gistfile1.pp

class my_app {
  $checkout = "/vagrant"
  $virtualenv = "/home/vagrant/env"

  python::virtualenv { $virtualenv:
   ensure       => present,
   owner        => "vagrant",
   version      => "system",
  }

  exec { 'setup':
   command     => "python setup.py develop",
   cwd         => $checkout,
   path        => "${virtualenv}/bin",
   refreshonly => true,
   subscribe   => Python::Virtualenv[$virtualenv],
  }

  Class['python'] -> Class['my-app'] # puppet 3 makes this nicer, ask me how
}

# ... 

class { 'python':
 version       => 'system',
 dev           => true,
 virtualenv    => true,
}
include 'my_app'