Form validation using an instance

I am validating an ImageField in a modelform. What I currently have is working --

# in model
class UserProfile(models.Model):
    user = models.ForeignKey(User, unique=True)
    avatar = models.ImageField(upload_to='images/%Y/%m/%d', blank=True)

class ProfilePictureForm(ModelForm):
    class Meta:
        model = UserProfile
        fields = ('avatar', 'headline')

    def clean_avatar(self):
        if not self.cleaned_data['avatar']:
            raise forms.ValidationError('Oops, looks like you forgot to attach an image!')
        else:
            return self.cleaned_data['avatar']

# in views
def getting_started_pic(request):
    username = request.session.get('username')
    profile = UserPr开发者_JAVA技巧ofile.objects.get(user=username)
    if request.method == 'POST':
        form = ProfilePictureForm(request.POST, request.FILES)
        if form.is_valid():
            form = ProfilePictureForm(request.POST, request.FILES, instance = profile)
            form.save()
            return render_to_response (...)

Note that I am setting the form variable twice, first without the instance and then with the instance. When I defined the form only once (including the instance = profile) the form world never return None, and thus the form would not properly validate (i.e., self.cleaned_data['avatar'] was always non-empty).

I have two questions related to this:

1 - Why does the (...instance = profile) affect the self.cleaned_data['avatar'] portion?

2 - How could I remove redundancy in this view function?

I don't know if you already checked this, but if your profile instance already has a valid avatar then cleaned_data will show this value if the user didn't enter another value.