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

[Alexa Villaume]

Thank you, everyone, for your helpful responses. 

If it’s of any interest, I ended up going with Tom A.’s solution because one of my columns holds arrays instead of single values. The dtype entry for that looks like ‘(‘SPEC’, ‘>f4’, (7925,)’ which was fully copied in Tom A.’s solution but not Tom R.’s. 

Best,
Alexa

On Apr 29, 2015, at 11:45 AM, Andrew Hearin <andrew.hearin at yale.edu> wrote:

> 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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