Why is pip install pyopenssl==0.13 failing?

Question

I'm trying to install PyOpenSSL 0.13 on my Macbook Pro (OSX version 10.11, El-Capitan). But it keeps failing. These are the steps I took

1. Download and install Command Line Tools (OSX 10.11) for Xcode 7.3.1 from here
2. $ virtualenv my-new-virtualenv
3. $ source my-new-virtualenv/bin/activate
4. $ pip install pyopenssl==0.13

When I do step #4, I get the following error:

OpenSSL/crypto/x509.h:17:10: fatal error: 'openssl/ssl.h' file not found
  #include <openssl/ssl.h>
       ^
  1 error generated.
  error: command 'cc' failed with exit status 1

  ----------------------------------------
  Failed building wheel for pyopenssl

Here is the entire trace showing that error.

Why am I getting this error and how can I fix it??

Answer 1

It appears you are missing the OpenSSL development headers, as mentioned by @Klaus D. This most likely happened because due to upgrading to El Capitan, these development headers were broken. It can usually be fixed by reinstalling your command line tools. If you have Homebrew, run this code: brew install openssl

Also, just out of curiosity, is there a particular reason why you want to use version 0.13? When I did $ pip install pyopenssl==0.14 I got no errors. See the bit on pyOpenSSL's documentation: