AssertionError with Cython modules (frozenlist._frozenlist) under Python 3.13 in PyDev debugger

[icarocd]

AssertionError with Cython modules (frozenlist._frozenlist) under Python 3.13 in PyDev debugger

Environment

• Python: 3.13.0 (official CPython)
• PyDev.Debugger: 13.0.2 (bundled with Eclipse)
• Eclipse
• OS: Linux (Kubuntu 25.04)
• Relevant C extension: frozenlist._frozenlist (used via aiohttp)

---

Problem

When running code with the debugger enabled in PyDev (Eclipse), I consistently hit this traceback when importing aiohttp, due to its internal use of the frozenlist Cython extension:

AssertionError: <code object _call_with_frames_removed at 0x..., file "<frozen importlib._bootstrap>", line 480> != <code object PyInit__frozenlist at 0x..., file "frozenlist/_frozenlist.pyx", line 1>

The traceback is triggered in:
_pydevd_sys_monitoring_cython.pyx, line 607, _get_func_code_info

---

Context

• The speedups were successfully compiled using:

  python3 setup_pydevd_cython.py build_ext --inplace

• Still, the debugger crashes when any Cython-compiled module (specifically frozenlist) is imported during debugging.

---

Workaround

I was able to continue by monkey-patching the module like this:

import sys
try:
    import frozenlist._frozenlist
except AssertionError:
    if 'frozenlist._frozenlist' in sys.modules:
        del sys.modules['frozenlist._frozenlist']
    sys.modules['frozenlist._frozenlist'] = None

This avoids triggering PyDev's internal assertion and allows the debugger to proceed, at the cost of reduced performance in aiohttp.

---

Request

Please confirm whether full support for Python 3.13 + Cython extensions is expected in an upcoming PyDev.Debugger version.

Thanks for your great work on this project!

[da-woods]

Here's the Cython issue for the same thing: cython/cython#7133.

I'm currently unsure if the issue is on PyDev.Debugger's side or on our side.

Note that this occurs just when stepping over the import. I wouldn't particularly expect to be able to step into Cython modules but just importing them while debugging shouldn't be an issue.

[icarocd]

For now I am adding the following environment variables as a workaround to make it work:
FROZENLIST_NO_EXTENSIONS=1
PYDEVD_DISABLE_FILE_VALIDATION=1

[fabioz]

I'm not entirely sure how to reproduce this... Can you elaborate a bit on how to reproduce? -- Is it just importing aiohttp in Python 3.13?

[da-woods]

A simple standalone version

Create a 1-line Cython module:

# cython: linetrace=True

Compile it with Cython 3.1.3 using cythonize -if your_cython_module.pyx.

Create a Python file

import your_cython_module
print("here")

Set a breakpoint on the line print("Here") and run it. You will get the reported crash on the import line.

---

I should say, I've seen and tested this on the version of PyDev debugger that Microsoft have vendored into VSCode, but the gist should be basically the same.

[da-woods]

And yes, use Python 3.13