How to use the testinfra.get_backend function in testinfra

def test_same_website_root():
    tree = {}
    for name in ("default", "production"):
        conn = testinfra.get_backend(
            name, connection="paramiko", ssh_config=".vagrant-ssh-config")
        Command = conn.get_module("Command")
        tree[name] = Command.check_output("tree /srv/website")

    assert tree["default"] == tree["production"]

''' This file starts with 000 to make it run first '''
import pytest
import testinfra

run_local = testinfra.get_backend(
    "local://"
).get_module("Command").run

@pytest.mark.parametrize("upstream,image,tag", [
    ( 'alpine:edge', 'alpine.docker', 'diginc/pi-hole:alpine' ),
    ( 'debian:jessie', 'debian.docker', 'diginc/pi-hole:debian' ),
    #( 'jsurf/rpi-raspbian', 'debian-armhf.docker', 'diginc/pi-hole:arm' ),
])
def test_build_pihole_image(upstream, image, tag):
    run_local('docker pull {}'.format(upstream))
    build_cmd = run_local('docker build -f {} -t {} .'.format(image, tag))
    if build_cmd.rc != 0:
        print build_cmd.stdout
        print build_cmd.stderr
    assert build_cmd.rc == 0

def _get_module(module_name, backend=default_backend):
    """Retrieve the correct module implementation determined by the backend
    being used.

    :param module_name: TestInfra module to retrieve
    :param backend: string representing backend for TestInfra
    :returns: desired TestInfra module object
    :rtype: object

    """
    backend_instance = testinfra.get_backend(backend)
    return backend_instance.get_module(_to_pascal_case(module_name))

# Copyright 2015 VMware, Inc.  All rights reserved.
# SPDX-License-Identifier: Apache-2.0 OR GPL-3.0-only

from __future__ import print_function
import testinfra.utils.ansible_runner


testinfra_hosts = testinfra.utils.ansible_runner.AnsibleRunner(
    '.molecule/ansible_inventory').get_hosts('all')

# Use testinfra to get a handy function to run commands locally
check_output = testinfra.get_backend(
    "local://"
).get_module("Command").check_output


def test_hosts_file(File):
    f = File('/root/.ssh/authorized_keys')
    assert f.exists
    assert f.user == 'root'
    assert f.group == 'root'

    command = check_output(
        'cat ~/.ssh/ansible_role_test_key.pub')
    assert command == f.content_string.rstrip()