django forms - overriding constructor for changing field type depending by input

hi everyone. i need to overrid the constructor of a form: if i have some value in input, some fields has to be ChoiceField, otherwise they have to be CharField. here's the code:

http://dpaste.com/750343/

but this way doesn't work. even if i declare fields before constructor  as CharField and then I ovverride them in __init__, they will not be renderized in template (I got this message: <django.forms.fields.ChoiceField object at 0x7ff0c0173ad0>).I don't understand where's the mistake (also because i have poor python skills...)

any help?

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