code: def pip_install_requirements(requirements, constraints=None, **options):
      
      command = ["install"]
  
      available_options = ('proxy', 'src', 'log', )
      for option in parse_options(options, available_options):
          command.append(option)
  
      command.append("-r {0}".format(requirements))
      if constraints:
          command.append("-c {0}".format(constraints))
          log("Installing from file: {} with constraints {} "
              "and options: {}".format(requirements, constraints, command))
      else:
          log("Installing from file: {} with options: {}".format(requirements,
                                                                 command))
      pip_execute(command)
docs: Install a requirements file.
  
      :param constraints: Path to pip constraints file.
      http://pip.readthedocs.org/en/stable/user_guide/#constraints-files