<p dir="auto">It seems that the scikit-learn test job that runs against numpy master found a breaking change in numpy:</p>
<p dir="auto"><a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="374460441" data-permission-text="Title is private" data-url="https://github.com/scikit-learn/scikit-learn/issues/12467" data-hovercard-type="pull_request" data-hovercard-url="/scikit-learn/scikit-learn/pull/12467/hovercard" href="https://github.com/scikit-learn/scikit-learn/pull/12467">scikit-learn/scikit-learn#12467</a></p>
<p dir="auto">Minimal reproduction case:</p>
<div class="highlight highlight-source-python notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="&gt;&gt;&gt; import numpy as np
&gt;&gt;&gt; np.__version__
'1.16.0.dev0+8aa1214'
&gt;&gt;&gt; np.vstack([0] for _ in range(3))
Traceback (most recent call last):
  File &quot;&lt;ipython-input-5-842ad8069d40&gt;&quot;, line 1, in &lt;module&gt;
    np.vstack([0] for _ in range(3))
  File &quot;/opt/venvs/py37/lib/python3.7/site-packages/numpy/core/overrides.py&quot;, line 151, in public_api
    implementation, public_api, relevant_args, args, kwargs)
  File &quot;/opt/venvs/py37/lib/python3.7/site-packages/numpy/core/overrides.py&quot;, line 96, in array_function_implementation_or_override
    return implementation(*args, **kwargs)
  File &quot;/opt/venvs/py37/lib/python3.7/site-packages/numpy/core/shape_base.py&quot;, line 260, in vstack
    return _nx.concatenate([atleast_2d(_m) for _m in tup], 0)
  File &quot;/opt/venvs/py37/lib/python3.7/site-packages/numpy/core/overrides.py&quot;, line 151, in public_api
    implementation, public_api, relevant_args, args, kwargs)
  File &quot;/opt/venvs/py37/lib/python3.7/site-packages/numpy/core/overrides.py&quot;, line 96, in array_function_implementation_or_override
    return implementation(*args, **kwargs)
  File &quot;/opt/venvs/py37/lib/python3.7/site-packages/numpy/core/multiarray.py&quot;, line 193, in concatenate
    return _multiarray_umath.concatenate(arrays, axis, out)
ValueError: need at least one array to concatenate"><pre class="notranslate"><span class="pl-c1">&gt;&gt;</span><span class="pl-c1">&gt;</span> <span class="pl-k">import</span> <span class="pl-s1">numpy</span> <span class="pl-k">as</span> <span class="pl-s1">np</span>
<span class="pl-c1">&gt;&gt;</span><span class="pl-c1">&gt;</span> <span class="pl-s1">np</span>.<span class="pl-s1">__version__</span>
<span class="pl-s">'1.16.0.dev0+8aa1214'</span>
<span class="pl-c1">&gt;&gt;</span><span class="pl-c1">&gt;</span> <span class="pl-s1">np</span>.<span class="pl-en">vstack</span>([<span class="pl-c1">0</span>] <span class="pl-k">for</span> <span class="pl-s1">_</span> <span class="pl-c1">in</span> <span class="pl-en">range</span>(<span class="pl-c1">3</span>))
<span class="pl-v">Traceback</span> (<span class="pl-s1">most</span> <span class="pl-s1">recent</span> <span class="pl-s1">call</span> <span class="pl-s1">last</span>):
  <span class="pl-v">File</span> <span class="pl-s">"&lt;ipython-input-5-842ad8069d40&gt;"</span>, <span class="pl-s1">line</span> <span class="pl-c1">1</span>, <span class="pl-s1">in</span> <span class="pl-c1">&lt;</span><span class="pl-s1">module</span><span class="pl-c1">&gt;</span>
    <span class="pl-s1">np</span>.<span class="pl-en">vstack</span>([<span class="pl-c1">0</span>] <span class="pl-k">for</span> <span class="pl-s1">_</span> <span class="pl-c1">in</span> <span class="pl-en">range</span>(<span class="pl-c1">3</span>))
  <span class="pl-v">File</span> <span class="pl-s">"/opt/venvs/py37/lib/python3.7/site-packages/numpy/core/overrides.py"</span>, <span class="pl-s1">line</span> <span class="pl-c1">151</span>, <span class="pl-s1">in</span> <span class="pl-s1">public_api</span>
    <span class="pl-s1">implementation</span>, <span class="pl-s1">public_api</span>, <span class="pl-s1">relevant_args</span>, <span class="pl-s1">args</span>, <span class="pl-s1">kwargs</span>)
  <span class="pl-v">File</span> <span class="pl-s">"/opt/venvs/py37/lib/python3.7/site-packages/numpy/core/overrides.py"</span>, <span class="pl-s1">line</span> <span class="pl-c1">96</span>, <span class="pl-s1">in</span> <span class="pl-s1">array_function_implementation_or_override</span>
    <span class="pl-k">return</span> <span class="pl-en">implementation</span>(<span class="pl-c1">*</span><span class="pl-s1">args</span>, <span class="pl-c1">**</span><span class="pl-s1">kwargs</span>)
