how to pull array of results from http Post result of ModelMultipleChoiceField in django view

how to pull array of results from http Post result of
ModelMultipleChoiceField in django view

I print the request POST after submitting a form with a
ModelMultipleChoiceField
if request.method == 'POST':
print 'form ' + str(request.POST)
for i in request.POST['groups_field']:
print i
This first print prints this:
form
The second print prints this:
4
Why does this only pull the last selected value?
The form field looks like this:
groups_field = forms.ModelMultipleChoiceField(queryset=Group.objects.none(),
widget=forms.CheckboxSelectMultiple)
How can I pull out the list of group fields selected?