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<section id="nep-34-disallow-inferring-dtype-object-from-sequences">
<span id="nep34"></span><h1>NEP 34 — Disallow inferring <code class="docutils literal notranslate"><span class="pre">dtype=object</span></code> from sequences<a class="headerlink" href="#nep-34-disallow-inferring-dtype-object-from-sequences" 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>Matti Picus</p>
</dd>
<dt class="field-even">Status<span class="colon">:</span></dt>
<dd class="field-even"><p>Final</p>
</dd>
<dt class="field-odd">Type<span class="colon">:</span></dt>
<dd class="field-odd"><p>Standards Track</p>
</dd>
<dt class="field-even">Created<span class="colon">:</span></dt>
<dd class="field-even"><p>2019-10-10</p>
</dd>
<dt class="field-odd">Resolution<span class="colon">:</span></dt>
<dd class="field-odd"><p><a class="reference external" href="https://mail.python.org/pipermail/numpy-discussion/2019-October/080200.html">https://mail.python.org/pipermail/numpy-discussion/2019-October/080200.html</a></p>
</dd>
</dl>
<section id="abstract">
<h2>Abstract<a class="headerlink" href="#abstract" title="Link to this heading">#</a></h2>
<p>When users create arrays with sequences-of-sequences, they sometimes err in
matching the lengths of the nested <a class="reference external" href="https://docs.python.org/3.7/glossary.html#term-sequence">sequences</a>, commonly called “ragged
arrays”. Here we will refer to them as ragged nested sequences. Creating such
arrays via <code class="docutils literal notranslate"><span class="pre">np.array([<ragged_nested_sequence>])</span></code> with no <code class="docutils literal notranslate"><span class="pre">dtype</span></code> keyword
argument will today default to an <code class="docutils literal notranslate"><span class="pre">object</span></code>-dtype array. Change the behaviour to
raise a <code class="docutils literal notranslate"><span class="pre">ValueError</span></code> instead.</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>Users who specify lists-of-lists when creating a <cite>numpy.ndarray</cite> via
<code class="docutils literal notranslate"><span class="pre">np.array</span></code> may mistakenly pass in lists of different lengths. Currently we
accept this input and automatically create an array with <code class="docutils literal notranslate"><span class="pre">dtype=object</span></code>. This
can be confusing, since it is rarely what is desired. Changing the automatic
dtype detection to never return <code class="docutils literal notranslate"><span class="pre">object</span></code> for ragged nested sequences (defined as a
recursive sequence of sequences, where not all the sequences on the same
level have the same length) will force users who actually wish to create
<code class="docutils literal notranslate"><span class="pre">object</span></code> arrays to specify that explicitly. Note that <code class="docutils literal notranslate"><span class="pre">lists</span></code>, <code class="docutils literal notranslate"><span class="pre">tuples</span></code>,
and <code class="docutils literal notranslate"><span class="pre">nd.ndarrays</span></code> are all sequences <a class="footnote-reference brackets" href="#id2" id="id1" role="doc-noteref"><span class="fn-bracket">[</span>0<span class="fn-bracket">]</span></a>. See for instance <a class="reference external" href="https://github.com/numpy/numpy/issues/5303">issue 5303</a>.</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>After this change, array creation with ragged nested sequences must explicitly
define a dtype:</p>
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">np</span><span class="o">.</span><span class="n">array</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="p">[</span><span class="mi">1</span><span class="p">]])</span>
<span class="go">ValueError: cannot guess the desired dtype from the input</span>
</pre></div>
</div>
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">np</span><span class="o">.</span><span class="n">array</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="p">[</span><span class="mi">1</span><span class="p">]],</span> <span class="n">dtype</span><span class="o">=</span><span class="nb">object</span><span class="p">)</span>
<span class="go"># succeeds, with no change from current behaviour</span>
</pre></div>
</div>
<p>The deprecation will affect any call that internally calls <code class="docutils literal notranslate"><span class="pre">np.asarray</span></code>. For
instance, the <code class="docutils literal notranslate"><span class="pre">assert_equal</span></code> family of functions calls <code class="docutils literal notranslate"><span class="pre">np.asarray</span></code>, so
users will have to change code like:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">np</span><span class="o">.</span><span class="n">assert_equal</span><span class="p">(</span><span class="n">a</span><span class="p">,</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>
</pre></div>
</div>
<p>to:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">np</span><span class="o">.</span><span class="n">assert_equal</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">np</span><span class="o">.</span><span class="n">array</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="n">dtype</span><span class="o">=</span><span class="nb">object</span><span class="p">))</span>
</pre></div>
</div>
</section>
<section id="detailed-description">
<h2>Detailed description<a class="headerlink" href="#detailed-description" title="Link to this heading">#</a></h2>
<p>To explicitly set the shape of the object array, since it is sometimes hard to
determine what shape is desired, one could use:</p>
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">arr</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">empty</span><span class="p">(</span><span class="n">correct_shape</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="nb">object</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">arr</span><span class="p">[</span><span class="o">...</span><span class="p">]</span> <span class="o">=</span> <span class="n">values</span>
</pre></div>
</div>
<p>We will also reject mixed sequences of non-sequence and sequence, for instance
all of these will be rejected:</p>
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">arr</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="n">np</span><span class="o">.</span><span class="n">arange</span><span class="p">(</span><span class="mi">10</span><span class="p">),</span> <span class="p">[</span><span class="mi">10</span><span class="p">]])</span>
<span class="gp">>>> </span><span class="n">arr</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([[</span><span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">),</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">),</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">)],</span> <span class="p">[</span><span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">),</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">]])</span>
