diff --git a/prompt_toolkit/completion/word_completer.py b/prompt_toolkit/completion/word_completer.py
index 28c133e92..caab51e2e 100644
--- a/prompt_toolkit/completion/word_completer.py
+++ b/prompt_toolkit/completion/word_completer.py
@@ -1,4 +1,4 @@
-from typing import Callable, Iterable, List, Mapping, Optional, Pattern, Union
+from typing import Callable, Iterable, Mapping, Optional, Pattern, Union
 
 from prompt_toolkit.completion import CompleteEvent, Completer, Completion
 from prompt_toolkit.document import Document
@@ -31,7 +31,7 @@ class WordCompleter(Completer):
 
     def __init__(
         self,
-        words: Union[List[str], Callable[[], List[str]]],
+        words: Union[Iterable[str], Callable[[], Iterable[str]]],
         ignore_case: bool = False,
         display_dict: Optional[Mapping[str, AnyFormattedText]] = None,
         meta_dict: Optional[Mapping[str, AnyFormattedText]] = None,
diff --git a/prompt_toolkit/formatted_text/base.py b/prompt_toolkit/formatted_text/base.py
index e88c5935a..3a05b091a 100644
--- a/prompt_toolkit/formatted_text/base.py
+++ b/prompt_toolkit/formatted_text/base.py
@@ -1,4 +1,4 @@
-from typing import TYPE_CHECKING, Any, Callable, Iterable, List, Tuple, Union, cast
+from typing import TYPE_CHECKING, Any, Callable, List, Iterable, Sequence, Tuple, Union, cast
 
 from prompt_toolkit.mouse_events import MouseEvent
 
@@ -8,6 +8,7 @@
 __all__ = [
     "OneStyleAndTextTuple",
     "StyleAndTextTuples",
+    "StyleAndTextTuplesType",
     "MagicFormattedText",
     "AnyFormattedText",
     "to_formatted_text",
@@ -21,9 +22,11 @@
     Tuple[str, str], Tuple[str, str, Callable[[MouseEvent], None]]
 ]
 
-# List of (style, text) tuples.
+# List (covariant version of list) of (style, text) tuples
+# Not there is a subclass of this 
 StyleAndTextTuples = List[OneStyleAndTextTuple]
-
+# For typing stuff, we prefer an Iterable as it is more flexible
+StyleAndTextTuplesType = Iterable[OneStyleAndTextTuple]
 
 if TYPE_CHECKING:
     from typing_extensions import TypeGuard
@@ -41,7 +44,7 @@ def __pt_formatted_text__(self) -> StyleAndTextTuples:
 AnyFormattedText = Union[
     str,
     "MagicFormattedText",
-    StyleAndTextTuples,
+    StyleAndTextTuplesTypes,
     # Callable[[], 'AnyFormattedText']  # Recursive definition not supported by mypy.
     Callable[[], Any],
     None,