href="#other-override-mechanisms" title="Link to this heading">#</a></h3>
<ul class="simple">
<li><p>NEP-18, the <code class="docutils literal notranslate"><span class="pre">__array_function__</span></code> protocol. <a class="footnote-reference brackets" href="#id21" id="id12" role="doc-noteref"><span class="fn-bracket">[</span>2<span class="fn-bracket">]</span></a></p></li>
<li><p>NEP-13, the <code class="docutils literal notranslate"><span class="pre">__array_ufunc__</span></code> protocol. <a class="footnote-reference brackets" href="#id22" id="id13" role="doc-noteref"><span class="fn-bracket">[</span>3<span class="fn-bracket">]</span></a></p></li>
<li><p>NEP-30, the <code class="docutils literal notranslate"><span class="pre">__duck_array__</span></code> protocol. <a class="footnote-reference brackets" href="#id28" id="id14" role="doc-noteref"><span class="fn-bracket">[</span>9<span class="fn-bracket">]</span></a></p></li>
</ul>
</section>
<section id="existing-numpy-like-array-implementations">
<h3>Existing NumPy-like array implementations<a class="headerlink" href="#existing-numpy-like-array-implementations" title="Link to this heading">#</a></h3>
<ul class="simple">
<li><p>Dask: <a class="reference external" href="https://dask.org/">https://dask.org/</a></p></li>
<li><p>CuPy: <a class="reference external" href="https://cupy.chainer.org/">https://cupy.chainer.org/</a></p></li>
<li><p>PyData/Sparse: <a class="reference external" href="https://sparse.pydata.org/">https://sparse.pydata.org/</a></p></li>
<li><p>Xnd: <a class="reference external" href="https://xnd.readthedocs.io/">https://xnd.readthedocs.io/</a></p></li>
<li><p>Astropy’s Quantity: <a class="reference external" href="https://docs.astropy.org/en/stable/units/">https://docs.astropy.org/en/stable/units/</a></p></li>
</ul>
</section>
<section id="existing-and-potential-consumers-of-alternative-arrays">
<h3>Existing and potential consumers of alternative arrays<a class="headerlink" href="#existing-and-potential-consumers-of-alternative-arrays" title="Link to this heading">#</a></h3>
<ul class="simple">
<li><p>Dask: <a class="reference external" href="https://dask.org/">https://dask.org/</a></p></li>
<li><p>scikit-learn: <a class="reference external" href="https://scikit-learn.org/">https://scikit-learn.org/</a></p></li>
<li><p>xarray: <a class="reference external" href="https://xarray.pydata.org/">https://xarray.pydata.org/</a></p></li>
<li><p>TensorLy: <a class="reference external" href="http://tensorly.org/">http://tensorly.org/</a></p></li>
</ul>
</section>
<section id="existing-alternate-dtype-implementations">
<h3>Existing alternate dtype implementations<a class="headerlink" href="#existing-alternate-dtype-implementations" title="Link to this heading">#</a></h3>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">ndtypes</span></code>: <a class="reference external" href="https://ndtypes.readthedocs.io/en/latest/">https://ndtypes.readthedocs.io/en/latest/</a></p></li>
<li><p>Datashape: <a class="reference external" href="https://datashape.readthedocs.io">https://datashape.readthedocs.io</a></p></li>
<li><p>Plum: <a class="reference external" href="https://plum-py.readthedocs.io/">https://plum-py.readthedocs.io/</a></p></li>
</ul>
</section>
<section id="alternate-implementations-of-parts-of-the-numpy-api">
<h3>Alternate implementations of parts of the NumPy API<a class="headerlink" href="#alternate-implementations-of-parts-of-the-numpy-api" title="Link to this heading">#</a></h3>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">mkl_random</span></code>: <a class="github reference external" href="https://github.com/IntelPython/mkl_random">IntelPython/mkl_random</a></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">mkl_fft</span></code>: <a class="github reference external" href="https://github.com/IntelPython/mkl_fft">IntelPython/mkl_fft</a></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">bottleneck</span></code>: <a class="github reference external" href="https://github.com/pydata/bottleneck">pydata/bottleneck</a></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">opt_einsum</span></code>: <a class="github reference external" href="https://github.com/dgasmith/opt_einsum">dgasmith/opt_einsum</a></p></li>
</ul>
</section>
</section>
<section id="implementation">
<h2>Implementation<a class="headerlink" href="#implementation" title="Link to this heading">#</a></h2>
<p>The implementation of this NEP will require the following steps:</p>
<ul class="simple">
<li><p>Implementation of <code class="docutils literal notranslate"><span class="pre">uarray</span></code> multimethods corresponding to the
NumPy API, including classes for overriding <code class="docutils literal notranslate"><span class="pre">dtype</span></code>, <code class="docutils literal notranslate"><span class="pre">ufunc</span></code>
and <code class="docutils literal notranslate"><span class="pre">array</span></code> objects, in the <code class="docutils literal notranslate"><span class="pre">unumpy</span></code> repository, which are usually
