Working with fixed format text db's

Frank Millman frank at chagford.com
Sat Jun 9 10:04:20 CEST 2007


On Jun 8, 5:50 pm, Neil Cerutti <horp... at yahoo.com> wrote:
> Many of the file formats I have to work with are so-called
> fixed-format records, where every line in the file is a record,
> and every field in a record takes up a specific amount of space.
>
> For example, one of my older Python programs contains the
> following to create a fixed-format text record for a batch of new
> students:
>
> new = file("new.dat", "w")
> if not new:
>     print "Error. Could not open file new.dat for writing."
>     raw_input("Press Return To Exit.")
>     sys.exit(1)
>
> for s in freshmen:
>     new.write(s.ssn.ljust(9))
>     new.write(s.id.ljust(10))
>     new.write(s.last[:16].ljust(16))
>     new.write(s.first[:11].ljust(11))
>     new.write(' '.ljust(10)) # Phone Number
>     new.write(' '.ljust(1254)) # Empty 'filler' space.
>     new.write('2813  ')
>     new.write(s.major.ljust(5))
>

I have to do this occasionally, and also find it cumbersome.

I toyed with the idea of posting a feature request for a new 'fixed
length' string formatting operator, with optional parameters for left/
right-justified and space/zero-filled.

We already have '%-12s' to space fill for a length of 12, but it is
not truly fixed-length, as if the value has a length greater than 12
you need it to be truncated, and this construction will not do that.

Assume we have a new flag '!n', which defaults to left-justified and
space-filled, but allows an optional 'r' and '0' to override the
defaults.

Then the above example could be written as

format = '%!9s%!10s%!16s%!11s%!10s%!1254s%!6s%!5s'
for s in freshmen:
    new.write (format %
       (s.ssn,s.id,s.last,s.first,
        ' ',' ','2813',s.major))

I never felt strongly enough about it to propose it, but I thought I
would mention it.

Frank Millman




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