@api_view(['POST'])
def user_login(request):
    """
    **User Login**
    Login an existing user.
    Used for authenticating the user.

    > POST

    * Requires following fields of users in JSON format:

        1. `email` - String
        2. `password` - String

    * Returns user profile data on successful login.
    * Also returns Authentication token to be used by frontend for further
     communication with backend.
    * On failure it returns appropriate HTTP status and message in JSON
    response.

    * Possible HTTP status codes and JSON response:

        * `HTTP_200_OK` on successful login.

                {
                  "message": Login successful,
                  "email": String,
                  "token": String,
                  "id": Integer
                }

        * `HTTP_401_UNAUTHORIZED` for failed login attempt.

                {
                 "message": "Invalid username or password"
                }

        * `HTTP_500_INTERNAL_SERVER_ERROR` - Internal server error.

        * `HTTP_404_NOT_FOUND` - When user is not found.

                {
                 "message": "User with specified email does not exist."
                }

    * Status code can be used from HTTP header. A separate status field in json
    data is not provided.
    :param request:

    """
    try:
        email = request.data['email']
        password = request.data['password']

    except KeyError:
        return Response(
                "Email and Password required",
                status=status.HTTP_400_BAD_REQUEST
        )

    response = utils.check_user_exists_for_login(email)
    if response is True:
        return authenticate_user(
                email, password, request=request)
    else:
        return response


def check_user_exists_for_login(email):
    try:
        User.objects.get(email=email)
        return True
    except User.DoesNotExist:  #'User' is User table in database
        return Response(
                "User with specified email does not exist.",
                status=status.HTTP_404_NOT_FOUND)



def authenticate_user(email, password, request=None):
    user = authenticate(email=email, password=password)
    if user:
        serializer_dict["message"] = "Login successful"

        return Response(serializer_dict, status=status.HTTP_200_OK)
    else:
        return Response("Invalid email or password", status=status.HTTP_401_UNAUTHORIZED)