very easy to create.</p></li>
<li><p>Moving backends from <code class="docutils literal notranslate"><span class="pre">unumpy</span></code> into the respective array libraries.</p></li>
</ul>
<p>Maintenance can be eased by testing over <code class="docutils literal notranslate"><span class="pre">{numpy,</span> <span class="pre">unumpy}</span></code> via parameterized
tests. If a new argument is added to a method, the corresponding argument
extractor and replacer will need to be updated within <code class="docutils literal notranslate"><span class="pre">unumpy</span></code>.</p>
<p>A lot of argument extractors can be re-used from the existing implementation
of the <code class="docutils literal notranslate"><span class="pre">__array_function__</span></code> protocol, and the replacers can be usually
re-used across many methods.</p>
<p>For the parts of the namespace which are going to be overridable by default,
the main method will need to be renamed and hidden behind a <code class="docutils literal notranslate"><span class="pre">uarray</span></code> multimethod.</p>
<p>Default implementations are usually seen in the documentation using the words
“equivalent to”, and thus, are easily available.</p>
<section id="uarray-primer">
<h3><code class="docutils literal notranslate"><span class="pre">uarray</span></code> Primer<a class="headerlink" href="#uarray-primer" title="Link to this heading">#</a></h3>
<p><strong>Note:</strong> <em>This section will not attempt to go into too much detail about
uarray, that is the purpose of the uarray documentation.</em> <a class="footnote-reference brackets" href="#id20" id="id15" role="doc-noteref"><span class="fn-bracket">[</span>1<span class="fn-bracket">]</span></a>
<em>However, the NumPy community will have input into the design of
uarray, via the issue tracker.</em></p>
<p><code class="docutils literal notranslate"><span class="pre">unumpy</span></code> is the interface that defines a set of overridable functions
(multimethods) compatible with the numpy API. To do this, it uses the
<code class="docutils literal notranslate"><span class="pre">uarray</span></code> library. <code class="docutils literal notranslate"><span class="pre">uarray</span></code> is a general purpose tool for creating
multimethods that dispatch to one of multiple different possible backend
implementations. In this sense, it is similar to the <code class="docutils literal notranslate"><span class="pre">__array_function__</span></code>
protocol but with the key difference that the backend is explicitly installed
by the end-user and not coupled into the array type.</p>
<p>Decoupling the backend from the array type gives much more flexibility to
end-users and backend authors. For example, it is possible to:</p>
<ul class="simple">
<li><p>override functions not taking arrays as arguments</p></li>
<li><p>create backends out of source from the array type</p></li>
<li><p>install multiple backends for the same array type</p></li>
</ul>
<p>This decoupling also means that <code class="docutils literal notranslate"><span class="pre">uarray</span></code> is not constrained to dispatching
over array-like types. The backend is free to inspect the entire set of
function arguments to determine if it can implement the function e.g. <code class="docutils literal notranslate"><span class="pre">dtype</span></code>
parameter dispatching.</p>
<section id="defining-backends">
<h4>Defining backends<a class="headerlink" href="#defining-backends" title="Link to this heading">#</a></h4>
<p><code class="docutils literal notranslate"><span class="pre">uarray</span></code> consists of two main protocols: <code class="docutils literal notranslate"><span class="pre">__ua_convert__</span></code> and
<code class="docutils literal notranslate"><span class="pre">__ua_function__</span></code>, called in that order, along with <code class="docutils literal notranslate"><span class="pre">__ua_domain__</span></code>.
<code class="docutils literal notranslate"><span class="pre">__ua_convert__</span></code> is for conversion and coercion. It has the signature
<code class="docutils literal notranslate"><span class="pre">(dispatchables,</span> <span class="pre">coerce)</span></code>, where <code class="docutils literal notranslate"><span class="pre">dispatchables</span></code> is an iterable of
<code class="docutils literal notranslate"><span class="pre">ua.Dispatchable</span></code> objects and <code class="docutils literal notranslate"><span class="pre">coerce</span></code> is a boolean indicating whether or
not to force the conversion. <code class="docutils literal notranslate"><span class="pre">ua.Dispatchable</span></code> is a simple class consisting
of three simple values: <code class="docutils literal notranslate"><span class="pre">type</span></code>, <code class="docutils literal notranslate"><span class="pre">value</span></code>, and <code class="docutils literal notranslate"><span class="pre">coercible</span></code>.
