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Message from discussion format difference for visual foxpro 9
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Ethan Furman  
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 More options Oct 31 2012, 11:27 am
From: Ethan Furman <et...@stoneleaf.us>
Date: Wed, 31 Oct 2012 08:27:42 -0700
Local: Wed, Oct 31 2012 11:27 am
Subject: Re: [python-dBase] Re: format difference for visual foxpro 9
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The Budiman wrote:
> Hello,

> Do you think it's right if we change in line 1871:

> if ord(null_data[byte]) >> bit & 1:

> to

> if ord(null_data[0]) >> bit & 1:

> Lets see the context:

>         nullable = flags & NULLABLE
>         binary = flags & BINARY
>         if nullable:
>             byte, bit = divmod(index, 8)
>             null_def = self._meta['_nullflags']
>             null_data = self._data[null_def[START]:null_def[END]]
>             try:
>                 if ord(null_data[0]) >> bit & 1:
>                     return Null
>             except IndexError:
>        print null_data
>                 print index
>                 print byte, bit
>                 print len(self._data), self._data
>                 print null_def
>                 print null_data
>                 raise

> As far as I can tell, the enclosing function (_retrieve_field_value)
> will return a field value. This part of code are checking if the field
> is nullable and if the field  value is in fact null. I'm guessing that
> null_def[START] and null_def[END] should contain the location of the
> null data or flag. In my cases, the length of the resulting null_data is
> always 1, so null_data[byte] usually throws the array out of range
> exception. I'm not sure what the 'byte' and 'bit' variable supposed to
> mean, but maybe in this case we just need to read that 1 character?

Each null byte stores the null state for up to eight fields; so if there are more than eight
nullable fields then null_data can be more than one byte long.  bit is which bit in the byte
represents the field in question.

So to make a long story short, no, changing to null_data[0] will not work once there are more than
eight nullable fields.

One possible problem, though, is that (I think) the varchar fields also use the null_data area to
keep track of varchar info, and I don't know how that interaction works.

~Ethan~


 
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