class TestForm(forms.Form):
    def __init__(self, desired_value=None, *args, **kwargs):
        self.desired_field = forms.ModelChoiceField(Queried_Model.objects.filter(param=desired_value))
        super(TestForm,self).__init__(*args, **kwargs)

class test():
    def __init__(self, value):
        self.var1 = value
    var2=var1
NameError: name 'var1' is not defined

class TestForm(forms.Form):
    def __init__(self, desired_value=None, *args, **kwargs):
        super(TestForm,self).__init__(*args, **kwargs)
        self.fields["desired_field"] = forms.ModelChoiceField(Queried_Model.objects.filter(param=desired_value))