How to use the pipupgrade._pip.call function in pipupgrade

if output:
                    assert rout
                else:
                    assert not rout
            else:
                assert rout == output

        _assert_outerr(rout, output)
        _assert_outerr(rerr, err)

    directory = tmpdir.mkdir("tmp")
    tempfile  = directory.join("tmp.log")
    path      = str(tempfile)

    _pip.call("install", "pipupgrade")
    assert_pip_call(_pip.call("install", "pipupgrade", quiet = True))
    
    _pip.call("install", "pipupgrade", log = path)
    assert tempfile.read()

def update_pipfile(pipfile, verbose = False):
	if not verbose:
		logger.setLevel(log.NOTSET)

	realpath = osp.realpath(pipfile)
	basepath = osp.dirname(realpath)

	logger.info("Searching for `pipenv`...")
	pipenv   = which("pipenv")

	if not pipenv:
		logger.info("Attempting to install pipenv...")

		_pip.call("install", "pipenv")

		pipenv = which("pipenv", raise_err = True)

	logger.info("`pipenv` found.")

	code     = popen("%s update" % pipenv, quiet = not verbose, cwd = basepath)

	return code == 0