On Wed, Oct 28, 2015 at 6:35 AM, Eric V. Smith <eric@trueblade.com> wrote:
In issue 25483 I'm adding an opcode to make f-string formatting more robust and faster. As part of that, I'm bumping the .pyc magic number.
While doing that, I notice Lib/importlib/_bootstrap_external.h includes this comment:
# Starting with the adoption of PEP 3147 in Python 3.2, every bump in magic # number also includes a new "magic tag", i.e. a human readable string used # to represent the magic number in __pycache__ directories. When you change # the magic number, you must also set a new unique magic tag. Generally this # can be named after the Python major version of the magic number bump, but # it can really be anything, as long as it's different than anything else # that's come before. The tags are included in the following table, starting # with Python 3.2a0.
The "following table" is a comment, that contains a few references to the tag "cpython-<version>", specifically cpython-32. It doesn't seem that the tag is routinely updated in the comment.
sys.implementation.cache_tag returns 'cpython-36', and is in fact implemented as 'cpython-{PY_MAJOR_VERSION}{PY_MINOR_VERSION}'.
Do I need to do anything else? Unlike what the comment in _boostrap_external.py suggests, this "magic tag" will not change every time a bytecode is added, but only on every minor release. Which implies that if we have a micro release that adds an opcode, we'll in fact break the promise in the comment.
You will want to bump the number on the following line: https://hg.python.org/cpython/file/default/Lib/importlib/_bootstrap_external... -eric
From my understanding on how this tag is used, this wouldn't be a problem (because the magic number in the file also changes). But I want to make sure I'm not misunderstanding something. I think the comment above is probably just misleading.
Eric.
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