<p dir="auto">Some NumPy C functions do not have signatures available. I have not audited the full API but I know this is true at least for <code class="notranslate">arange</code>.</p>
<p dir="auto">It <a href="https://stackoverflow.com/a/25847066/907060" rel="nofollow">sounds like</a> you can add signatures for functions defined in C by adding a <code class="notranslate">__text_signature__</code> attribute. Or you can add a <code class="notranslate">__signature__</code> attribute to return a <a href="https://docs.python.org/3/library/inspect.html#inspect.Signature" rel="nofollow"><code class="notranslate">Signature</code> instance</a>.</p>
<h3 dir="auto">Reproducing code example:</h3>

<div class="highlight highlight-source-python notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="import numpy as np
import inspect

inspect.signature(np.arange)"><pre class="notranslate"><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-k">import</span> <span class="pl-s1">inspect</span>

<span class="pl-s1">inspect</span>.<span class="pl-en">signature</span>(<span class="pl-s1">np</span>.<span class="pl-s1">arange</span>)</pre></div>
<h3 dir="auto">Error message:</h3>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
&lt;ipython-input-3-5338db87f2d9&gt; in &lt;module&gt;
----&gt; 1 signature(np.arange)

/usr/local/Caskroom/miniconda/base/envs/python-record-api/lib/python3.8/inspect.py in signature(obj, follow_wrapped)
   3091 def signature(obj, *, follow_wrapped=True):
   3092     &quot;&quot;&quot;Get a signature object for the passed callable.&quot;&quot;&quot;
-&gt; 3093     return Signature.from_callable(obj, follow_wrapped=follow_wrapped)
   3094 
   3095 

/usr/local/Caskroom/miniconda/base/envs/python-record-api/lib/python3.8/inspect.py in from_callable(cls, obj, follow_wrapped)
   2840     def from_callable(cls, obj, *, follow_wrapped=True):
   2841         &quot;&quot;&quot;Constructs Signature for the given callable object.&quot;&quot;&quot;
-&gt; 2842         return _signature_from_callable(obj, sigcls=cls,
   2843                                         follow_wrapper_chains=follow_wrapped)
   2844 

/usr/local/Caskroom/miniconda/base/envs/python-record-api/lib/python3.8/inspect.py in _signature_from_callable(obj, follow_wrapper_chains, skip_bound_arg, sigcls)
   2294 
   2295     if _signature_is_builtin(obj):
-&gt; 2296         return _signature_from_builtin(sigcls, obj,
   2297                                        skip_bound_arg=skip_bound_arg)
   2298 

/usr/local/Caskroom/miniconda/base/envs/python-record-api/lib/python3.8/inspect.py in _signature_from_builtin(cls, func, skip_bound_arg)
   2105     s = getattr(func, &quot;__text_signature__&quot;, None)
   2106     if not s:
-&gt; 2107         raise ValueError(&quot;no signature found for builtin {!r}&quot;.format(func))
   2108 
   2109     return _signature_fromstr(cls, func, s, skip_bound_arg)

ValueError: no signature found for builtin &lt;built-in function arange&gt;"><pre class="notranslate"><code class="notranslate">---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
&lt;ipython-input-3-5338db87f2d9&gt; in &lt;module&gt;
----&gt; 1 signature(np.arange)

/usr/local/Caskroom/miniconda/base/envs/python-record-api/lib/python3.8/inspect.py in signature(obj, follow_wrapped)
   3091 def signature(obj, *, follow_wrapped=True):
   3092     """Get a signature object for the passed callable."""
-&gt; 3093     return Signature.from_callable(obj, follow_wrapped=follow_wrapped)
   3094 
   3095 

/usr/local/Caskroom/miniconda/base/envs/python-record-api/lib/python3.8/inspect.py in from_callable(cls, obj, follow_wrapped)
   2840     def from_callable(cls, obj, *, follow_wrapped=True):
   2841         """Constructs Signature for the given callable object."""
-&gt; 2842         return _signature_from_callable(obj, sigcls=cls,
   2843                                         follow_wrapper_chains=follow_wrapped)
   2844 

/usr/local/Caskroom/miniconda/base/envs/python-record-api/lib/python3.8/inspect.py in _signature_from_callable(obj, follow_wrapper_chains, skip_bound_arg, sigcls)
   2294 
   2295     if _signature_is_builtin(obj):
-&gt; 2296         return _signature_from_builtin(sigcls, obj,
   2297                                        skip_bound_arg=skip_bound_arg)
   2298 

/usr/local/Caskroom/miniconda/base/envs/python-record-api/lib/python3.8/inspect.py in _signature_from_builtin(cls, func, skip_bound_arg)
   2105     s = getattr(func, "__text_signature__", None)
   2106     if not s:
-&gt; 2107         raise ValueError("no signature found for builtin {!r}".format(func))
   2108 
   2109     return _signature_fromstr(cls, func, s, skip_bound_arg)

ValueError: no signature found for builtin &lt;built-in function arange&gt;
</code></pre></div>
<h3 dir="auto">Numpy/Python version information:</h3>
<div class="highlight highlight-source-python notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="&gt;&gt;&gt; import sys, numpy; print(numpy.__version__, sys.version)
1.18.4 3.8.2 | packaged by conda-forge | (default, Apr 16 2020, 17:20:38) 
[Clang 9.0.1 ]"><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">sys</span>, <span class="pl-s1">numpy</span>; <span class="pl-en">print</span>(<span class="pl-s1">numpy</span>.<span class="pl-s1">__version__</span>, <span class="pl-s1">sys</span>.<span class="pl-s1">version</span>)
<span class="pl-c1">1.18</span>.<span class="pl-c1">4</span> <span class="pl-c1">3.8</span>.<span class="pl-c1">2</span> <span class="pl-c1">|</span> <span class="pl-s1">packaged</span> <span class="pl-s1">by</span> <span class="pl-s1">conda</span><span class="pl-c1">-</span><span class="pl-s1">forge</span> <span class="pl-c1">|</span> (<span class="pl-s1">default</span>, <span class="pl-v">Apr</span> <span class="pl-c1">16</span> <span class="pl-c1">2020</span>, <span class="pl-c1">17</span>:<span class="pl-c1">20</span>:<span class="pl-c1">38</span>) 
[<span class="pl-v">Clang</span> <span class="pl-c1">9.0</span><span class="pl-c1">.1</span> ]</pre></div>
<p dir="auto">Which involves including <code class="notranslate">--\n\n</code> in the docstrings. See <a href="http://stackoverflow.com/a/41245451/102441" rel="nofollow">http://stackoverflow.com/a/41245451/102441</a></p>