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# Source code for astropy.units.format.fits

    # Licensed under a 3-clause BSD style license - see LICENSE.rst

    """
    Handles the "FITS" unit format.
    """

    from __future__ import (absolute_import, division, print_function,
                            unicode_literals)
    from ...extern.six.moves import zip

    import keyword
    import warnings

    from ...utils.exceptions import AstropyDeprecationWarning

    from . import generic
    from . import utils

    class Fits(generic.Generic):
        """
        The FITS standard unit format.

        This supports the format defined in the Units section of the `FITS
        Standard <http://fits.gsfc.nasa.gov/fits_standard.html>`_.
        """
        name = 'fits'

        def __init__(self):
            # Build this on the class, so it only gets generated once.
            if '_parser' not in Fits.__dict__:
                Fits._parser, Fits._lexer = self._make_parser()

            if not '_units' in Fits.__dict__:
                Fits._units, Fits._deprecated_units = self._generate_unit_names()

        @staticmethod
        def _generate_unit_names():
            from ... import units as u
            names = {}
            deprecated_names = set()

            bases = [
                'm', 'g', 's', 'rad', 'sr', 'K', 'A', 'mol', 'cd',
                'Hz', 'J', 'W', 'V', 'N', 'Pa', 'C', 'Ohm', 'S',
                'F', 'Wb', 'T', 'H', 'lm', 'lx', 'a', 'yr', 'eV',
                'pc', 'Jy', 'mag', 'R', 'bit', 'byte'
            ]
            deprecated_bases = ['G', 'barn']
            prefixes = [
                'y', 'z', 'a', 'f', 'p', 'n', 'u', 'm', 'c', 'd',
                '', 'da', 'h', 'k', 'M', 'G', 'T', 'P', 'E', 'Z', 'Y']

            for base in bases + deprecated_bases:
                for prefix in prefixes:
                    key = prefix + base
                    if keyword.iskeyword(key):
                        continue
                    names[key] = getattr(u, key)
            for base in deprecated_bases:
                for prefix in prefixes:
                    deprecated_names.add(prefix + base)

            simple_units = [
                'deg', 'arcmin', 'arcsec', 'mas', 'min', 'h', 'd', 'Ry',
                'solMass', 'u', 'solLum', 'solRad', 'AU', 'lyr', 'count',
                'photon', 'ph', 'pixel', 'pix', 'D', 'Sun', 'chan', 'bin',
                'voxel', 'adu', 'beam'
            ]
            deprecated_units = ['erg', 'Angstrom']

            for unit in simple_units + deprecated_units:
                names[unit] = getattr(u, unit)
            for unit in deprecated_units:
                deprecated_names.add(unit)

            return names, deprecated_names

        @classmethod
        def _parse_unit(cls, unit):
            if unit not in cls._units:
                raise ValueError(
                    "Unit {0!r} not supported by the FITS standard.".format(unit))

            if unit in cls._deprecated_units:
                warnings.warn(
                    "The unit {0!r} has been deprecated in the FITS "
                    "standard.".format(unit),
                    AstropyDeprecationWarning)

            return cls._units[unit]

        def _get_unit_name(self, unit):
            name = unit.get_format_name('fits')

            if name not in self._units:
                raise ValueError(
                    "Unit {0!r} is not part of the FITS standard".format(name))

            if name in self._deprecated_units:
                warnings.warn(
                    "The unit {0!r} has been deprecated in the FITS "
                    "standard.".format(name),
                    AstropyDeprecationWarning)

            return name

        def to_string(self, unit):
            from .. import core

            # Remove units that aren't known to the format
            unit = utils.decompose_to_known_units(unit, self._get_unit_name)

            if isinstance(unit, core.CompositeUnit):
                if unit.scale != 1:
                    raise ValueError(
                        "The FITS unit format is not able to represent scale. "
                        "Multiply your data by {0:e}.".format(unit.scale))

                pairs = list(zip(unit.bases, unit.powers))
                pairs.sort(key=lambda x: x[1], reverse=True)

                s = self._format_unit_list(pairs)
            elif isinstance(unit, core.NamedUnit):
                s = self._get_unit_name(unit)

            return s