added new typing and changed error response

backoff/_decorator.py

@@ -18,7 +18,7 @@
     _Jitterer,
     _MaybeCallable,
     _MaybeLogger,
-    _MaybeSequence,
+    _MaybeTuple,
     _Predicate,
     _WaitGenerator,
 )
@@ -120,8 +120,23 @@ def decorate(target):
     return decorate
 
 
+def _check_exception_type(exception: _MaybeTuple[Type[Exception]]) -> None:
+    """
+    Raise TypeError if exception is not a valid type, otherwise return None
+    """
+    if isinstance(exception, Type) and issubclass(exception, Exception):
+        return
+    if isinstance(exception, tuple) and all(isinstance(e, Type)
+            and issubclass(e, Exception) for e in exception):
+        return
+    raise TypeError(
+        f"exception '{exception}' of type {type(exception)} is not"
+        " an Exception type or a tuple of Exception types"
+    )
+
+
 def on_exception(wait_gen: _WaitGenerator,
-                 exception: _MaybeSequence[Type[Exception]],
+                 exception: _MaybeTuple[Type[Exception]],
                  *,
                  max_tries: Optional[_MaybeCallable[int]] = None,
                  max_time: Optional[_MaybeCallable[float]] = None,
@@ -180,6 +195,7 @@ def on_exception(wait_gen: _WaitGenerator,
             args will first be evaluated and their return values passed.
             This is useful for runtime configuration.
     """
+    _check_exception_type(exception)
     def decorate(target):
         nonlocal logger, on_success, on_backoff, on_giveup
 

backoff/_typing.py

@@ -1,7 +1,7 @@
 # coding:utf-8
 import logging
 import sys
-from typing import (Any, Callable, Coroutine, Dict, Generator, Sequence, Tuple,
+from typing import (Any, Callable, Coroutine, Dict, Generator, Tuple,
                     TypeVar, Union)
 
 if sys.version_info >= (3, 8):  # pragma: no cover
@@ -39,6 +39,6 @@ class Details(_Details, total=False):
 _Jitterer = Callable[[float], float]
 _MaybeCallable = Union[T, Callable[[], T]]
 _MaybeLogger = Union[str, logging.Logger, None]
-_MaybeSequence = Union[T, Sequence[T]]
+_MaybeTuple = Union[T, Tuple[T]]
 _Predicate = Callable[[T], bool]
 _WaitGenerator = Callable[..., Generator[float, None, None]]

tests/test_typing.py

@@ -32,3 +32,17 @@ def bar():
 )
 def baz():
     pass
+
+
+# Type Successes
+for exception in OSError, tuple([OSError]), (OSError, ValueError):
+    backoff.on_exception(backoff.expo, exception)
+
+
+# Type Failures
+for exception in OSError(), [OSError], (OSError, ValueError()), "hi", (2, 3):
+    try:
+        backoff.on_exception(backoff.expo, exception)
+        raise AssertionError(f"Expected TypeError for {exception}")
+    except TypeError:
+        pass