Bytecode compilation output depends on order of files compiled

Bug report

Bug description:

This is minimal reproduction of this downstream bug report: https://github.com/astral-sh/uv/issues/10619

The output of compileall.compile_file depends on the order in which the files are compiled. This means compilation is non-deterministic if builds are distributed over a process pool.

This becomes a problem when building docker images, where you usually bytecode compile ahead of time for faster startup, and where the hash of the image depends on all files in the image, including the .pyc files.

Specifically, the output of

a = {"foo", 2, 3}

def f():
    b = {"foo", 2, 3}

is different if we previously compiled another file with

import foo

Reproducer script:

#!/bin/bash

set -e

script=$(cat << EOF
import compileall
import sys

for path in sys.argv[1:]:
    compileall.compile_file(path)
EOF
)

cat << EOF > a.py
import foo
EOF

cat << EOF > b.py
a = {"foo", 2, 3}

def f():
    b = {"foo", 2, 3}
EOF

# Both files
rm -rf __pycache__
python3.14 -c "$script" a.py b.py
sha256sum __pycache__/b.cpython-314.pyc

# For debugging
cp __pycache__/b.cpython-314.pyc b1.cpython-314.pyc

# Single file only
rm -rf __pycache__
python3.14 -c "$script" b.py
sha256sum __pycache__/b.cpython-314.pyc

# For debugging
cp __pycache__/b.cpython-314.pyc b2.cpython-314.pyc

This is caused be different refcounts in the marshalled files:

import marshal
import sys

with open("b1.cpython-313.pyc", "rb") as f:
  f.read(16)  # Skip header
  pyc1 = marshal.load(f)

with open("b2.cpython-313.pyc", "rb") as f:
  f.read(16)  # Skip header
  pyc2 = marshal.load(f)

print(sys.getrefcount(pyc1.co_consts[0]))
print(sys.getrefcount(pyc2.co_consts[0]))

This prints 2 and 3.

The original report is from 3.13, i've reproduced it with 3.14.0a4. It happens at least on linux and windows.

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-156862

Start with the provided compileall.compile_file reproducer and compare the generated .pyc hashes and unmarshalled constants when compiling a.py before b.py versus b.py alone. Trace the bytecode compilation and marshal output paths to determine why compilation order changes refcounts. Done means equivalent source produces deterministic .pyc output regardless of compilation order, with regression coverage for the reported cases.