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Comment by mbakke | Hacker News Reader

Typically part of a "version string":

    $ python3
    Python 3.10.7 (main, Jan  1 1970, 00:00:01) [GCC 11.3.0] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    >>>

Perhaps a relic from when software had to be manually updated?

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On NixOS, I think the release time or commit time is used:

    $ python3
    Python 3.10.11 (main, Apr  4 2023, 22:10:32) [GCC 12.2.0] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    >>>

That is more useful than the build time.

How is that possible? Is nixpkgs an input to the Python derivation? Or do packagers "hard code" a value every time they modify the Python build code? Automated tooling that sets it after pull requests? Something else? :-)

GCC respects SOURCE_DATE_EPOCH, and Nixpkgs has specific support for setting that environment variable: https://github.com/NixOS/nixpkgs/blob/92fdbd284c262f3e478033... (although I haven't proved that this is actually how it works for cpython's build).

Irrelevant spelunking details follow:

That string is output by cpython to contain the contents of the __DATE__ C macro (https://github.com/python/cpython/blob/fa35b9e89b2e207fc8bae... which calls to https://github.com/python/cpython/blob/fa35b9e89b2e207fc8bae... which uses the __DATE__ macro at https://github.com/python/cpython/blob/fa35b9e89b2e207fc8bae... ).

Cpython is defined in nixpkgs at https://github.com/NixOS/nixpkgs/blob/92fdbd284c262f3e478033... which I imagine (but haven't proved) uses GCC.

Thank you! Setting SOURCE_DATE_EPOCH to the most recent file timestamp found in the source input is a clever hack.

2023-04-04T22:10:32 is the timestamp of Python-3.10.11/Misc/NEWS from https://www.python.org/ftp/python/3.10.11/Python-3.10.11.tar...

The source for the cpython build is the release tarball (https://github.com/NixOS/nixpkgs/blob/master/pkgs/developmen...).

In that case, NixOS sets SOURCE_DATE_EPOCH (which I suspect will be picked up by the python build) to the latest timestamp found in that archive (https://github.com/NixOS/nixpkgs/blob/master/pkgs/build-supp...)

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