fix: add warning to retry target to avoid incorrect use

google/api_core/retry.py

@@ -62,6 +62,7 @@ def check_if_exists():
 import random
 import sys
 import time
+import inspect
 from typing import Any, Callable, TypeVar, TYPE_CHECKING
 
 import requests.exceptions
@@ -201,7 +202,10 @@ def retry_target(
 
     for sleep in sleep_generator:
         try:
-            return target()
+            result = target()
+            if inspect.isawaitable(result):
+                raise exceptions.GoogleAPICallError("Warning: Using the synchronous google.api_core.retry.Retry with asynchronous calls may lead to unexpected results. Please use google.api_core.retry_async.AsyncRetry instead.")
+            return result
 
         # pylint: disable=broad-except
         # This function explicitly must deal with broad exceptions.

tests/unit/test_retry.py

@@ -128,6 +128,24 @@ def test_retry_target_non_retryable_error(utcnow, sleep):
     assert exc_info.value == exception
     sleep.assert_not_called()
 
+@mock.patch("asyncio.sleep", autospec=True)
+@mock.patch(
+    "google.api_core.datetime_helpers.utcnow",
+    return_value=datetime.datetime.min,
+    autospec=True,
+)
+@pytest.mark.asyncio
+async def test_retry_target_warning_for_retry(utcnow, sleep):
+    predicate = retry.if_exception_type(ValueError)
+    target = mock.AsyncMock(spec=["__call__"])
+
+    with pytest.raises(exceptions.GoogleAPICallError) as exc_info:
+        retry.retry_target(target, predicate, range(10), None)
+
+    assert exc_info.match("Warning: Using the synchronous google.api_core.retry.Retry with asynchronous calls may lead to unexpected results. Please use google.api_core.retry_async.AsyncRetry instead.")
+    assert exc_info.type == exceptions.GoogleAPICallError
+    sleep.assert_not_called()
+
 
 @mock.patch("time.sleep", autospec=True)
 @mock.patch("google.api_core.datetime_helpers.utcnow", autospec=True)