[Python-Dev] Bytcode "magic tag"

Eric Snow ericsnowcurrently at gmail.com
Wed Oct 28 10:22:19 EDT 2015

On Wed, Oct 28, 2015 at 6:35 AM, Eric V. Smith <eric at 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:


> 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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