Description

moontc

opened on Nov 6, 2026

Issue body actions

Bug report

The pure-Python loop in traceback._compute_suggestion_error() gives different results from _Py_CalculateSuggestions() in Python/suggestions.c:

1. Initial best distance: len(wrong_name) in Python, PY_SSIZE_T_MAX in C.
2. Candidate limit: len(d) > 750 in Python, >= 750 in C.
3. Python returns None for names longer than 40 characters; C only checks the length after trimming the common prefix and suffix.

import sys, traceback, _suggestions

class A:
    def __init__(self, names): self.names = names
    def __dir__(self): return self.names

def pure_python(names, wrong):
    sys.modules['_suggestions'] = None
    try:
        getattr(A(names), wrong)
    except AttributeError as e:
        return traceback._compute_suggestion_error(e, e.__traceback__, wrong)
    finally:
        sys.modules['_suggestions'] = _suggestions

for names, wrong in [
    (['tuple', 'True'], 'ture'),
    (['x' * 41 + 'y'], 'x' * 41),
    ([f'name{i:03}' for i in range(749)] + ['vvvvv'], 'vvvvx'),
]:
    print(_suggestions._generate_suggestions(sorted(names), wrong),
          pure_python(names, wrong))

tuple None
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxy None
None vvvvv

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-161495: Make traceback suggestion fallback match the C implementation #161497

Activity

1. moontc

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Nov 6, 2026
2. moontc

   added a commit that references this issue on Nov 6, 2026

   pythongh-161495: Optimize common-affix trimming in traceback

   144707f
3. NekoAsakura

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on Nov 6, 2026
4. picnixz commented on Nov 7, 2026

   

   picnixz

   on Nov 7, 2026

   Member

   More actions

   Are these functions interchangeable? they are also private so you are not meant to use them outside CPython. When do they produce different meaningful results?

   Reacted by moontc
5. moontc commented on Nov 8, 2026

   

   moontc

   on Nov 8, 2026

   ContributorAuthor

   More actions

   Before gh-97008, the default exception display was mainly done in C, and the suggestions only existed in Python/suggestions.c. Then gh-97008 pointed out:

   This is implemented in Python/suggestions.c and unavailable in the pure Python implementation of traceback formatting in Lib/traceback.py. This makes the output of both different, which is problematic for tooling using traceback.py, as well as for alternative implementations of Python like PyPy and GraalPy.

   Then a pure Python implementation of suggestions was added to traceback.py.

   Let's implement the equivalent of suggestions.c in Lib/traceback.py to bring both in line.

   so its design goal was to "implement the equivalent of suggestions.c in Lib/traceback.py".

   after #110702, default exception display prioritizes the use of python's traceback.
   after #113712, _compute_suggestion_error() can call the C implementation through _suggestions and also contains a pure-Python implementation

   "Are these functions interchangeable?"
   Yes, more precisely, the pure-Python suggestion calculation inside _compute_suggestion_error() should give the same results as _Py_CalculateSuggestions()

   “When do they produce different meaningful results?”
   They produce different suggestions depending on whether the _suggestions module is available.
   For example, the same code produces different suggestions on CPython and PyPy, because PyPy doesn't have the _suggestions module and uses the pure Python implementation in traceback.py.

   Reacted by Bénédikt Tran

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