[Python-Dev] Intention to accept PEP 552 soon (deterministic pyc files)

[Antoine Pitrou]

On Tue, 3 Oct 2017 08:15:04 -0700
Guido van Rossum <gvanrossum at gmail.com> wrote:
> It's really not that hard. You just check the magic number and if it's the
> new one, skip 4 words. No need to understand the internals of the header.

Still, I agree with Barry that an API would be nice.

Regards

Antoine.

> 
> On Oct 3, 2017 08:06, "Barry Warsaw" <barry at python.org> wrote:
> 
> > Guido van Rossum wrote:  
> > > There have been no further comments. PEP 552 is now accepted.
> > >
> > > Congrats, Benjamin! Go ahead and send your implementation for  
> > review.Oops.  
> > > Let me try that again.  
> >
> > While I'm very glad PEP 552 has been accepted, it occurs to me that it
> > will now be more difficult to parse the various pyc file formats from
> > Python.  E.g. I used to be able to just open the pyc in binary mode,
> > read all the bytes, and then lop off the first 8 bytes to get to the
> > code object.  With the addition of the source file size, I now have to
> > (maybe, if I have to also read old-style pyc files) lop off the front 12
> > bytes, but okay.
> >
> > With PEP 552, I have to do a lot more work to just get at the code
> > object.  How many bytes at the front of the file do I need to skip past?
> >  What about all the metadata at the front of the pyc, how do I interpret
> > that if I want to get at it from Python code?
> >
> > Should the PEP 552 implementation add an API, probably to
> > importlib.util, that would understand all current and future formats?
> > Something like this perhaps?
> >
> > class PycFileSpec:
> >     magic_number: bytes
> >     timestamp: Optional[bytes] # maybe an int? datetime?
> >     source_size: Optional[bytes]
> >     bit_field: Optional[bytes]
> >     code_object: bytes
> >
> > def parse_pyc(path: str) -> PycFileSpec:
> >
> > Cheers,
> > -Barry
> >
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>