msgspec tests crash with PYTHON_LAZY_IMPORTS=all on 3.15

On the latest 3.15-dev I am using: PYTHON_LAZY_IMPORTS=all pytest -o testpaths=tests/unit --assert=plain -vv

It produces:

Current thread 0x000000020e02a0c0 (most recent call first):
  File "<string>", line 6 in <module>
  File "/Users/sobolev/Desktop/msgspec/tests/unit/utils.py", line 20 in temp_module
  File "/Users/sobolev/.pyenv/versions/3.15-dev/lib/python3.15/contextlib.py", line 203 in __enter__
  File "/Users/sobolev/Desktop/msgspec/tests/unit/test_struct.py", line 2608 in test_classvar
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/_pytest/python.py", line 167 in pytest_pyfunc_call
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/pluggy/_callers.py", line 121 in _multicall
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/pluggy/_manager.py", line 120 in _hookexec
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/pluggy/_hooks.py", line 512 in __call__
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/_pytest/python.py", line 1707 in runtest
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/_pytest/runner.py", line 184 in pytest_runtest_call
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/pluggy/_callers.py", line 121 in _multicall
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/pluggy/_manager.py", line 120 in _hookexec
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/pluggy/_hooks.py", line 512 in __call__
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/_pytest/runner.py", line 250 in <lambda>
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/_pytest/runner.py", line 361 in from_call
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/_pytest/runner.py", line 249 in call_and_report
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/_pytest/runner.py", line 139 in runtestprotocol
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/_pytest/runner.py", line 118 in pytest_runtest_protocol
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/pluggy/_callers.py", line 121 in _multicall
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/pluggy/_manager.py", line 120 in _hookexec
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/pluggy/_hooks.py", line 512 in __call__
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/_pytest/main.py", line 408 in pytest_runtestloop
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/pluggy/_callers.py", line 121 in _multicall
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/pluggy/_manager.py", line 120 in _hookexec
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/pluggy/_hooks.py", line 512 in __call__
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/_pytest/main.py", line 384 in _main
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/_pytest/main.py", line 330 in wrap_session
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/_pytest/main.py", line 377 in pytest_cmdline_main
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/pluggy/_callers.py", line 121 in _multicall
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/pluggy/_manager.py", line 120 in _hookexec
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/pluggy/_hooks.py", line 512 in __call__
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/_pytest/config/__init__.py", line 229 in _main
  File "/Users/sobolev/Desktop/msgspec/.venv/lib/python3.15/site-packages/_pytest/config/__init__.py", line 253 in _console_main
  File "/Users/sobolev/Desktop/msgspec/.venv/bin/pytest", line 10 in <module>

Current thread's C stack trace (most recent call first):
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _Py_DumpStack+0x44 [0x1030620c4]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at faulthandler_fatal_error+0x210 [0x10307c370]
  Binary file "/usr/lib/system/libsystem_platform.dylib", at _sigtramp+0x38 [0x1a00d16a4]
  Binary file "/Users/sobolev/Desktop/msgspec/src/msgspec/_core.cpython-315-darwin.so", at structmeta_is_classvar+0x1c4 [0x10424ad3c]
  Binary file "/Users/sobolev/Desktop/msgspec/src/msgspec/_core.cpython-315-darwin.so", at structmeta_collect_fields+0x470 [0x104248de0]
  Binary file "/Users/sobolev/Desktop/msgspec/src/msgspec/_core.cpython-315-darwin.so", at StructMeta_new_inner+0x42c [0x104247bf8]
  Binary file "/Users/sobolev/Desktop/msgspec/src/msgspec/_core.cpython-315-darwin.so", at StructMeta_new+0x20c [0x1042900e4]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at type_call+0x54 [0x102f0eb70]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _PyObject_MakeTpCall+0x130 [0x102e7a2fc]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at builtin___build_class__+0x640 [0x102fc120c]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at PyObject_Vectorcall+0x58 [0x102e7aa54]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _Py_VectorCallInstrumentation_StackRefSteal+0x12c [0x102fc6ed4]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _PyEval_EvalFrameDefault+0x19f4 [0x102fc9670]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at PyEval_EvalCode+0x1ac [0x102fc6734]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at run_mod+0x278 [0x103048fc8]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _PyRun_StringFlagsWithName+0xfc [0x10304702c]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at builtin_exec+0x59c [0x102fc2e3c]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _PyEval_EvalFrameDefault+0x2888 [0x102fca504]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at gen_send_ex+0x214 [0x102e97e40]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at gen_iternext+0x1c [0x102e96698]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at builtin_next+0x64 [0x102fc3cc8]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _PyEval_EvalFrameDefault+0x25f4 [0x102fca270]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _PyEval_Vector+0x20c [0x102fc697c]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _PyObject_VectorcallPrepend+0xa8 [0x102e7bc34]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _PyVectorcall_Call+0x74 [0x102e7a920]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _PyEval_EvalFrameDefault+0x2b70 [0x102fca7ec]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _PyEval_Vector+0x20c [0x102fc697c]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _PyObject_VectorcallDictTstate+0x90 [0x102e7a0ac]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _PyObject_Call_Prepend+0x98 [0x102e7b0a8]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at slot_tp_call+0x7c [0x102f168fc]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _PyObject_MakeTpCall+0x130 [0x102e7a2fc]
  Binary file "/Users/sobolev/.pyenv/versions/3.15-dev/lib/libpython3.15.dylib", at _Py_VectorCall_StackRefSteal+0x114 [0x102fc6bb0]
  <truncated rest of calls>

