✨ Add support for standard tracebacks via the env `TYPER_STANDARD_TRACEBACK` by colin-nolan · Pull Request #1299 · fastapi/typer

colin-nolan · 2025-09-08T16:44:42Z

This PR allows standard tracebacks to be enabled using the environment variable: TYPER_STANDARD_TRACEBACK.

The work is a follow up to the discussion with @svlandeg: #1284.

The existing environment variable _TYPER_STANDARD_TRACEBACK is retained but referred to as "deprecated" because:

It is non-standard for an advertised env to start with an underscore.
Environment variables that start with an underscore have been observed as causing issues in some execution environments (e.g. with AWS Lambda).

Manual testing with:

import typer

def main():
    raise Exception("Hello World")

if __name__ == "__main__":
    typer.run(main)

Produces expected results:

$ python will-fail.py 
╭───────────────────────────────────────── Traceback (most recent call last) ─────────────────────────────────────────╮
│ /Users/cnolan/repositories/typer/will-fail.py:5 in main                                                             │
│                                                                                                                     │
│   2 import typer                                                                                                    │
│   3                                                                                                                 │
│   4 def main():                                                                                                     │
│ ❱ 5 │   raise Exception("Hello World")                                                                              │
│   6                                                                                                                 │
│   7 if __name__ == "__main__":                                                                                      │
│   8 │   typer.run(main)                                                                                             │
╰─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
Exception: Hello World

$ TYPER_STANDARD_TRACEBACK=1 python will-fail.py 
Traceback (most recent call last):
  File "/Users/cnolan/repositories/typer/will-fail.py", line 8, in <module>
    typer.run(main)
    ~~~~~~~~~^^^^^^
  File "/Users/cnolan/repositories/typer/typer/main.py", line 1073, in run
    app()
    ~~~^^
  File "/Users/cnolan/repositories/typer/typer/main.py", line 332, in __call__
    raise e
  File "/Users/cnolan/repositories/typer/typer/main.py", line 315, in __call__
    return get_command(self)(*args, **kwargs)
           ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/Users/cnolan/repositories/typer/.venv/lib/python3.13/site-packages/click/core.py", line 1442, in __call__
    return self.main(*args, **kwargs)
           ~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/Users/cnolan/repositories/typer/typer/core.py", line 700, in main
    return _main(
        self,
    ...<6 lines>...
        **extra,
    )
  File "/Users/cnolan/repositories/typer/typer/core.py", line 193, in _main
    rv = self.invoke(ctx)
  File "/Users/cnolan/repositories/typer/.venv/lib/python3.13/site-packages/click/core.py", line 1226, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cnolan/repositories/typer/.venv/lib/python3.13/site-packages/click/core.py", line 794, in invoke
    return callback(*args, **kwargs)
  File "/Users/cnolan/repositories/typer/typer/main.py", line 692, in wrapper
    return callback(**use_params)
  File "/Users/cnolan/repositories/typer/will-fail.py", line 5, in main
    raise Exception("Hello World")
Exception: Hello World

$ _TYPER_STANDARD_TRACEBACK=1 python will-fail.py 
Traceback (most recent call last):
  File "/Users/cnolan/repositories/typer/will-fail.py", line 8, in <module>
    typer.run(main)
    ~~~~~~~~~^^^^^^
  File "/Users/cnolan/repositories/typer/typer/main.py", line 1073, in run
    app()
    ~~~^^
  File "/Users/cnolan/repositories/typer/typer/main.py", line 332, in __call__
    raise e
  File "/Users/cnolan/repositories/typer/typer/main.py", line 315, in __call__
    return get_command(self)(*args, **kwargs)
           ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/Users/cnolan/repositories/typer/.venv/lib/python3.13/site-packages/click/core.py", line 1442, in __call__
    return self.main(*args, **kwargs)
           ~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/Users/cnolan/repositories/typer/typer/core.py", line 700, in main
    return _main(
        self,
    ...<6 lines>...
        **extra,
    )
  File "/Users/cnolan/repositories/typer/typer/core.py", line 193, in _main
    rv = self.invoke(ctx)
  File "/Users/cnolan/repositories/typer/.venv/lib/python3.13/site-packages/click/core.py", line 1226, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cnolan/repositories/typer/.venv/lib/python3.13/site-packages/click/core.py", line 794, in invoke
    return callback(*args, **kwargs)
  File "/Users/cnolan/repositories/typer/typer/main.py", line 692, in wrapper
    return callback(**use_params)
  File "/Users/cnolan/repositories/typer/will-fail.py", line 5, in main
    raise Exception("Hello World")
Exception: Hello World

