diff --git a/sphinx/ext/autodoc/__init__.py b/sphinx/ext/autodoc/__init__.py
index 1bd49fc38..657455c7c 100644
--- a/sphinx/ext/autodoc/__init__.py
+++ b/sphinx/ext/autodoc/__init__.py
@@ -961,7 +961,7 @@ class ModuleDocumenter(Documenter):
     def __init__(self, *args: Any) -> None:
         super().__init__(*args)
         merge_members_option(self.options)
-        self.__all__ = None
+        self.__all__ = None  # type: Optional[Sequence[str]]
 
     @classmethod
     def can_document_member(cls, member: Any, membername: str, isattr: bool, parent: Any
@@ -985,26 +985,16 @@ class ModuleDocumenter(Documenter):
         return ret
 
     def import_object(self, raiseerror: bool = False) -> bool:
-        def is_valid_module_all(__all__: Any) -> bool:
-            """Check the given *__all__* is valid for a module."""
-            if (isinstance(__all__, (list, tuple)) and
-                    all(isinstance(e, str) for e in __all__)):
-                return True
-            else:
-                return False
-
         ret = super().import_object(raiseerror)
 
-        if not self.options.ignore_module_all:
-            __all__ = getattr(self.object, '__all__', None)
-            if is_valid_module_all(__all__):
-                # valid __all__ found. copy it to self.__all__
-                self.__all__ = __all__
-            elif __all__:
-                # invalid __all__ found.
-                logger.warning(__('__all__ should be a list of strings, not %r '
-                                  '(in module %s) -- ignoring __all__') %
-                               (__all__, self.fullname), type='autodoc')
+        try:
+            if not self.options.ignore_module_all:
+                self.__all__ = inspect.getall(self.object)
+        except ValueError as exc:
+            # invalid __all__ found.
+            logger.warning(__('__all__ should be a list of strings, not %r '
+                              '(in module %s) -- ignoring __all__') %
+                           (exc.args[0], self.fullname), type='autodoc')
 
         return ret
 
diff --git a/sphinx/util/inspect.py b/sphinx/util/inspect.py
index 4f4f12e1d..c7375aa57 100644
--- a/sphinx/util/inspect.py
+++ b/sphinx/util/inspect.py
@@ -20,7 +20,7 @@ import warnings
 from functools import partial, partialmethod
 from inspect import Parameter, isclass, ismethod, ismethoddescriptor, ismodule  # NOQA
 from io import StringIO
-from typing import Any, Callable, Dict, List, Mapping, Optional, Tuple, cast
+from typing import Any, Callable, Dict, List, Mapping, Optional, Sequence, Tuple, cast
 
 from sphinx.deprecation import RemovedInSphinx40Warning, RemovedInSphinx50Warning
 from sphinx.pycode.ast import ast  # for py35-37
@@ -137,6 +137,22 @@ def unwrap_all(obj: Any, *, stop: Callable = None) -> Any:
             return obj
 
 
+def getall(obj: Any) -> Optional[Sequence[str]]:
+    """Get __all__ attribute of the module as dict.
+
+    Return None if given *obj* does not have __all__.
+    Raises ValueError if given *obj* have invalid __all__.
+    """
+    __all__ = safe_getattr(obj, '__all__', None)
+    if __all__ is None:
+        return None
+    else:
+        if (isinstance(__all__, (list, tuple)) and all(isinstance(e, str) for e in __all__)):
+            return __all__
+        else:
+            raise ValueError(__all__)
+
+
 def getslots(obj: Any) -> Optional[Dict]:
     """Get __slots__ attribute of the class as dict.