def test_xspecvar_zero_handling(bexp, yexp, dyexp):
"""How does XSPEC variance handle 0 in source and/or background?
The values were calculated using XSPEC 12.10.1m (HEASOFT 6.26.1)
using the following commands to create the file foo.dat which
contains (after three 'header' lines) the data 'x 0.5 y dy'
data foo.fits
iplot data
wplot foo.dat
quit
where foo.fits is a fake PHA file set up to have the channel/count
values used below (a CSC-style PHA file was used so that source
and background were in the same file but a separate bgnd PHA
file could also have been used).
"""
stat = Chi2XspecVar()
chans = numpy.arange(1, 10, dtype=numpy.int16)
scnts = numpy.asarray([0, 0, 0, 1, 3, 1, 1, 3, 3], dtype=numpy.int16)
bcnts = numpy.asarray([0, 1, 3, 0, 0, 1, 3, 1, 3], dtype=numpy.int16)
s = DataPHA('src', chans, scnts, exposure=1)
b = DataPHA('bkg', chans, bcnts, exposure=bexp)
s.set_background(b)
s.subtract()
y, dy, other = s.to_fit(staterrfunc=stat.calc_staterror)
assert other is None
assert y == pytest.approx(yexp)
assert dy == pytest.approx(dyexp)
