The format of the header of pyc files was stable for long time and
changed only few times. First time it was changed in 3.3: added the size
of the corresponding source mod 2**32. [1] Second time it was changed
in 3.7: added the 32-bit flags field and support of hash-based pyc files
(PEP 552). [2] [3]
I think that it is worth to make more changed.
1. More stable file signature. Currently the magic number is changed in
every feature release. Only the third and the forth bytes are stable
(b'\r\n'), the first bytes are changed non-predicable. The 'py' launcher
and third-party software like the 'file' command should support the list
of magic numbers for all existing Python releases, and they can't detect
pyc file for future versions. There is also a chance the pyc file
signature will match the signature of other file type by accident. It
would be better if the first 4 bytes of pyc files be same for all Python
versions (or at least for all Python versions with the same major number).
2. Include the Python version. Currently the 'py' launcher needs to
support the table that maps magic numbers to Python version. It can
recognize only Python versions released before building the launcher. If
the two major numbers of Python version be included in the version, it
would not need such table.
3. The number of compatible subversion. Currently the interpreter
supports only a single magic number. If the updated version of the
compiler produces more optimal or more correct but compatible bytecode
(like ), there is no way to say that the new bytecode is preferable, but
the old bytecode can be used too. Changing the magic number causes
invalidating all pyc files compiled by the old compiler (see [4] for the
example of problems caused by this). The header could contain two magic
numbers: the major magic number should be bumped for incompatible
changes, the minor magic number should be reset to 0 when the major
magic number is bumped, and should be bumped when the compiler become
producing different but compatible bytecode. If the import system reads
the pyc file with the minor magic number equal or greater than current,
it just uses the pyc file. If it reads the pyc file with the minor magic
number lesser than current, it can regenerate the pyc file if it is
writeable. And the compileall module should regenerate all pyc files
with minor magic numbers lesser than current.
[1] https://bugs.python.org/issue13645
[2] https://bugs.python.org/issue31650
[3] http://www.python.org/dev/peps/pep-0552/
[4] https://bugs.python.org/issue27286
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