<code class="docutils literal notranslate"><span class="pre">__ua_convert__</span></code> returns an iterable of the converted values, or
<code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code> in the case of failure.</p>
<p><code class="docutils literal notranslate"><span class="pre">__ua_function__</span></code> has the signature <code class="docutils literal notranslate"><span class="pre">(func,</span> <span class="pre">args,</span> <span class="pre">kwargs)</span></code> and defines
the actual implementation of the function. It receives the function and its
arguments. Returning <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code> will cause a move to the default
implementation of the function if one exists, and failing that, the next
backend.</p>
<p>Here is what will happen assuming a <code class="docutils literal notranslate"><span class="pre">uarray</span></code> multimethod is called:</p>
<ol class="arabic simple">
<li><p>We canonicalise the arguments so any arguments without a default
are placed in <code class="docutils literal notranslate"><span class="pre">*args</span></code> and those with one are placed in <code class="docutils literal notranslate"><span class="pre">**kwargs</span></code>.</p></li>
<li><p>We check the list of backends.</p>
<ol class="loweralpha simple">
<li><p>If it is empty, we try the default implementation.</p></li>
</ol>
</li>
<li><p>We check if the backend’s <code class="docutils literal notranslate"><span class="pre">__ua_convert__</span></code> method exists. If it exists:</p>
<ol class="loweralpha simple">
<li><p>We pass it the output of the dispatcher,
which is an iterable of <code class="docutils literal notranslate"><span class="pre">ua.Dispatchable</span></code> objects.</p></li>
<li><p>We feed this output, along with the arguments,
to the argument replacer. <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code> means we move to 3
with the next backend.</p></li>
<li><p>We store the replaced arguments as the new arguments.</p></li>
</ol>
</li>
<li><p>We feed the arguments into <code class="docutils literal notranslate"><span class="pre">__ua_function__</span></code>, and return the output, and
exit if it isn’t <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>.</p></li>
<li><p>If the default implementation exists, we try it with the current backend.</p></li>
<li><p>On failure, we move to 3 with the next backend. If there are no more
backends, we move to 7.</p></li>
<li><p>We raise a <code class="docutils literal notranslate"><span class="pre">ua.BackendNotImplementedError</span></code>.</p></li>
</ol>
</section>
<section id="defining-overridable-multimethods">
<h4>Defining overridable multimethods<a class="headerlink" href="#defining-overridable-multimethods" title="Link to this heading">#</a></h4>
<p>To define an overridable function (a multimethod), one needs a few things:</p>
<ol class="arabic simple">
<li><p>A dispatcher that returns an iterable of <code class="docutils literal notranslate"><span class="pre">ua.Dispatchable</span></code> objects.</p></li>
<li><p>A reverse dispatcher that replaces dispatchable values with the supplied
ones.</p></li>
<li><p>A domain.</p></li>
<li><p>Optionally, a default implementation, which can be provided in terms of
other multimethods.</p></li>
</ol>
<p>As an example, consider the following:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">uarray</span> <span class="k">as</span> <span class="nn">ua</span>
<span class="k">def</span> <span class="nf">full_argreplacer</span><span class="p">(</span><span class="n">args</span><span class="p">,</span> <span class="n">kwargs</span><span class="p">,</span> <span class="n">dispatchables</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">full</span><span class="p">(</span><span class="n">shape</span><span class="p">,</span> <span class="n">fill_value</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">order</span><span class="o">=</span><span class="s1">'C'</span><span class="p">):</span>
<span class="k">return</span> <span class="p">(</span><span class="n">shape</span><span class="p">,</span> <span class="n">fill_value</span><span class="p">),</span> <span class="nb">dict</span><span class="p">(</span>
<span class="n">dtype</span><span class="o">=</span><span class="n">dispatchables</span><span class="p">[</span><span class="mi">0</span><span class="p">],</span>
<span class="n">order</span><span class="o">=</span><span class="n">order</span>
<span class="p">)</span>
<span class="k">return</span> <span class="n">full</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
<span class="nd">@ua</span><span class="o">.</span><span class="n">create_multimethod</span><span class="p">(</span><span class="n">full_argreplacer</span><span class="p">,</span> <span class="n">domain</span><span class="o">=</span><span class="s2">"numpy"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">full</span><span class="p">(</span><span class="n">shape</span><span class="p">,</span> <span class="n">fill_value</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">order</span><span class="o">=</span><span class="s1">'C'</span><span class="p">):</span>
<span class="k">return</span> <span class="p">(</span><span class="n">ua</span><span class="o">.</span><span class="n">Dispatchable</span><span class="p">(</span><span class="n">dtype</span><span class="p">,</span> <span class="n">np</span><span class="o">.</span><span class="n">dtype</span><span class="p">),)</span>
</pre></div>
</div>