Django - Form not valid but no error

Question

Model:

class Session(models.Model):
    tutor = models.ForeignKey(User)
    start_time = models.DateTimeField()
    end_time = models.DateTimeField()
    def __unicode__(self):
        return u'%s - %s' % (self.tutor, self.start_time)

Form:

class SessionForm(forms.ModelForm):
    class Meta:
        model = Session
        exclude = ['tutor']

Testing:

>>> ses = Session.objects.get(pk=1)
>>> ses
<Session: Robert - 2012-03-22 13:00:00>
>>> form = SessionForm(instance=ses)
>>> form.is_valid()
false
>>> form.errors
{}

What's wrong ?

Answer 1

You have not passed any data to the form, so it is not valid. The instance argument is not used to set the form data, just the initial data.

Answer 2

This is a humble attempt to provide a little background for Daniel Roseman's answer above.

As you can see in the source, BaseForm.is_valid() does the following:

return self.is_bound and not self.errors

So, if is_valid() returns False, even though there are no errors, then is_bound must be False.

Now, the value for is_bound is assigned in BaseForm.__init__() (source):

self.is_bound = data is not None or files is not None

From the forms documentation:

A Form instance is either bound to a set of data, or unbound.

• If it’s bound to a set of data, it’s capable of validating that data and rendering the form as HTML with the data displayed in the HTML.
• If it’s unbound, it cannot do validation (because there’s no data to validate!), but it can still render the blank form as HTML.

Also note that errors is a property, which calls the full_clean() method (source), which does the actual validation.