If the package is available as a wheel, you don't need to execute code to see what the requirements are; you just need to parse the "METADATA" file. However, the only way to get the METADATA for a wheel (using PyPA standard APIs, anyway) is to download the whole wheel.

For comparison, pacman (the Arch Linux package manager) packages have fairly similar ".PKGINFO" file in them; but in order to support resolving dependencies without downloading the packages, the server's repository index includes not just a listing of the (name, version) tuple for each package, it also includes each package's full .PKGINFO.

Enhancing the PyPA "Simple repository API" to allow fetching the METADATA independently of the wheel would be a relatively simple enhancement that would make a big difference.

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As I was writing this comment, I discovered that PyPA did this; they adopted PEP 658 in March of this year! https://github.com/pypa/packaging.python.org/commit/1ebb57b7...