[AstroPy] Constructing an astropy table with colnames and dtypes of existing table

[Andrew Hearin]

You beat me to the punch, Tom, I was going to suggest exactly this. Numpys built-in empty_like method is extremely convenient, and it would be great to have an analogous astropy table method. 


On Apr 29, 2015, at 2:21 PM, Thomas Robitaille <thomas.robitaille at gmail.com> wrote:

> On 29 April 2015 at 19:19, Aldcroft, Thomas
> <aldcroft at head.cfa.harvard.edu> wrote:
>> 
>> 
>> On Wed, Apr 29, 2015 at 2:13 PM, Aldcroft, Thomas
>> <aldcroft at head.cfa.harvard.edu> wrote:
>>> 
>>> 
>>> 
>>> On Wed, Apr 29, 2015 at 1:40 PM, Alexa Villaume <avillaum at ucsc.edu> wrote:
>>>> 
>>>> Hi everybody,
>>>> 
>>>> I’m trying to construct an astropy table using the exact same columns and
>>>> corresponding dtypes of an existing table. The documentation goes into using
>>>> existing columns but it wasn’t as clear to me on how to handle the data
>>>> types. For example, I have an existing table ‘irtf’ and I want to make a
>>>> table ‘irtf_hb’ if I do,
>>>> 
>>>> irtf_hb = Table(names=irtf.colnames)
>>>> 
>>>> the table is initialized with all the right column names but the dtypes
>>>> are all floats. However, I can’t do this,
>>>> 
>>>> irtf_hb = Table(names=irtf.colnames, dtype=irtf.dtype)
>>>> 
>>>> Because I get an error, “ValueError: dtype must be a list or None”. I
>>>> haven’t had any luck with trying to change the type of irtf.dtype or using
>>>> list comprehension to extract a list of dtypes.
>>>> 
>>>> Is there a simple way to use the dtypes of an existing table to
>>>> initialize a new table?
>>> 
>>> 
>>> You should be able to do:
>>> 
>>>>>> irtf_hb = Table(irtf[0:0])
>>> 
>>> This is a slight hack, but it's pretty simple and should give you a
>>> zero-length table with all the original table properties.  I tested on a
>>> simple case but haven't checked in detail.
>> 
>> 
>> BTW, "all the original table properties" includes all column and table meta
>> data (e.g. units), so this should be the preferred idiom.
> 
> How about a new class method that does something like
> ``np.empty_like`` called ``Table.empty_like``?
> 
> Cheers,
> Tom
> 
>> 
>> - Tom
>> 
>>> 
>>> 
>>> In [3]: from astropy.table import Table
>>> 
>>> In [4]: t = Table([[1]])
>>> 
>>> In [5]: t2 = Table(t[0:0])
>>> 
>>> In [6]: t2
>>> Out[6]:
>>> <Table masked=False length=0>
>>> col0
>>> int64
>>> -----
>>> 
>>> In [7]:
>>> 
>>> - Tom
>>> 
>>>> 
>>>> 
>>>> Thank you,
>>>> Alexa
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>> 
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