  <span class="pl-v">File</span> <span class="pl-s">"/opt/venvs/py37/lib/python3.7/site-packages/numpy/core/shape_base.py"</span>, <span class="pl-s1">line</span> <span class="pl-c1">260</span>, <span class="pl-s1">in</span> <span class="pl-s1">vstack</span>
    <span class="pl-k">return</span> <span class="pl-s1">_nx</span>.<span class="pl-en">concatenate</span>([<span class="pl-en">atleast_2d</span>(<span class="pl-s1">_m</span>) <span class="pl-k">for</span> <span class="pl-s1">_m</span> <span class="pl-c1">in</span> <span class="pl-s1">tup</span>], <span class="pl-c1">0</span>)
  <span class="pl-v">File</span> <span class="pl-s">"/opt/venvs/py37/lib/python3.7/site-packages/numpy/core/overrides.py"</span>, <span class="pl-s1">line</span> <span class="pl-c1">151</span>, <span class="pl-s1">in</span> <span class="pl-s1">public_api</span>
    <span class="pl-s1">implementation</span>, <span class="pl-s1">public_api</span>, <span class="pl-s1">relevant_args</span>, <span class="pl-s1">args</span>, <span class="pl-s1">kwargs</span>)
  <span class="pl-v">File</span> <span class="pl-s">"/opt/venvs/py37/lib/python3.7/site-packages/numpy/core/overrides.py"</span>, <span class="pl-s1">line</span> <span class="pl-c1">96</span>, <span class="pl-s1">in</span> <span class="pl-s1">array_function_implementation_or_override</span>
    <span class="pl-k">return</span> <span class="pl-en">implementation</span>(<span class="pl-c1">*</span><span class="pl-s1">args</span>, <span class="pl-c1">**</span><span class="pl-s1">kwargs</span>)
  <span class="pl-v">File</span> <span class="pl-s">"/opt/venvs/py37/lib/python3.7/site-packages/numpy/core/multiarray.py"</span>, <span class="pl-s1">line</span> <span class="pl-c1">193</span>, <span class="pl-s1">in</span> <span class="pl-s1">concatenate</span>
    <span class="pl-k">return</span> <span class="pl-s1">_multiarray_umath</span>.<span class="pl-en">concatenate</span>(<span class="pl-s1">arrays</span>, <span class="pl-s1">axis</span>, <span class="pl-s1">out</span>)
<span class="pl-v">ValueError</span>: <span class="pl-s1">need</span> <span class="pl-s1">at</span> <span class="pl-s1">least</span> <span class="pl-s1">one</span> <span class="pl-s1">array</span> <span class="pl-s1">to</span> <span class="pl-s1">concatenate</span></pre></div>
<p dir="auto">This used to work in past versions of numpy:</p>
<div class="highlight highlight-source-python notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="&gt;&gt;&gt; import numpy as np
&gt;&gt;&gt; np.__version__
'1.14.5'
&gt;&gt;&gt; np.vstack([0] for _ in range(3))
array([[0],
       [0],
       [0]])"><pre class="notranslate"><span class="pl-c1">&gt;&gt;</span><span class="pl-c1">&gt;</span> <span class="pl-k">import</span> <span class="pl-s1">numpy</span> <span class="pl-k">as</span> <span class="pl-s1">np</span>
<span class="pl-c1">&gt;&gt;</span><span class="pl-c1">&gt;</span> <span class="pl-s1">np</span>.<span class="pl-s1">__version__</span>
<span class="pl-s">'1.14.5'</span>
<span class="pl-c1">&gt;&gt;</span><span class="pl-c1">&gt;</span> <span class="pl-s1">np</span>.<span class="pl-en">vstack</span>([<span class="pl-c1">0</span>] <span class="pl-k">for</span> <span class="pl-s1">_</span> <span class="pl-c1">in</span> <span class="pl-en">range</span>(<span class="pl-c1">3</span>))
<span class="pl-en">array</span>([[<span class="pl-c1">0</span>],
       [<span class="pl-c1">0</span>],
       [<span class="pl-c1">0</span>]])</pre></div>
<p dir="auto">As reported by <a class="user-mention notranslate" data-hovercard-type="user" data-hovercard-url="/users/ilayn/hovercard" data-octo-click="hovercard-link-click" data-octo-dimensions="link_type:self" href="https://github.com/ilayn">@ilayn</a> over in <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="374172452" data-permission-text="Title is private" data-url="https://github.com/numpy/numpy/issues/12262" data-hovercard-type="issue" data-hovercard-url="/numpy/numpy/issues/12262/hovercard" href="https://github.com/numpy/numpy/issues/12262">#12262</a>:</p>
<blockquote>
<p dir="auto">There are other backwards incompatible changes for hstack and column_stack which were explicitly accepting generator expressions but now they don't.</p>
<p dir="auto">Xref : <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="374158906" data-permission-text="Title is private" data-url="https://github.com/scipy/scipy/issues/9405" data-hovercard-type="pull_request" data-hovercard-url="/scipy/scipy/pull/9405/hovercard" href="https://github.com/scipy/scipy/pull/9405">scipy/scipy#9405</a></p>
</blockquote>