Django Invalid HTTP_HOST header: 'testserver'. You may need to add u'testserver' to ALLOWED_HOSTS

I started learning Django, I'm in the middle of implementing "Test a view" functionality. When I use test Client in the shell, the exception has occurred as follows.

Invalid HTTP_HOST header: 'testserver'. You may need to add u'testserver' to ALLOWED_HOSTS.

I run the command in the shell as follows.

>>> from django.test.utils import setup_test_environment
>>> setup_test_environment()
>>> from django.test import Client
>>> client = Client()
>>> response = client.get('/')
>>> response.status_code
400

In the tutorial, 404 should be appeared, but I get 400. When I continue running the command as follows, same exception has occurred.

>>> response = client.get(reverse('polls:index'))
>>> response.status_code
400

but the result must be 200.I guess I should declare ALLOWED_HOSTS in the settings.py, but how can I? I run the server on localhost using $python manage.py runserver.

I wanna know the reason and solution.

Here is settings.py as follows.

import os
BASE_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
SECRET_KEY = '8v57o6wyupthi^#41_yfg4vsx6s($1$x0xmu*95_u93wwy0_&u'
DEBUG = True
ALLOWED_HOSTS = [127.0.0.1,'localhost']
INSTALLED_APPS = [
    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    'polls',
]
....    (MIDDLEWARE)
ROOT_URLCONF = 'tutorial.urls'

TEMPLATES = [
    {
        'BACKEND': 'django.template.backends.django.DjangoTemplates',
        'DIRS': [],
        'APP_DIRS': True,
        'OPTIONS': {
            'context_processors': [
                'django.template.context_processors.debug',
                'django.template.context_processors.request',
                'django.contrib.auth.context_processors.auth',
                'django.contrib.messages.context_processors.messages',
            ],
        },
    },
]

WSGI_APPLICATION = 'tutorial.wsgi.application'
....    (DATABASES, AUTH_PASSWORD_VALIDATORS)
LANGUAGE_CODE = 'en-us'

TIME_ZONE = 'UTC'

USE_I18N = True

USE_L10N = True

USE_TZ = True

Edit the following line in your settings.py file:

ALLOWED_HOSTS = ['127.0.0.1', 'localhost']

Restart your server afterwards