Assertion `year >= 1' failed in 'days_before_year'` when unpickling datetime with year 0

Start in Modules/_datetimemodule.c, where datetime objects are reconstructed from pickle state, and inspect the related datetime tests. Reproduce the report with the supplied corrupted pickle bytes in a development build, then check that loading rejects year 0 with the expected ValueError while valid dates still work.

Crash report

_datetimemodule.c does not validate that MINYEAR <= year <= MAXYEAR when reconstructing a datetime.datetime object from serialized bytes (unpickling).

This allows instantiating an invalid datetime object with year == 0. Subsequent calls to methods such as .toordinal() hit the C-level assertion assert(year >= 1) in days_before_year(), crashing the interpreter process with SIGABRT in debug / -X dev builds.

Steps to Reproduce

Run the following script with ./python -X dev:

import datetime
import pickle

dt = datetime.datetime(2023, 10, 1)
data = pickle.dumps(dt)

# Replace year 2023 (0x07E7) with 0 (0x0000)
corrupted_data = data.replace(b'\x07\xe7', b'\x00\x00')

obj = pickle.loads(corrupted_data)
obj.toordinal()

Actual Behavior

The process aborts due to an assertion failure:

python: ./Modules/_datetimemodule.c:477: int days_before_year(int): Assertion `year >= 1' failed.
Fatal Python error: Aborted

Current thread 0x00007f8943602400 [python] (most recent call first):
  File "<python-input-0>", line 11 in <module>
  File "/home/boss/projects/oss/cpython/main/Lib/_pyrepl/console.py", line 231 in runcode
  File "/home/boss/projects/oss/cpython/main/Lib/_pyrepl/console.py", line 288 in runsource
  File "/home/boss/projects/oss/cpython/main/Lib/code.py", line 324 in push
  File "/home/boss/projects/oss/cpython/main/Lib/_pyrepl/simple_interact.py", line 149 in run_multiline_interactive_console
  File "/home/boss/projects/oss/cpython/main/Lib/_pyrepl/main.py", line 58 in interactive_console

Current thread's C stack trace (most recent call first):
  Binary file "./python", at __interceptor_backtrace+0x4c [0x55a217378b2c]
  Binary file "./python", at _Py_DumpStack+0x10c [0x55a21803e9c4]
  Binary file "./python", at +0x1a523c1 [0x55a2180b83c1]
  Binary file "/usr/lib/libc.so.6", at +0x3e6f0 [0x7f894323e6f0]
  Binary file "/usr/lib/libc.so.6", at +0x9a11c [0x7f894329a11c]
  Binary file "/usr/lib/libc.so.6", at gsignal+0x20 [0x7f894323e5d0]
  Binary file "/usr/lib/libc.so.6", at abort+0x26 [0x7f8943225685]
  Binary file "/usr/lib/libc.so.6", at +0x26704 [0x7f8943226704]
  Binary file "/usr/lib/libc.so.6", at __assert_fail+0xc2 [0x7f89432361f2]
  Binary file "./python", at +0x1ab4039 [0x55a21811a039]
  Binary file "./python", at +0x115f759 [0x55a2177c5759]
  Binary file "./python", at +0x111c356 [0x55a217782356]
  Binary file "./python", at _Py_VectorCallInstrumentation_StackRefSteal+0x180 [0x55a217c96a58]
  Binary file "./python", at +0x1671568 [0x55a217cd7568]
  Binary file "./python", at _PyEval_EvalFrameDefault+0x4a1 [0x55a217c9a059]
  Binary file "./python", at PyEval_EvalCode+0x2d9 [0x55a217c956e1]
  Binary file "./python", at +0x1621beb [0x55a217c87beb]
  Binary file "./python", at _Py_BuiltinCallFastWithKeywords_StackRef+0x155 [0x55a217c9773d]
  Binary file "./python", at +0x16bcdf4 [0x55a217d22df4]
  Binary file "./python", at _PyEval_EvalFrameDefault+0x4a1 [0x55a217c9a059]
  Binary file "./python", at +0x162fc40 [0x55a217c95c40]
  Binary file "./python", at +0x111f20a [0x55a21778520a]
  Binary file "./python", at +0x1a1c6ca [0x55a2180826ca]
  Binary file "./python", at +0x1a1ae13 [0x55a218080e13]
  Binary file "./python", at Py_RunMain+0xd79 [0x55a21807f051]
  Binary file "./python", at +0x1a19cf9 [0x55a21807fcf9]
  Binary file "./python", at Py_BytesMain+0xd5 [0x55a21807ffbd]
  Binary file "/usr/lib/libc.so.6", at +0x27781 [0x7f8943227781]
  Binary file "/usr/lib/libc.so.6", at __libc_start_main+0x89 [0x7f89432278b9]
  Binary file "./python", at _start+0x25 [0x55a217328fa5]
fish: Job 1, './python -X dev' terminated by signal SIGABRT (Abort)

Expected Behavior

pickle.loads() should validate year against MINYEAR (1) and MAXYEAR (9999) during state unpacking and raise ValueError: year 0 is out of range, consistent with the standard datetime(year, month, day) constructor.

Environment

• CPython version: 3.16.0a1+dev (heads/main:ad101c3b307)
• OS: Linux
• Build: CC=clang CXX=clang++ LDFLAGS='-fuse-ld=lld' ./configure --with-tail-call-interp --with-pydebug --with-address-sanitizer --with-undefined-behavior-sanitizer

CPython versions tested on:

CPython main branch, 3.16, 3.15, 3.14, 3.13, 3.12, 3.11

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.16.0a1+dev (heads/main:ad101c3b307, Oct 22 2026, 23:57:40) [Clang 23.1.3 ]