Merge pull request #2023 from krngrvr09/fits

Improved error message for scale units in FITS.

astropy/io/fits/connect.py

@@ -15,6 +15,7 @@
 from ...table import Table
 from ...utils import OrderedDict
 from ...utils.exceptions import AstropyUserWarning
+from astropy.units.format.fits import UnitScaleError
 
 from . import HDUList, TableHDU, BinTableHDU, GroupsHDU
 from .hdu.hdulist import fitsopen as fits_open
@@ -238,7 +239,15 @@ def write_table_fits(input, output, overwrite=False):
     # Set units for output HDU
     for col in table_hdu.columns:
         if input[col.name].unit is not None:
-            col.unit = input[col.name].unit.to_string(format='fits')
+            try:
+                col.unit = input[col.name].unit.to_string(format='fits')
+            except UnitScaleError:
+                scale = input[col.name].unit.scale
+                raise UnitScaleError(
+                    "The column '{0}' could not be stored in FITS format "
+                    "because it has a scale '({1})' that "
+                    "is not recognized by the FITS standard. Either scale "
+                    "the data or change the units.".format(col.name, str(scale)))
 
     for key, value in input.meta.items():
 

astropy/io/fits/tests/test_connect.py

@@ -10,6 +10,7 @@
 from .... import units as u
 from .... import log
 from ....tests.helper import pytest, catch_warnings
+from astropy.units.format.fits import UnitScaleError
 
 PY3 = sys.version_info[0] >= 3
 DATA = os.path.join(os.path.dirname(__file__), 'data')
@@ -221,3 +222,15 @@ def test_masking_regression_1795():
     assert np.all(t['c2'].data == np.array(['abc', 'xy ']))
     assert_allclose(t['c3'].data, np.array([3.70000007153, 6.6999997139]))
     assert np.all(t['c4'].data == np.array([False, True]))
+
+def test_scale_error():
+    from astropy.table import Table
+    a = [1, 4, 5]
+    b = [2.0, 5.0, 8.2]
+    c = ['x', 'y', 'z']
+    t = Table([a, b, c], names=('a', 'b', 'c'), meta={'name': 'first table'})
+    t['a'].unit='percent'
+    with pytest.raises(UnitScaleError) as exc:
+        t.write('t.fits',format='fits', overwrite=True)
+    assert exc.value.args[0]=="The column 'a' could not be stored in FITS format because it has a scale '(1.0)' that is not recognized by the FITS standard. Either scale the data or change the units."
+

astropy/units/format/fits.py

@@ -17,6 +17,11 @@
 from . import utils
 from ...utils.misc import did_you_mean
 
+class UnitScaleError(ValueError):
+    """
+    Used to catch the errors involving scaled units, 
+    which are not recognized by FITS format.
+    """
 
 class Fits(generic.Generic):
     """
@@ -119,7 +124,7 @@ def to_string(self, unit):
 
         if isinstance(unit, core.CompositeUnit):
             if unit.scale != 1:
-                raise ValueError(
+                raise UnitScaleError(
                     "The FITS unit format is not able to represent scale. "
                     "Multiply your data by {0:e}.".format(unit.scale))