import markdown

def render_markdown(sender, instance, *args, **kwargs):
    '''
        Renders markdown from a text field to a dedicated html field.
        This is meant to be used with the pre_save signal.
    '''
    md = markdown.Markdown(safe_mode='escape')

    for fieldname, _ in instance.__dict__.iteritems():
        if fieldname.endswith("_html"):
            fieldname_raw = fieldname[:fieldname.find('_html')]
            field_raw = getattr(instance, fieldname_raw, None)

            if field_raw:
                setattr(instance, fieldname, md.convert(field_raw))

...

from django.db import models
from django.db.models.signals import pre_save

class SomeModel(models.Model):
    ...
    biography = models.TextField()
    biography_html = models.TextField()
    ...

# render markdown from biography to biography_html on save
pre_save.connect(render_markdown, sender=SomeModel)