List of all packages that are installed in this environment in
        the format [(name, ver), ..].

def installed_packages(self):
        """
        List of all packages that are installed in this environment in
        the format [(name, ver), ..].
        """
        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))))