forms.Form silently won't bind to POST data (under certain combinations of key and label)

[anonymous]

In this example:

class TestFormX(forms.Form):
    test_one = forms.CharField(required=True)

    def __init__(self, *args, **kwargs):
        super(TestFormX, self).__init__(*args, **kwargs)

        self.fields['test_two'] = forms.IntegerField(required=False)
        for key in (3,4):
            self.fields[key]    = forms.IntegerField(required=False, label='must have a label')


def sandbox2(request):
    if request.method == 'POST':
        form = TestFormX(request.POST)
        if form.is_valid():
            print 'Form is valid'
        else:
            print form.errors
    else:
        form = TestFormX()
    return render(request, 'xxx_sandbox2.html', {'form' : form}, status=200)

Form fields one and two act normally. Fields 3 and 4 appear to work, but
don't bind data from the request.POST. Thus, any input by the user is lost.

This was found on a dynamically built from.Form, where the database primary key was used as the field name.