render_template メソッドをオーバーライドして、別の方法でそれを行いました。ここに私の解決策があります。process_step() について知りませんでした...
def render_template(self, request, form, previous_fields, step, context):
if not step == 0:
# A workarround to find the type value!
attr = 'name="0-type" value='
attr_pos = previous_fields.find(attr) + len(attr)
val = previous_fields[attr_pos:attr_pos+4]
type = int(val.split('"')[1])
if step == 2 and (not type == 1 and not type == 2 and not type == 3):
form = self.get_form(step+1)
return super(ProductWizard, self).render_template(request, form, previous_fields, step+1, context)
return super(ProductWizard, self).render_template(request, form, previous_fields, step, context)