Serverspec fails to verify any package installed by pip on OS X target

Using serverspec-2.36.0 I cannot verify any package installed by pip on OS X El Capitan virtual machine.

Testing the command executed by Serverspec gives correct results.

The following example is for ansible installed with pip install ansible --user on user vagrant.

• My ansible_spec.rb:

  require 'spec_helper'
  
  describe command('whoami') do
    let(:disable_sudo) { true }
    its(:stdout) { should match 'vagrant' }
  end
  
  describe package('ansible') do
    let(:disable_sudo) { true }
    it { should be_installed.by('pip') }
  end
  

• Result:

  Command "whoami"
    stdout
      should match "vagrant"
  
  Package "ansible"
    should be installed by "pip" (FAILED - 1)
  

  Details of the second task:

  Failures:
  
    1) Package "ansible" should be installed by "pip"
       On host `osx-01'
       Failure/Error: it { should be_installed.by('pip') }
         expected Package "ansible" to be installed by "pip"
         /bin/sh -c pip\ list\ \|\ grep\ -iw\ --\ \\\^ansible
  
       # ./spec/ansible_spec.rb:10:in `block (2 levels) in <top (required)>'
  

• I login to machine and run:

  $ whoami
  vagrant
  $ pip list | grep -iw -- ^ansible
  ansible (2.1.0.0)
  $ /bin/sh -c pip\ list\ \|\ grep\ -iw\ --\ \\\^ansible
  ansible (2.1.0.0)
  

I am clueless regarding both: the reason and next possible steps in troubleshooting.

troubleshooting

• I added a task to check which python (from inside the Serverspec) and it fails with:

  1) Command "which python" stdout should match "/usr/local/bin/python"
     On host `ansible-osx-devops-environment-01'
     Failure/Error: its(:stdout) { should match '/usr/local/bin/python' }
       expected "" to match "/usr/local/bin/python"
       env PATH="/usr/local/bin" /bin/sh -c which\ python
  
     # ./spec/ansible_spec.rb:11:in `block (2 levels) in <top (required)>'
  

For reasons beyond my understanding Serverspec could not find either OS X's native Python (in /usr/bin/python) or the one installed with Homebrew (in /usr/local/bin/python).

As a workaround I added:

set :path, '/usr/local/bin:$PATH'

to spec_helper.rb.