[Pytables-users] Re: setattr and tables.IsDescription

[Francesc A. <fa...@op...>]

A Dijous 10 Juliol 2003 19:45, vareu escriure:
> I'm working on converting python data structures to pytables commands.
>
> So if you have:
>
> data={'fred':[1,2,3,4,5,6,7,8,9,10],
> 'barney':[11,12,13,14,15],
> 'wilma':[99,99,3,22,66]
> }
>
>
> Just do something like:
> save(data,'file1.hdf')
>
> and it will automatically save the dictionary in pytables format for
> you.
> I think it will make pytables more accessible.

That sounds nice

>
>
> Anyways, if I define a class like:
>
> class Row(tables.IsDescription): pass
>
> I cannot use setattr to do this:
>
> setattr(Row, 'data0', tables.Col('Int32'))
>
> The definition of data0 has to be present at class defintion time. I
> think this is because of the metaclass that helps create
> tables.IsDescription. (the metaclass stuff they've added to python is
> cool!)
>
> I've tried some tactics like defining  my own metaclass for Row, but
> ended up
> resorting to using compile/exec on strings to build the class. It seems
> awkward, can you think of a better way to at runtime define the data
> that forms a table?

I don't know if you are aware that pytables can also build a Table from a
dictionary like:

RecordDescriptionDict = {
    'var1': Col("CharType", 4),   # 4-character String
    'var2': Col("Int32", 1),      # integer
    'var3': Col("Int16", 1),      # short integer 
    'var4': Col("Float64", 2),    # double (double-precision)
    'var5': Col("Float32", 4),    # float  (single-precision)
    'var6': Col("Int16", 1),      # short integer
    'var7': Col("CharType", 1),   # 1-character String
    }


and then call the Table constructor as always:

h5file.createTable(group, 'table', RecordDescritionDict, title = self.title)

So, you can pass both a metaclass descentant or a dictionary to Table and
both will be understood (for more information see
http://pytables.sourceforge.net/html-doc/usersguide-html4.html#subsection4.4.2
and
http://pytables.sourceforge.net/html-doc/usersguide-html3.html#secondExample)

I think that this should be the way to go for you. However, it is indeed
possible to build-up a metaclass dynamically and you can see examples of
that use in the Table._newRecArray() and Table._open() methods in the
sources. But this is a more lower level approach and not recommended unless
you absolutely need it.

Good luck!,

-- 
Francesc Alted