[Python-ideas] Add more information in the header of pyc files

[Serhiy Storchaka]

10.04.18 18:58, Antoine Pitrou пише:
> On Tue, 10 Apr 2018 18:49:36 +0300
> Serhiy Storchaka <storchaka at gmail.com>
> wrote:
>> 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.
> 
> -1.  This is a risky move (and costly, in maintenance terms).  It's easy
> to overlook subtle differencies that may translate into
> incompatibilities in some production uses.  The rule « one Python
> feature release == one bytecode version » is easy to remember and
> understand, and is generally very well accepted.

A bugfix release can fix bugs in bytecode generation. See for example 
issue27286. [1]  The part of issue33041 backported to 3.7 and 3.6 is an 
other example. [2]  There were other examples of compatible changing the 
bytecode. Without bumping the magic number these fixes can just not have 
any effect if existing pyc files were generated by older compilers. But 
bumping the magic number in a bugfix release can lead to rebuilding 
every pyc file (even unaffected by the fix) in distributives.

[1] https://bugs.python.org/issue27286
[2] https://bugs.python.org/issue33041