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Friday, February 26, 2010

Numeric Formatting in Python

Sometimes I hate Python. Here's the difference between Perl and Python: Perl accepts, even embraces the fact that language is messy. Python sometimes gets this wrong.

Here are some samples of dealing with formatting numbers:

from decimal import Decimal

>>> f= Decimal('1200.1')
>>> "%s"%f.normalize()
Huzzah, it does the right thing!

>>> f= Decimal('1200')
>>> "%s"%f.normalize()
 Doh!  But clearly, removing the normalize() will fix things, right?

>>> "%s"%f
Yee-haw! Now we're cooking!

>>> f= Decimal('1200.100')
>>> "%s"%f
Hm, users don't want the trailing zeroes.  The only thing that I can think to do is use a regular expression, and now I have two problems.

 But wait! If the number doesn't have anything after the decimal, then we do one, and if it does, we do the other!

def dec_string(dec):
    if dec:
        if dec.normalize().as_tuple().exponent > 0:
            return "%d"%dec
            return "%s"%dec.normalize()
        return 0
 And that seems to be working.  Note that the final version uses "%d"%dec, rather than %s, since we want to make sure and truncate past the decimal.

Update: check out my post about integrating it with Django forms to make it seamless.

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