Numeric Formatting in Django

You might have seen my previous post lamenting the weirdnesses around number formatting in python.  This spills over into Django; if you have DecimalFields in your models (especially if you are using the ModelForm object to create your forms.


The previous "format Decimal object as a string that any 10-year-old would expect" method is:

def dec_string(dec):
    if isinstance(dec, Decimal):
        if dec:
            if dec.normalize().as_tuple().exponent > 0:
                return "%d"%dec
            else:
                return "%s"%dec.normalize()
        else:
            return 0
    else:
        return dec

(I added the isinstance() bit to protect me from myself =)

Since we know how to format this, all that remains to be done is to create a widget that automatically formats DecimalFields correctly.

class DecimalInput(TextInput):
  def render(self, name, value, attrs=None):
    value = dec_string(value)
    return super(DecimalInput, self).render(name, value, attrs)

And then just use that in your form:

class EnergyForm(ModelForm):
    kwh = DecimalField(widget=DecimalInput(attrs={'size':'6'}))

There is more to the ModelForm, but that's what the docs are for, right? Read more...

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()
'1200.1'

Huzzah, it does the right thing!

>>> f= Decimal('1200')
>>> "%s"%f.normalize()
'1.2E+3'

 Doh!  But clearly, removing the normalize() will fix things, right?

>>> "%s"%f
'1200'

Yee-haw! Now we're cooking!

>>> f= Decimal('1200.100')
>>> "%s"%f
'1200.100'

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
        else:
            return "%s"%dec.normalize()
    else:
        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. Read more...