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Fix annotations support on 3.14 #852

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Merged
merged 1 commit into from
Oct 19, 2025
+69 −6
Merged

Fix annotations support on 3.14 #852

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59 changes: 57 additions & 2 deletions msgspec/_core.c
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Original file line number Diff line number Diff line change
Expand Up @@ -452,6 +452,7 @@ typedef struct {
#endif
PyObject *astimezone;
PyObject *re_compile;
PyObject *get_annotate_from_class_namespace;
uint8_t gc_cycle;
} MsgspecState;

Expand Down Expand Up @@ -5814,12 +5815,45 @@ structmeta_is_classvar(

static int
structmeta_collect_fields(StructMetaInfo *info, MsgspecState *mod, bool kwonly) {
PyObject *annotations = PyDict_GetItemString(
PyObject *annotations = PyDict_GetItemString( // borrowed reference
info->namespace, "__annotations__"
);
if (annotations == NULL) return 0;
if (annotations == NULL) {
if (mod->get_annotate_from_class_namespace != NULL) {
PyObject *annotate = PyObject_CallOneArg(
mod->get_annotate_from_class_namespace, info->namespace
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@JelleZijlstra JelleZijlstra May 26, 2025 •
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This is the portable approach, which should continue to work on future Python versions. If you value performance over portability, you can instead inline this function https://github.com/python/cpython/blob/9ddc7c548d45b73c84131e6d75b03c26a3e8b6e8/Lib/annotationlib.py#L824 ; it's just dict operations.

I made it a separate function in CPython so that we can be free to optimize the internal representation of annotate functions in the future. For example, perhaps in 3.15 the class will store just a code object instead of a function.

);
if (annotate == NULL) {
return -1;
}
if (annotate == Py_None) {
Py_DECREF(annotate);
return 0;
}
PyObject *format = PyLong_FromLong(1); /* annotationlib.Format.VALUE */
if (format == NULL) {
Py_DECREF(annotate);
return -1;
}
annotations = PyObject_CallOneArg(
annotate, format
);
Py_DECREF(annotate);
Py_DECREF(format);
if (annotations == NULL) {
return -1;
}
}
else {
return 0; // No annotations, nothing to do
}
}
else {
Py_INCREF(annotations);
}

if (!PyDict_Check(annotations)) {
Py_DECREF(annotations);
PyErr_SetString(PyExc_TypeError, "__annotations__ must be a dict");
return -1;
}
Expand Down Expand Up @@ -5869,6 +5903,7 @@ structmeta_collect_fields(StructMetaInfo *info, MsgspecState *mod, bool kwonly)
}
return 0;
error:
Py_DECREF(annotations);
Py_XDECREF(module_ns);
return -1;
}
Expand Down Expand Up @@ -22225,6 +22260,26 @@ PyInit__core(void)
Py_DECREF(temp_module);
if (st->re_compile == NULL) return NULL;

/* annotationlib.get_annotate_from_class_namespace */
temp_module = PyImport_ImportModule("annotationlib");
if (temp_module == NULL) {
if (PyErr_ExceptionMatches(PyExc_ModuleNotFoundError)) {
// Below Python 3.14
PyErr_Clear();
st->get_annotate_from_class_namespace = NULL;
}
else {
return NULL;
}
}
else {
st->get_annotate_from_class_namespace = PyObject_GetAttrString(
temp_module, "get_annotate_from_class_namespace"
);
Py_DECREF(temp_module);
if (st->get_annotate_from_class_namespace == NULL) return NULL;
}

/* Initialize cached constant strings */
#define CACHED_STRING(attr, str) \
if ((st->attr = PyUnicode_InternFromString(str)) == NULL) return NULL
Expand Down
16 changes: 12 additions & 4 deletions msgspec/_utils.py
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
# type: ignore
import collections
import inspect
import sys
import typing

Expand Down Expand Up @@ -71,6 +72,13 @@ def _eval_type(t, globalns, localns):
_eval_type = typing._eval_type


if sys.version_info >= (3, 10):
from inspect import get_annotations as _get_class_annotations
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We can't use this module as it introduces a large regression in terms of startup overhead:

❯ docker run --rm -it python:3.13 bash
root@fe224bc3d8c0:/# pip install -qqq msgspec
root@fe224bc3d8c0:/# python -m timeit -n 1 -r 1 "import msgspec"
1 loop, best of 1: 11.7 msec per loop
root@fe224bc3d8c0:/# python -m timeit -n 1 -r 1 "import inspect"
1 loop, best of 1: 7.84 msec per loop

It's worth noting that as of 3.14 this function moved to annotationlib (which your C code imports directly) and inspect re-exports it. That module loads twice as fast but ideally we wouldn't incur any unnecessary overhead:

❯ docker run --rm -it python:3.14 bash
root@8130942e2641:/# python -m timeit -n 1 -r 1 "import annotationlib"
1 loop, best of 1: 3.34 msec per loop

How would you recommend we proceed? I'm going to merge but this will be considered a known regression and will have to be fixed in a patch release right after.

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Discussion in #880

else:
def _get_class_annotations(cls):
return cls.__dict__.get("__annotations__", {})


def _apply_params(obj, mapping):
if isinstance(obj, typing.TypeVar):
return mapping.get(obj, obj)
Expand Down Expand Up @@ -149,17 +157,17 @@ def get_class_annotations(obj):
cls_locals = dict(vars(cls))
cls_globals = getattr(sys.modules.get(cls.__module__, None), "__dict__", {})

ann = cls.__dict__.get("__annotations__", {})
ann = _get_class_annotations(cls)
for name, value in ann.items():
if name in hints:
continue
if value is None:
value = type(None)
elif isinstance(value, str):
if isinstance(value, str):
value = _forward_ref(value)
value = _eval_type(value, cls_locals, cls_globals)
if mapping is not None:
value = _apply_params(value, mapping)
if value is None:
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I changed _eval_type so it no longer turns None into NoneType.

value = type(None)
hints[name] = value
return hints

Expand Down