title: How to list dependencies for a python library without installing?
body: <p>Is there a way to get a list of dependencies for a given python package without installing it first?</p>
  <p>I can currently get a list of requirements, but it requires installing the packages.  For example, I can use pip to show basic requirements info, but it doesn't include version information:</p>
  <pre><code>$ pip show pytest
  Name: pytest
  Version: 3.0.6
  ...
  Requires: colorama, setuptools, py
  </code></pre>
  <p>I've tried a library called <a href="https://pypi.python.org/pypi/pipdeptree/0.7.0" rel="noreferrer"><code>pipdeptree</code></a> that includes much better output on requirements, but it also requires installation of the packages</p>
  <pre><code>$ pipdeptree -p pytest
  pytest==3.0.6
  - colorama [required: Any, installed: 0.3.7]
  - py [required: &gt;=1.4.29, installed: 1.4.32]
  - setuptools [required: Any, installed: 34.0.0]
    - appdirs [required: &gt;=1.4.0, installed: 1.4.0]
  ...
  </code></pre>
  <p>Ideally, I would get the level of detail that <code>pipdeptree</code> provides.  Also, being able to produce a <code>requirements.txt</code> file from a python <code>wheel</code> or from pypi with <code>pip</code> would suffice as well.</p>
  <p>I'm interested in the dependency <em>constraints</em> for a given package, not the final downloaded packages after resolving the dependency requirements.  For example, I don't really care that pip downloaded <code>package-2.3.4</code>, I would rather know that <code>package&gt;=2.1</code> was a requirement.</p>