A Saturday 04 December 2010 06:02:40 Craig the Demolishor escrigué:
> Hi folks,
> Sorry if this has been addressed before, I nosed around a little
> bit but didn't see anything related to what I'm currently
> experiencing. I've been trying to find a way to store some
> variable-length data in my IsDescription-derived table row, and I
> tried to do it by creating a StringCol with a maximum width, and
> then byte-packing my data with struct.pack() and writing the
> bytestring to the table row. However, when I read back the written
> string from the file, the bytestring has been truncated. I've
> attached an example where I try to write out a length-240 string and
> get back a length 237 string.
What you are trying to do is not safe because NumPy is truncating
strings for all trailing zeros. Look at this:
>>> s = struct.pack("ii", 1, 0)
>>> s
'\x01\x00\x00\x00\x00\x00\x00\x00'
>>> len(s)
8
>>> np.array([(1,s)], "i4,S8")['f1']
array(['\x01'],
dtype='|S8')
>>> len(np.array([(1,s)], "i4,S8")['f1'])
1
As PyTables uses NumPy under the hood, this behaviour is inherited. If
you want to use variable length data, your best bet is to use a VLArray
object. You can still mix tabular data with variable length data by
putting all columns that are variable length in VLArrays.
Hope this helps,
--
Francesc Alted
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