diff --git a/src/flask_digest_auth/auth.py b/src/flask_digest_auth/auth.py
index bb53c8a..9d0b744 100644
--- a/src/flask_digest_auth/auth.py
+++ b/src/flask_digest_auth/auth.py
@@ -23,6 +23,7 @@ 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
@@ -35,6 +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."""
 
@@ -51,11 +82,43 @@ 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]] \
-            = lambda x: None
-        self.__get_user: t.Callable[[str], t.Optional] = lambda x: None
         self.app: t.Optional[Flask] = 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(
-            authorization.username)
+        password_hash: t.Optional[str] \
+            = 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.