Extension modules: msgspec._core, msgpack._cmsgpack (total: 2)
[1]    67332 segmentation fault  PYTHON_LAZY_IMPORTS=all pytest -o testpaths=tests/unit --assert=plain

The offending test: tests/unit/test_struct.py::TestClassVar::test_classvar[True-2]

msgspec/tests/unit/test_struct.py

Lines 2602 to 2610 in e18fab4

    
           @pytest.mark.parametrize("case", [1, 2, 3, 4, 5]) 
        
           @pytest.mark.parametrize("future_annotations", [True, False]) 
        
           def test_classvar(self, case, future_annotations): 
        
               source = getattr(self, f"case{case}")() 
        
               if future_annotations: 
        
                   source = "        from __future__ import annotations\n" + source 
        
               with temp_module(source) as mod: 
        
                   assert mod.Ex.__struct_fields__ == ("a", "b") 
        
                   assert mod.Ex.cv2 == 1

Not sure for now what is crashing: msgspec or CPython :)

This is a msgspec issue:

Repro:

# ex.py
from __future__ import annotations
import typing
from msgspec import Struct

class Ex(Struct):
    a: int
    cv1: typing.ClassVar
    b: int
    cv2: typing.ClassVar[int] = 1

Output after my initial fix:

» PYTHON_LAZY_IMPORTS=all python ex.py
Traceback (most recent call last):
  File "/Users/sobolev/Desktop/msgspec/ex.py", line 5, in <module>
    class Ex(Struct):
    ...<3 lines>...
        cv2: typing.ClassVar[int] = 1
AttributeError: 'lazy_import' object has no attribute 'ClassVar'

There's no clear API to work with lazy imports from the C code 🙈

I can't even use PyLazyImport_CheckExact or PyLazyImport_Type because they are internal: https://github.com/python/cpython/blob/8270ae560c632f2ee88ab6c6b33562227d0deaae/Include/internal/pycore_lazyimportobject.h

So, I would suggest using PyImport_ImportModule instead of

if (*module_ns == NULL) {
    *module_ns = structmeta_get_module_ns(mod, info);
}
if (*module_ns == NULL) return 0;
PyObject *temp = PyDict_GetItemString(*module_ns, "typing");

there's no other way, as far as I understand 🤔

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`msgspec` tests crash with `PYTHON_LAZY_IMPORTS=all` on 3.15 #1095

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	@pytest.mark.parametrize("case", [1, 2, 3, 4, 5])
	@pytest.mark.parametrize("future_annotations", [True, False])
	def test_classvar(self, case, future_annotations):
	source = getattr(self, f"case{case}")()
	if future_annotations:
	source = " from __future__ import annotations\n" + source
	with temp_module(source) as mod:
	assert mod.Ex.__struct_fields__ == ("a", "b")
	assert mod.Ex.cv2 == 1

msgspec tests crash with PYTHON_LAZY_IMPORTS=all on 3.15 #1095

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sobolevn commented on Jun 21, 2026

sobolevn commented on Jun 21, 2026

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`msgspec` tests crash with `PYTHON_LAZY_IMPORTS=all` on 3.15 #1095