code: def installed_packages(self):
          
          freeze_options = ['-l', '--all'] if self.pip_version >= (8, 1, 0) else ['-l']
          return list(map(split_package_name, filter(None, self._execute_pip(
                  ['freeze'] + freeze_options).split(linesep))))
docs: List of all packages that are installed in this environment in
          the format [(name, ver), ..].