Environment details

• OS type and version: macOS 26.1
• Python version: 3.12.12
• pip version: 25.3
• google-api-core version: HEAD =f8bf6f9610f3e0e7580f223794c3906513e1fa73

Description

The retry mechanism clears the__cause__ attribute when re-raising non-retryable exceptions, breaking explicit exception chaining created withraise ... from ....

Steps to reproduce

1. Use the@retry_async.AsyncRetry decorator
2. Raise a non-retryable exception with a cause
3. Catch the exception and try to read or log its cause

Code example

importasynciofromgoogle.api_coreimportretry_async,exceptions@retry_async.AsyncRetry(predicate=retry_async.if_exception_type(exceptions.InternalServerError))asyncdefexample():try:raiseexceptions.Unauthenticated("Invalid credentials")exceptexceptions.Unauthenticatedasexc:raiseValueError("Access denied")fromexc# Explicit chainingasyncdefmain():# After going through retry:try:awaitexample()exceptValueErrorase:print(e.__cause__)# Expected: "401 Invalid credentials"; Actual: Noneif__name__=="__main__":asyncio.run(main())

Seehttps://github.com/googleapis/python-api-core/pull/879/changes for a unit test that fails with the current code and passes with the proposed fix.

Root Cause

Inretry_base.py:168,_default_exception_factory() returns(exc_list[-1], None), which causes raisefinal_exc fromNone on line 214, explicitly clearing__cause__.

Proposed fix

Preserve the exception'scause attribute:

# Beforereturnexc_list[-1],None# Afterfinal_exc=exc_list[-1]cause=getattr(final_exc,'__cause__',None)returnfinal_exc,cause

This maintains exception chains for better debugging and meets developer expectations for exception handling.

Draft pull request

#879