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

Barry Warsaw barry at python.org
Tue Oct 3 11:03:30 EDT 2017

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:


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