pbuilder and debhelper: Test suite run before dependencies satisfied

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

Howdy mentors,

Why does 'dh build' attempt to run the package's test suite, but
doesn't satisfy the build dependencies before doing so? How can this
be made to work within a 'pbuilder' environment?

I'm packaging a Python application that has a unit test suite. This
unit test suite, naturally, imports each of the modules in the
application in order to test them. That, in turn, causes any of the
'import bar' statements in the application to be executed. So, the
unit test suite will fail in the absence of any one of the package's
Python library dependencies.

Yet, when I attempt to 'pdebuild' this package, it runs the 'build'
target of 'debian/rules', which runs 'dh build', which runs
'dh_auto_test', which runs the application unit test suite. At this
point in the process, though, the pbuilder environment does not have
the package dependencies installed, so the unit test suite fails,
failing the 'pdebuild'.

Running 'debian/rules build' shouldn't need the package dependencies
installed, because the package isn't being installed yet, only built
into a Debian binary package. So why is the package unit test suite
run from 'dh build'?

How is this supposed to work?

-- 
 \            “Beware of bugs in the above code; I have only proved it |
  `\                 correct, not tried it.” —Donald Knuth, 1977-03-29 |
_o__)                                                                  |
Ben Finney