When trying to cross compile python3.9, `configure` attempts to find a strict python 3.9 version match, however if it fails it still attempts to use `python` in PYTHON_FOR_BUILD instead of failing outright like the code implies it should

$/python3/targetbuild# which python3.9 python3 python
/usr/bin/python3

$/python3/targetbuild# python3 --version
Python 3.7.3

$/python3/targetbuild# PYTHON_FOR_REGEN=/python3/hostbuild/python \
ac_cv_file__dev_ptmx=yes \
ac_cv_file__dev_ptc=no \
ac_cv_buggy_getaddrinfo=no \
../configure --build=x86-linux-gnu \
	--host=aarch64-linux-gnu \
	--enable-loadable-sqlite-extensions \
	--enable-option-checking=fatal \
	--enable-shared \
	--with-system-expat \
	--with-system-ffi \
	--without-ensurepip
checking build system type... x86-unknown-linux-gnu
checking host system type... aarch64-unknown-linux-gnu
checking for python3.9... /python3/hostbuild/python
checking for python interpreter for cross build... python
...

$/python3/targetbuild# grep PYTHON_FOR_BUILD config.log 
PYTHON_FOR_BUILD='_PYTHON_PROJECT_BASE=$(abs_builddir) _PYTHON_HOST_PLATFORM=$(_PYTHON_HOST_PLATFORM) PYTHONPATH=$(shell test -f pybuilddir.txt && echo $(abs_builddir)/`cat pybuilddir.txt`:)$(srcdir)/Lib _PYTHON_SYSCONFIGDATA_NAME=_sysconfigdata_$(ABIFLAGS)_$(MACHDEP)_$(MULTIARCH) python'


in configure

if test "$cross_compiling" = yes; then
    AC_MSG_CHECKING([for python interpreter for cross build])
    if test -z "$PYTHON_FOR_BUILD"; then
        for interp in python$PACKAGE_VERSION python3 python; do
            which $interp >/dev/null 2>&1 || continue
            if $interp -c "import sys;sys.exit(not '.'.join(str(n) for n in sys.version_info@<:@:2@:>@) == '$PACKAGE_VERSION')"; then
                break
            fi
            interp=
        done
        if test x$interp = x; then
            AC_MSG_ERROR([python$PACKAGE_VERSION interpreter not found])
        fi
        AC_MSG_RESULT($interp)
        PYTHON_FOR_BUILD='_PYTHON_PROJECT_BASE=$(abs_builddir) _PYTHON_HOST_PLATFORM=$(_PYTHON_HOST_PLATFORM) PYTHONPATH=$(shell test -f pybuilddir.txt && echo $(abs_builddir)/`cat pybuilddir.txt`:)$(srcdir)/Lib _PYTHON_SYSCONFIGDATA_NAME=_sysconfigdata_$(ABIFLAGS)_$(MACHDEP)_$(MULTIARCH) '$interp
    fi
elif test "$cross_compiling" = maybe; then
    AC_MSG_ERROR([Cross compiling required --host=HOST-TUPLE and --build=ARCH])
else
    PYTHON_FOR_BUILD='./$(BUILDPYTHON) -E'
fi
AC_SUBST(PYTHON_FOR_BUILD)


The issue is a failing edge case here:

        for interp in python$PACKAGE_VERSION python3 python; do
            which $interp >/dev/null 2>&1 || continue

where interp keeps it's last value doesn't trigger the empty check here:

        if test x$interp = x; then
            AC_MSG_ERROR([python$PACKAGE_VERSION interpreter not found])
        fi

Note that there's an explicit clearing of interp when the python version isn't a match:

            if $interp -c "import sys;sys.exit(not '.'.join(str(n) for n in sys.version_info@<:@:2@:>@) == '$PACKAGE_VERSION')"; then
                break
            fi
            interp=


The fix should be pretty straightforward:

        for interp in python$PACKAGE_VERSION python3 python ''; do

adding '' as the last possible interpreter means one hasn't been found up to that point and allows configure to properly fail

$/python3/targetbuild# PYTHON_FOR_REGEN=/python3/hostbuild/python ac_cv_file__dev_ptmx=yes ac_cv_file__dev_ptc=no ac_cv_buggy_getaddrinfo=no ../configure --build=x86-linux-gnu --host=aarch64-linux-gnu --enable-loadable-sqlite-extensions --enable-option-checking=fatal --enable-shared --with-system-expat --with-system-ffi --without-ensurepip
checking build system type... x86-unknown-linux-gnu
checking host system type... aarch64-unknown-linux-gnu
checking for python3.9... /python3/hostbuild/python
checking for python interpreter for cross build... configure: error: python3.9 interpreter not found

It will continue to work when a proper interpreter is found

$/python3/targetbuild# PATH=/python3/hostbuild:$PATH PYTHON_FOR_REGEN=/python3/hostbuild/python ac_cv_file__dev_ptmx=yes ac_cv_file__dev_ptc=no ac_cv_buggy_getaddrinfo=no ../configure --build=x86-linux-gnu --host=aarch64-linux-gnu --enable-loadable-sqlite-extensions --enable-option-checking=fatal --enable-shared --with-system-expat --with-system-ffi --without-ensurepip
checking build system type... x86-unknown-linux-gnu
checking host system type... aarch64-unknown-linux-gnu
checking for python3.9... /python3/hostbuild/python
checking for python interpreter for cross build... Could not find platform dependent libraries <exec_prefix>
Consider setting $PYTHONHOME to <prefix>[:<exec_prefix>]
python


This should help highlight any inconsistent environment between configuring and building.