title: Set up virtualenv using a requirements.txt generated by conda
body: <p>I'm setting up a python project, using an Anaconda virtual environment. I'm generating a requirements.txt so other people can easily set up their own virtual environment for the project.</p>
  
  <p>I was wondering though, when other developers want to contribute to the project, but want to use virtualenv instead of Anaconda, can they do that?</p>
  
  <p>I tried the following: </p>
  
  <ul>
  <li><p>I set up an empty project in an Anaconda environment and installed the aiohttp module. Then <code>conda list --export &gt; requirements.txt</code> generates the following:</p>
  
  <pre><code># This file may be used to create an environment using:
  # $ conda create --name &lt;env&gt; --file &lt;this file&gt;
  # platform: win-64
  aiohttp=2.3.9=py36_0
  async-timeout=2.0.0=py36hc3e01a3_0
  certifi=2018.1.18=py36_0
  chardet=3.0.4=py36h420ce6e_1
  multidict=3.3.2=py36h72bac45_0
  pip=9.0.1=py36h226ae91_4
  python=3.6.4=h6538335_1
  setuptools=38.4.0=py36_0
  vc=14=h0510ff6_3
  vs2015_runtime=14.0.25123=3
  wheel=0.30.0=py36h6c3ec14_1
  wincertstore=0.2=py36h7fe50ca_0
  yarl=0.14.2=py36h27d1bf2_0
  </code></pre></li>
  <li><p>I set up an empty project in a virtualenv environment and installed the aiohttp module there too. <code>pip freeze &gt; requirements.txt</code> then generates:</p>
  
  <pre><code>aiohttp==3.0.1
  async-timeout==2.0.0
  attrs==17.4.0
  chardet==3.0.4
  idna==2.6
  idna-ssl==1.0.0
  multidict==4.1.0
  yarl==1.1.0
  </code></pre></li>
  </ul>
  
  <p>So apparently both outputs are different, and my theory is: once I generate my requirements.txt with conda on my project, other developers can't choose virtualenv instead - at least not if they're not prepared to install a long list requirements by hand (it will be more than just the aiohttp module of course).</p>
  
  <p>A first sight, importing the conda-generated requirements.txt in a project on virtualenv (<code>pip install -r requirements-conda.txt</code>) throws this error:</p>
  
  <pre><code>Invalid requirement: 'aiohttp=2.3.9=py36_0'
  = is not a valid operator. Did you mean == ?
  </code></pre>
  
  <p>Am I right when I think that if developers would like to do this, they would need to programmatically change the package list to the format that virtualenv understands, or they would have to import all packages manually? Meaning that I impose them to choose conda as virtual environment as well if they want to save themselves some extra work?</p>