[AstroPy] Changing dtype when copying masked table throws TypeError

[Gregory Simonian]

Hi all,

I have a Table with several columns which were read in as strings
because they had a combination of numeric and non-numeric characters.
After cleaning out the non-numeric characters, I'd like to convert the
dtype of the column from string to int.

Looking at the astropy documentation for modifying tables [0], under the
caveats section it states:

"The data type for a column cannot be changed in place. In order to do
this you must make a copy of the table with the column type changed
appropriately."

So I try making a new table (the table is small, so I don't mind the
temporary memory hit), but I run into the error:
"TypeError: Cannot set fill value of string with array of dtype int64"
When checking the fill value of the column (which is masked from the
reader), it's 'N/A', which makes sense that it wouldn't work. But when I
change it to '0', I still get the same problem. I managed to find a
minimal working example as:

import numpy as np
t = Table({'a': ['1']}, masked=True)
t['a'].set_fill_value('0')
u = Table(t, names=['a'], dtype=[np.int])
> TypeError: Cannot set fill value of string with array of dtype int64

When debugging the exception, it seems like the Table constructor
doesn't convert the fill value of masked tables when generating a new
table, which seems to defeat the ability to change the dtype of masked
columns. I was wondering if other people have run by this, and if
there's a more straightforward way of changing the dtype of a column.
This is with Astropy 1.0.3 on Python 2.7.10 through Anaconda on CentOS 6.6.

When checking to see if this was reported, I ran into issue #3809 [1],
which might end up being the ultimate solution to my problem. It seems
like a workaround would be:

temp_column = np.ma.asanyarray(t['a'], np.int)
del(t['a'])
t['a'] = temp_column

Thanks!

                                            Gregory Simonian

[0]http://docs.astropy.org/en/stable/table/modify_table.html#caveats
[1] https://github.com/astropy/astropy/issues/3809
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