raise_from don't work

Description

from future.utils import raise_from

def fa():
    try:
        b()
    except Exception as ex:
        raise_from(Exception('fa'), ex)

def b():
    raise Exception('b')

if __name__ == "__main__":
    fa()

Traceback (most recent call last):
  File ".\mod.py", line 20, in <module>
    fa()
  File ".\mod.py", line 8, in fa
    raise_from(Exception('fa'), ex)
  File "C:\Python27\lib\site-packages\future\utils\__init__.py", line 454, in raise_from
    raise e
Exception: fa

Contributor guide

Tech stack

python

Domain

backend

Issue type

bug

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

3

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

half day

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

stale

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

Python exception handlingfuture library internals

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

40

Research direction

Investigate the raise from function in future/utils/ init .py (line 454). The issue reports that the original exception chain is lost when using raise from. In Python 2, raise from is not natively supported; the library attempts to simulate it. Study the current implementation and compare with Python 3's raise from behavior. Ensure the raised exception includes the cause attribute set to the original exception. Test with the provided script in Python 2.7. Consider improving the implementation to preserve the traceback chain.