code: def pip(self, package_names, raise_on_error=True):
          
          if isinstance(package_names, basestring):
              package_names = [package_names]
          cmd = "pip install -U %s" % (' '.join(package_names))
          return self.wait(cmd, raise_on_error=raise_on_error)
docs: Install specified python packages using pip. -U option added
          Waits for command to finish.
  
          Parameters
          ----------
          package_names: list-like of str
          raise_on_error: bool, default True
              If True then raise ValueError if stderr is not empty