Using AuthenticationForm in Django
>>> from django.contrib.auth.forms import AuthenticationForm
>>> POST = { 'username': 'test', 'password': 'me', }
>>> form = AuthenticationForm(POST)
>>> form.is_valid()
False

form = AuthenticationForm(data=request.POST)

return self.is_bound and not bool(self.errors)


>>> form.errors
{'__all__': [u'Please enter a correct username and password. Note that both fields are case-sensitive.']}

def clean(self):
    username = self.cleaned_data.get('username')
    password = self.cleaned_data.get('password')

    if username and password:
        self.user_cache = authenticate(username=username, password=password)
        if self.user_cache is None:
            raise forms.ValidationError(_("Please enter a correct username and password. Note that both fields are case-sensitive."))
        elif not self.user_cache.is_active:
            raise forms.ValidationError(_("This account is inactive."))
    self.check_for_test_cookie()
    return self.cleaned_data