Create a script x.py with:

def func():
    constant_obj = b'x'
    runtime_obj = 'x'.encode()
    if constant_obj is not runtime_obj:
        print("marshal failed to get the singleton")
    else:
        print("all good")

func()

The script works as expected when run with ./python x.py since this command doesn't use cached PYC file, it always parse the script:

$ ./python x.py 
all good
$ ./python x.py 
all good

When using ./python -m x instead, the script fails if a cached PYC is loaded instead of parsing the .py source, so the second run:

$ ./python -m x  # no PYC cache
all good
$ ./python -m x  # use PYC cache (loaded by marshal)
marshal failed to get the singleton

Good idea! I add a loop on all marshal versions!

Merged. Thanks for the review @serhiy-storchaka, as usual!

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖

GH-157432 is a backport of this pull request to the 3.14 branch.

GH-157433 is a backport of this pull request to the 3.15 branch.