Revised the DigestAuth class to deal with the dummy get_password_hash and get_user functions when they are not registered yet.

README.rst

@@ -75,6 +75,7 @@ In your ``my_app.py``:
     ... (Configure the Flask application) ...
 
     auth: DigestAuth = DigestAuth(realm="Admin")
+    auth.init_app(app)
 
     @auth.register_get_password
     def get_password_hash(username: str) -> t.Optional[str]:
@@ -113,6 +114,7 @@ In your ``my_app/__init__.py``:
         ... (Configure the Flask application) ...
 
         auth.realm = app.config["REALM"]
+        auth.init_app(app)
 
         @auth.register_get_password
         def get_password_hash(username: str) -> t.Optional[str]:
@@ -156,6 +158,9 @@ module that requires log in, without specifying the authentication
 mechanism.  The Flask application can specify the actual
 authentication mechanism as it sees fit.
 
+``login_manager.init_app(app)`` must be called before
+``auth.init_app(app)``.
+
 
 Example for Simple Applications with Flask-Login Integration
 ------------------------------------------------------------

src/flask_digest_auth/auth.py

@@ -23,12 +23,12 @@ from __future__ import annotations
 
 import sys
 import typing as t
+from abc import ABC, abstractmethod
 from functools import wraps
 from random import random
 from secrets import token_urlsafe
 
-from flask import g, request, Response, session, abort, Flask, Request, \
+from flask import g, request, Response, session, abort, Flask, Request
-    current_app
 from itsdangerous import URLSafeTimedSerializer, BadData
 from werkzeug.datastructures import Authorization
 
@@ -36,6 +36,36 @@ from flask_digest_auth.algo import calc_response
 from flask_digest_auth.exception import UnauthorizedException
 
 
+class BasePasswordHashGetter(ABC):
+    """The base password hash getter."""
+
+    @staticmethod
+    @abstractmethod
+    def __call__(username: str) -> t.Optional[str]:
+        """Returns the password hash of a user.
+
+        :param username: The username.
+        :return: The password hash, or None if the user does not exist.
+        :raise UnboundLocalError: When the password hash getter function is
+            not registered yet.
+        """
+
+
+class BaseUserGetter(ABC):
+    """The base user getter."""
+
+    @staticmethod
+    @abstractmethod
+    def __call__(username: str) -> t.Optional[t.Any]:
+        """Returns a user.
+
+        :param username: The username.
+        :return: The user, or None if the user does not exist.
+        :raise UnboundLocalError: When the user getter function is not
+            registered yet.
+        """
+
+
 class DigestAuth:
     """The HTTP digest authentication."""
 
@@ -52,9 +82,42 @@ class DigestAuth:
         self.use_opaque: bool = True
         self.domain: t.List[str] = []
         self.qop: t.List[str] = ["auth", "auth-int"]
-        self.__get_password_hash: t.Callable[[str], t.Optional[str]] \
+        self.app: t.Optional[Flask] = None
-            = lambda x: None
+
-        self.__get_user: t.Callable[[str], t.Optional] = lambda x: None
+        class DummyPasswordHashGetter(BasePasswordHashGetter):
+            """The dummy password hash getter."""
+
+            @staticmethod
+            def __call__(username: str) -> t.Optional[str]:
+                """Returns the password hash of a user.
+
+                :param username: The username.
+                :return: The password hash, or None if the user does not exist.
+                :raise UnboundLocalError: When the password hash getter function
+                    is not registered yet.
+                """
+                raise UnboundLocalError("The function to return the password"
+                                        " hash was not registered yet.")
+
+        self.__get_password_hash: BasePasswordHashGetter \
+            = DummyPasswordHashGetter()
+
+        class DummyUserGetter(BaseUserGetter):
+            """The dummy user getter."""
+
+            @staticmethod
+            def __call__(username: str) -> t.Optional[t.Any]:
+                """Returns a user.
+
+                :param username: The username.
+                :return: The user, or None if the user does not exist.
+                :raise UnboundLocalError: When the user getter function is not
+                    registered yet.
+                """
+                raise UnboundLocalError("The function to return the user"
+                                        " was not registered yet.")
+
+        self.__get_user: BaseUserGetter = DummyUserGetter()
 
     def login_required(self, view) -> t.Callable:
         """The view decorator for HTTP digest authentication.
@@ -127,8 +190,8 @@ class DigestAuth:
             except BadData:
                 raise UnauthorizedException("Invalid opaque")
             state.opaque = authorization.opaque
-        password_hash: t.Optional[str] = self.__get_password_hash(
+        password_hash: t.Optional[str] \
-            authorization.username)
+            = self.__get_password_hash(authorization.username)
         if password_hash is None:
             raise UnauthorizedException(
                 f"No such user \"{authorization.username}\"")
@@ -187,7 +250,20 @@ class DigestAuth:
             hash, or None if the user does not exist.
         :return: None.
         """
-        self.__get_password_hash = func
+
+        class PasswordHashGetter(BasePasswordHashGetter):
+            """The base password hash getter."""
+
+            @staticmethod
+            def __call__(username: str) -> t.Optional[str]:
+                """Returns the password hash of a user.
+
+                :param username: The username.
+                :return: The password hash, or None if the user does not exist.
+                """
+                return func(username)
+
+        self.__get_password_hash = PasswordHashGetter()
 
     def register_get_user(self, func: t.Callable[[str], t.Optional[t.Any]])\
             -> None:
@@ -197,7 +273,20 @@ class DigestAuth:
             or None if the user does not exist.
         :return: None.
         """
-        self.__get_user = func
+
+        class UserGetter(BaseUserGetter):
+            """The user getter."""
+
+            @staticmethod
+            def __call__(username: str) -> t.Optional[t.Any]:
+                """Returns a user.
+
+                :param username: The username.
+                :return: The user, or None if the user does not exist.
+                """
+                return func(username)
+
+        self.__get_user = UserGetter()
 
     def init_app(self, app: Flask) -> None:
         """Initializes the Flask application.
@@ -205,13 +294,12 @@ class DigestAuth:
         :param app: The Flask application.
         :return: None.
         """
+        app.digest_auth = self
+        self.app = app
 
-        try:
+        if hasattr(app, "login_manager"):
             from flask_login import LoginManager, login_user
 
-            if not hasattr(app, "login_manager"):
-                raise AttributeError(
-                    "Please run the Flask-Login init-app() first")
             login_manager: LoginManager = getattr(app, "login_manager")
 
             @login_manager.unauthorized_handler
@@ -252,12 +340,7 @@ class DigestAuth:
                         app.logger.warning(str(e))
                     return None
 
-        except ModuleNotFoundError:
+    def logout(self) -> None:
-            raise ModuleNotFoundError(
-                "init_app() is only for Flask-Login integration")
-
-    @staticmethod
-    def logout() -> None:
         """Logs out the user.
         This actually causes the next authentication to fail, which forces
         the browser to ask the user for the username and password again.
@@ -267,7 +350,7 @@ class DigestAuth:
         if "user" in session:
             del session["user"]
         try:
-            if hasattr(current_app, "login_manager"):
+            if hasattr(self.app, "login_manager"):
                 from flask_login import logout_user
                 logout_user()
         except ModuleNotFoundError:

tests/test_auth.py

@@ -49,6 +49,7 @@ class AuthenticationTestCase(TestCase):
         app.test_client_class = Client
 
         auth: DigestAuth = DigestAuth(realm=_REALM)
+        auth.init_app(app)
         user_db: t.Dict[str, str] \
             = {_USERNAME: make_password_hash(_REALM, _USERNAME, _PASSWORD)}