How to set django model field by name?

This seems like it should be dead simple, so I must be missing something. I just want to set the value of a field in my model instance by name. Say I have:

class Foo(Model):
  bar = CharField()

f = Foo()

I want to set the value of bar by name, not by accessing the field. So something like:

f.fields['bar'] = 'BAR"

instead of

f.bar = 'BAR'

I've tried setattr but it doesn't persist the value in the database. I also tried going through _meta.fields but got various errors along the way.