Feedback wanted for my first django custom field!

Question

Hi I've created my first custom django field. Its a social security number field (Cpr) following danish syntax. As I'm new to both python and django i would love to get some feedback on the code!

import django.forms as f
from datetime import datetime
import json
from django.utils.safestring import mark_safe
from django.core.exceptions import ValidationError

# Globals
YEAR = datetime.now().year          
FIELDS = (
    {'name' : "cpr_date",   'size': 2, 'maxlength': 2, 'renderfunc': None, 
        'field': f.IntegerField(min_value=1, max_value=31)
    },
    {'name' : "cpr_month",  'size': 2, 'maxlength': 2, 'renderfunc': None,
        'field': f.IntegerField(min_value=1, max_value=12),
    },    
    {'name' : "cpr_year",   'size': 4, 'maxlength': 4, 
        'renderfunc': 'render_age',
        'field': f.IntegerField(min_value=YEAR-120, max_value=YEAR),
    },
    {'name' : "cpr_digits", 'size': 4, 'maxlength': 4, 
        'renderfunc': 'render_gender',
        'field': f.IntegerField(min_value=1000, max_value=9999),
    },
)

class CprWidget(f.widgets.MultiWidget):
    """Widget showing date, month, year and 4-last-digit fields for a danish cpr 
    number.
    """
    def __init__(self, attrs=None):
        widgets = [f.widgets.TextInput(field) for field in FIELDS]
        super(CprWidget, self).__init__(widgets, attrs)

    def decompress(self, v):
        """Decompresses widget values to json before save"""
        if v: return json.loads(v)
        return [None, None, None, None]

    def render(self, name, value, attrs=None):
        """Extends with details rendered from parts of Cpr number"""
        values = self.decompress_value(value)
        output = super(CprWidget, self).render(name, value, attrs)
        extra_output = ""
        all_is_valid = True
        i = 0
        for field in FIELDS:
            try:
                value = values[i]
                if value == None:
                    raise ValueError
                fieldinst = field['field']
                fieldinst.validate(value)
                fieldinst.run_validators(value)
                if field['renderfunc']:
                    func = getattr(self, field['renderfunc'])
                    extra_output = extra_output + func(value)
            except (IndexError, ValueError, ValidationError):
                all_is_valid = False
            finally:
                i = i + 1
        if all_is_valid:
            extra_output = extra_output + self.render_cpr(values)
        return output + mark_safe(extra_output)

    def render_gender(self, value):
        """Renders male or female based on last digit of Cpr"""
        last_digit = int(str(value)[3])
        gender = (('Male','Female')[last_digit % 2 == 0])
        return self.render_line("Gender", gender)

    def render_age(self, value):
        """Renders age"""
        return self.render_line("Age", YEAR-value)

    def render_cpr(self, values):
        """Renders full Cpr number"""
        cpr = "%02d%02d%s-%s" % (
            int(values[0]), int(values[1]), str(values[2])[2:4], values[3])
        return self.render_line("Cpr", cpr)

    def render_line(self, title, value):
        """Renders a line with title and value"""
        return "<br /><b>%s: </b>%s" % (title, value)

    def decompress_value(self, value):
        """Decompress values from value if needed"""
        if not isinstance(value, list):
            value = self.decompress(value)
        return value

    def format_output(self, rendered_widgets):
        """Format html for widgets"""
        labels = [u'Date', u'Month', u'Year', u'Digits']
        labels.reverse()
        html = u''
        for w in rendered_widgets: html += labels.pop() + ': ' + str(w)
        return html

class CprField(f.MultiValueField):
    """Field extending MultiValueField with date, month, year and 4-last-digit 
    fields for a danish cpr number.
    """
    widget = CprWidget
    def __init__(self, required=False, widget=None, label=None, initial=None):
        fields = [field['field'] for field in FIELDS]
        super(CprField, self).__init__(fields=fields, widget=widget, label=label
            ,initial=initial, required=required)

    def compress(self, data_list):
        """Compress field values to json"""
        return json.dumps(data_list)

Answer 1

Is there some particular reason you are combining a Date widget with the Cpr number? Seems that they are two separate things, and Django already has a decent SelectDateWidget. I'm not familiar with the Danish Cpr number, but unless the date is part of the number, they should really be two separate fields.