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Description of problem:
Less inscrutably: rebuild of rpm package of rpm (more precisely of rpm-python,
but the source package is simply `rpm') fails because it tries to install the
Python module according to PYTHONPATH (or PYTHONHOME, or maybe others).

This is because python/Makefile.am uses

  pylibdir = $(shell python -c 'import sys; print sys.path[1]')

to find where the python module should be installed. When the package is built
by someone who has PYTHONPATH set to his/her personal Python module tree, the
first path in PYTHONPATH becomes sys.path[1], and the module is not installed to
$RPM_BUILDROOT/lib/python..., but somewhere into the deep space, and the package
build fails.

Version-Release number of selected component (if applicable):
rpm-4.2-0.69

How reproducible:
Always

Steps to Reproduce:
1. export PYTHONPATH=$HOME
2. rpmbuild --rebuild rpm-4.2-0.69.src.rpm


Additional info:

There are several possible fixes:

1. unset all the PYTHON* environment variables in the spec file.  This works but
is ugly.

2a. use

  pylibdir = $(shell python -E -c 'import sys; print sys.path[1]')

in python/Makefile.am.  This however may break builds when one really wants to
install rpm somewhere to home.

2b. patch python/Makefile.in in the spec file to use the -E option. Normal
builds then use PYTHONPATH, PYTHONHOME, etc., but rpm package builds are not
affected by it.