</pre></div>
</div>
</section>
<section id="related-work">
<h2>Related work<a class="headerlink" href="#related-work" title="Link to this heading">#</a></h2>
<p><a class="reference external" href="https://github.com/numpy/numpy/pull/14341">PR 14341</a> tried to raise an error when ragged nested sequences were specified
with a numeric dtype <code class="docutils literal notranslate"><span class="pre">np.array,</span> <span class="pre">[[1],</span> <span class="pre">[2,</span> <span class="pre">3]],</span> <span class="pre">dtype=int)</span></code> but failed due to
false-positives, for instance <code class="docutils literal notranslate"><span class="pre">np.array([1,</span> <span class="pre">np.array([5])],</span> <span class="pre">dtype=int)</span></code>.</p>
</section>
<section id="implementation">
<h2>Implementation<a class="headerlink" href="#implementation" title="Link to this heading">#</a></h2>
<p>The code to be changed is inside <code class="docutils literal notranslate"><span class="pre">PyArray_GetArrayParamsFromObject</span></code> and the
internal <code class="docutils literal notranslate"><span class="pre">discover_dimensions</span></code> function. The first implementation in <a class="reference external" href="https://github.com/numpy/numpy/pull/14794">PR
14794</a> caused a number of downstream library failures and was reverted before
the release of 1.18. Subsequently downstream libraries fixed the places they
were using ragged arrays. The reimplementation became <a class="reference external" href="https://github.com/numpy/numpy/pull/15119">PR 15119</a> which was
merged for the 1.19 release.</p>
</section>
<section id="backward-compatibility">
<h2>Backward compatibility<a class="headerlink" href="#backward-compatibility" title="Link to this heading">#</a></h2>
<p>Anyone depending on creating object arrays from ragged nested sequences will
need to modify their code. There will be a deprecation period during which the
current behaviour will emit a <code class="docutils literal notranslate"><span class="pre">DeprecationWarning</span></code>.</p>
</section>
<section id="alternatives">
<h2>Alternatives<a class="headerlink" href="#alternatives" title="Link to this heading">#</a></h2>
<ul class="simple">
<li><p>We could continue with the current situation.</p></li>
<li><p>It was also suggested to add a kwarg <code class="docutils literal notranslate"><span class="pre">depth</span></code> to array creation, or perhaps
to add another array creation API function <code class="docutils literal notranslate"><span class="pre">ragged_array_object</span></code>. The goal
was to eliminate the ambiguity in creating an object array from <code class="docutils literal notranslate"><span class="pre">array([[1,</span>
<span class="pre">2],</span> <span class="pre">[1]],</span> <span class="pre">dtype=object)</span></code>: should the returned array have a shape of
<code class="docutils literal notranslate"><span class="pre">(1,)</span></code>, or <code class="docutils literal notranslate"><span class="pre">(2,)</span></code>? This NEP does not deal with that issue, and only
deprecates the use of <code class="docutils literal notranslate"><span class="pre">array</span></code> with no <code class="docutils literal notranslate"><span class="pre">dtype=object</span></code> for ragged nested
sequences. Users of ragged nested sequences may face another deprecation
cycle in the future. Rationale: we expect that there are very few users who
intend to use ragged arrays like that, this was never intended as a use case
of NumPy arrays. Users are likely better off with <a class="reference external" href="https://github.com/scikit-hep/awkward-array">another library</a> or just
using list of lists.</p></li>
<li><p>It was also suggested to deprecate all automatic creation of <code class="docutils literal notranslate"><span class="pre">object</span></code>-dtype
arrays, which would require adding an explicit <code class="docutils literal notranslate"><span class="pre">dtype=object</span></code> for something
like <code class="docutils literal notranslate"><span class="pre">np.array([Decimal(10),</span> <span class="pre">Decimal(10)])</span></code>. This too is out of scope for
the current NEP. Rationale: it’s harder to asses the impact of this larger
change, we’re not sure how many users this may impact.</p></li>
</ul>
</section>
<section id="discussion">
<h2>Discussion<a class="headerlink" href="#discussion" title="Link to this heading">#</a></h2>
<p>Comments to <a class="reference external" href="https://github.com/numpy/numpy/issues/5303">issue 5303</a> indicate this is unintended behaviour as far back as
2014. Suggestions to change it have been made in the ensuing years, but none
have stuck. The WIP implementation in <a class="reference external" href="https://github.com/numpy/numpy/pull/14794">PR 14794</a> seems to point to the
viability of this approach.</p>
</section>
<section id="references-and-footnotes">
<h2>References and footnotes<a class="headerlink" href="#references-and-footnotes" title="Link to this heading">#</a></h2>
<aside class="footnote-list brackets">
<aside class="footnote brackets" id="id2" role="doc-footnote">
<span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id1">0</a><span class="fn-bracket">]</span></span>
<p><code class="docutils literal notranslate"><span class="pre">np.ndarrays</span></code> are not recursed into, rather their shape is used
directly. This will not emit warnings:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">ragged</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([[</span><span class="mi">1</span><span class="p">],</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="n">dtype</span><span class="o">=</span><span class="nb">object</span><span class="p">)</span>
<span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="n">ragged</span><span class="p">,</span> <span class="n">ragged</span><span class="p">])</span> <span class="c1"># no dtype needed</span>
</pre></div>
</div>
</aside>
</aside>
</section>
<section id="copyright">
<h2>Copyright<a class="headerlink" href="#copyright" title="Link to this heading">#</a></h2>
<p>This document has been placed in the public domain.</p>
</section>
</section>
</article>
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<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#motivation-and-scope">Motivation and scope</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#usage-and-impact">Usage and impact</a></li>
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<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#implementation">Implementation</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#backward-compatibility">Backward compatibility</a></li>
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