code: def detect_envlist(ini):
      
      # Find the envs that tox knows about
      declared_envs = get_declared_envs(ini)
  
      # Find all the envs for all the desired factors given
      desired_factors = get_desired_factors(ini)
  
      # Reduce desired factors
      desired_envs = ['-'.join(env) for env in product(*desired_factors)]
  
      # Find matching envs
      return match_envs(declared_envs, desired_envs,
                        passthru=len(desired_factors) == 1)
docs: Default envlist automatically based on the Travis environment.