svlandeg

As discussed, this PR adds the option to set an env var TYPER_STANDARD_TRACEBACK while still supporting the old var _TYPER_STANDARD_TRACEBACK too. We need to support both because we don't want to break existing workflows, but that does complicate the code slightly. I think that's an acceptable trade-off to ensure this variable can be properly set in AWS Lambda etc.

typer/main.py

svlandeg · 2025-09-09T08:03:22Z

I will leave this PR for a final review by Tiangolo 🙏

tiangolo

Makes sense, thank you! 🚀

tiangolo · 2025-09-20T07:25:41Z

It seems that I can't push to your branch GenPAxCo:feature/non-underscore-traceback-env to update with the changes in master, which would probably solve the failing tests. Maybe you can update it to "allow maintainers to push to the branch", or you can update it with master yourself, I think that should do it. 🤓

colin-nolan · 2025-10-03T09:20:51Z

@tiangolo I've updated and tests are now passing.

svlandeg · 2025-10-03T14:54:43Z

Thanks @colin-nolan! I've moved this back to Tiangolo's internal review queue 😉

colin-nolan · 2025-10-16T14:31:15Z

Waiting on @tiangolo

svlandeg · 2025-11-25T10:48:48Z

@colin-nolan: can you please turn on the option that maintainers can edit this PR?

Keeps support fo `_TYPER_STANDARD_TRACEBACK` but notes as deprecated because: - It is non-standard for an advertised env to start with an underscore. - Environment variables that start with an underscore have been observed as causing issues in some execution environments (e.g. with AWS Lambda). See: fastapi#1284

github-actions · 2025-11-25T11:13:55Z

📝 Docs preview

Last commit a75d349 at: https://55fbf697.typertiangolo.pages.dev

Modified Pages

https://55fbf697.typertiangolo.pages.dev/tutorial/exceptions/ - (before)

colin-nolan · 2025-11-25T11:15:52Z

@svlandeg I've resolved the merge conflict.

I've tried to enable maintainers PR edits but, for some reason, the option to do this doesn't exist(?) (https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/allowing-changes-to-a-pull-request-branch-created-from-a-fork)

Edit: On reading the docs from GH again, I note (emphasis mine):

Pull request authors can give these permissions when they initially create a pull request from a fork in a personal account or after they create the pull request.

The fork is from a company account. Its ambiguous as to whether permission can be added post creation but that could well the be issue.

svlandeg · 2025-11-25T12:44:05Z

Ok, thanks @colin-nolan! As Tiangolo had already approved earlier, and nothing changed since then other than merging in master, I'm going to go ahead and merge this in. Thanks again!

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svlandeg added the feature New feature, enhancement or request label Sep 9, 2025

svlandeg changed the title ~~Adds support for standard tracebacks via the env TYPER_STANDARD_TRACEBACK~~ ✨ Add support for standard tracebacks via the env TYPER_STANDARD_TRACEBACK Sep 9, 2025

svlandeg self-assigned this Sep 9, 2025

svlandeg approved these changes Sep 9, 2025

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colin-nolan force-pushed the feature/non-underscore-traceback-env branch from b77ece1 to 0a41362 Compare September 23, 2025 22:55

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colin-nolan added 2 commits November 25, 2025 11:09

Test new and legacy envs for setting standard tb

a75d349

colin-nolan force-pushed the feature/non-underscore-traceback-env branch from 0a41362 to a75d349 Compare November 25, 2025 11:12

github-actions bot removed the conflicts Automatically generated when a PR has a merge conflict label Nov 25, 2025

svlandeg merged commit de159dc into fastapi:master Nov 25, 2025
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