On Oct 3, 2017 9:55 AM, "Serhiy Storchaka" <storchaka@gmail.com> wrote: While PEP 552 is accepted, I would want to see some changes. 1. Increase the size of the constant part of the signature to at least 32 bits. Currently only the third and forth bytes are constant, and they are '\r\n', that is often occurred in text files. The first two bytes can be different in every Python version. This make hard detecting pyc files by utilities like file (1). 2. Split the "version" of pyc files by "major" and "minor" parts. Every major version is incompatible with other major versions, the interpreter accepts only one particular major version. It can't be changed in a bugfix release. But all minor versions inside the same major version are forward and backward compatible. The interpreter should be able to execute pyc file with arbitrary minor version, but it can use minor version of pyc file to handle errors in older versions. Minor version can be changed in a bugfix release. I hope this can help us with issues like https://bugs.python.org/issue29537. Currently 3.5 supports two magic numbers. If we change the pyc format, it would be easy to make the above changes. IIUC the PEP doesn't commit to any particular magic word format, so this can be negotiated separately, on the tracker (unless there's a PEP specifying the internal structure of the magic word?). --Guido