# -*- coding: utf-8 -*-
"""
unit_conversion.py
==================
Functions for checking and sanitizing units that do not follow the FITS specification.
This uses functions from ``astropy.unit`` to parse and handle units.
"""
import warnings
from astropy.units import Unit, UnrecognizedUnit
from astropy.io.fits.verify import VerifyWarning
class UnitWarning(VerifyWarning):
"""
Unit warning class
Used when units do not parse or parse oddly
"""
def fits_to_units(unit_str):
""" Do a lookup from a astropy unit and return a fits unit string
unit_str (str): a FITS unit string
returns an astropy.units.Unit(), or UnrecognizedUnit()
Notes
-----
This will attempt to correct some common mistakes in the FITS format.
"""
unit_lookup = {
'meters': 'm',
'meter': 'm',
'degrees': 'deg',
'degree': 'deg',
'hz': 'Hz',
'hertz': 'Hz',
'second': 's',
'sec': 's',
'secs': 's',
'days': 'd',
'day': 'd',
'steradians': 'sr',
'steradian': 'sr',
'radians': 'rad',
'radian': 'rad',
'jy': 'Jy',
'au': 'AU',
}
try:
new_units = ""
if unit_str is None:
unit_str = ''
unit_str = unit_str.lower()
unit_list = unit_str.split("/")
for uu in unit_list:
if uu.endswith("s") and len(uu) > 1:
uu = uu[:-1]
corrected_unit = unit_lookup.get(uu, uu)
new_units += corrected_unit
new_units += " / "
new_units = new_units[:-3]
unit = Unit(new_units)
return unit
except ValueError:
warnings.warn("Unknown unit: %s" % new_units, UnitWarning)
return UnrecognizedUnit(unit_str)
def units_to_fits(unit):
""" Convert an astropy unit to a FITS format string.
uses the to_string() method built-in to astropy Unit()
Notes
-----
The output will be the format defined in the FITS standard:
http://fits.gsfc.nasa.gov/fits_standard.html
A roundtrip from fits_to_units -> units_to_fits may not return
the original string, as people often don't follow the standard.
"""
if unit is None:
unit = Unit('')
return unit.to_string("fits").upper()
