[Numpy-discussion] Saving and loading a structured array from a TEXT file

Derek Homeier derek at astro.physik.uni-goettingen.de
Mon Jan 23 16:28:47 EST 2012

On 23 Jan 2012, at 22:07, Derek Homeier wrote:

>> In [4]: r = np.ones(3,dtype=[('name', '|S5'), ('foo', '<i8'), ('bar', '<f8')])
>> In [5]: r.tofile('toto.txt',sep='\n')
>> bash-4.2$ cat toto.txt
>> ('1', 1, 1.0)
>> ('1', 1, 1.0)
>> ('1', 1, 1.0)
> cnv =  {0: lambda s: s.lstrip('('), -1: lambda s: s.rstrip(')')}
> r2 = np.loadtxt('toto.txt', delimiter=',', converters=cnv, dtype=r.dtype)
> Generally loadtxt works more smoothly together with savetxt, but the latter unfortunately 
> does not offer an easy way to save structured arrays (note to self and others currently 
> working on npyio: definitely room for improvement!).

For the record, in that example

np.savetxt('toto.txt', r, fmt='%s,%d,%f')

would work as well, saving you the custom converter for loadtxt - it could just become tedious 
to work out the format for more complex structures, so an option to construct this automatically 
from r.dtype could certainly be a nice enhancement. 
Just wondering, is there something like the inverse operator to np.format_parser, i.e. 
mapping each dtype to a default print format specifier?


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