# Licensed under a 3-clause BSD style license - see LICENSE.rst
"""Units and Quantity related helper functions"""
from __future__ import absolute_import, division, print_function, unicode_literals
import logging
import astropy.units as u
__all__ = ["standardise_unit", "unit_from_fits_image_hdu"]
log = logging.getLogger("__name__")
[docs]def standardise_unit(unit):
"""Standardise unit.
Changes applied by this function:
* Drop "photon" == "ph" from the unit
* Drop "count" == "ct" from the unit
Parameters
----------
unit : `~astropy.units.Unit` or str
Any old unit
Returns
-------
unit : `~astropy.units.Unit`
Shiny new, standardised unit
Examples
--------
>>> from gammapy.utils.units import standardise_unit
>>> standardise_unit('ph cm-2 s-1')
Unit("1 / (cm2 s)")
>>> standardise_unit('ct cm-2 s-1')
Unit("1 / (cm2 s)")
>>> standardise_unit('cm-2 s-1')
Unit("1 / (cm2 s)")
"""
unit = u.Unit(unit)
bases, powers = [], []
for base, power in zip(unit.bases, unit.powers):
if str(base) not in {"ph", "ct"}:
bases.append(base)
powers.append(power)
return u.CompositeUnit(scale=unit.scale, bases=bases, powers=powers)
[docs]def unit_from_fits_image_hdu(header):
"""Read unit from a FITS image HDU.
- The ``BUNIT`` key is used.
- `astropy.units.Unit` is called.
If the ``BUNIT`` value is invalid, a log warning
is emitted and the empty unit is used.
- `standardise_unit` is called
"""
unit = header.get("BUNIT", "")
try:
u.Unit(unit)
except ValueError:
log.warning(
"Invalid value BUNIT={!r} in FITS header. Setting empty unit.".format(unit)
)
unit = ""
return standardise_unit(unit)