Does “${python3:Depends}” reliably generate correct dependencies?

[Ben Finney <ben+debian@benfinney.id.au>]

Howdy all,

If we set “Depends: ${python3:Depends}” in the binary package, and use
‘dh_python3’, is that all that is needed to ensure the correct
dependencies on Python 3 versions?

Recently I received this bug report (Bug#802889):

    please don't depend on all python3 versions, forcing the
    installation of all python versions shouldn't be enforced for a
    module.

    $ apt-cache show python3-coverage | grep Depends
    Depends: python3-pkg-resources, python3 (<< 3.6), python3 (>= 3.4~),
    python3.4, python3.5, libc6 (>= 2.4)

    <URL:https://bugs.debian.org/802889>

So far as I can tell, these dependencies are generated because
“Depends: […] ${python3:Depends}” is in that “Package” definition.

On a Debian Sid system today, building a package for Python 3, along
with its extension modules, means building it for all supported Python 3
versions. Are the above dependencies wrong for that?

If the dependencies are incorrect, what should be used in the Depends
field instead of “${python3:Depends}”?

If the dependencies are correct, exactly what should I say in
explanation to the reporter of Bug#802889?

-- 
 \         “Smoking cures weight problems. Eventually.” —Steven Wright |
  `\                                                                   |
_o__)                                                                  |
Ben Finney