Again chained validators

Hi all,
I'm still trying to wrap my head around chained validators, I hope you  
can help me a little.

Suppose I have a form with several fields in it, and many of them are  
somewhat coupled together. For example I have a field "foo" whose  
value must be one out of a list **but** if another field "bar" has a  
certain value then "foo" must have a specific value.

In a case like this I build up a schema like the following:

...
foo = OneOf(allowed_list)
bar = String()
chained_validator = [CheckFooBar()]

where CheckFooBar() is:

class CheckFooBar(FormValidator):
     def validate_python(self, values, state):
         # validation code here

So far so good. The problem arise when I have several fields in a form  
with many different chained validators like CheckFooBar() attached to  
the schema. If one of them fails the following ones are not run and it  
could take many re-submission to the user to discover all the  
mistakes. That's not user friendly at all.

As a possible solution I could define my chained validator like this:

class CheckFooBar(FormValidator):
     validate_partial_form = True
     def validate_partial(self, values, state):
         # validation code here

In this case all the chained validators are run no matter if they fail  
or not, however these chained validator are also run even if one of  
the primitive validators (those declared in the schema) fails.

As a consequence of this, the values passed to validate_partial() are  
the original values coming from the user input, leaving to  
validate_partial() the task of checking if the those values are there  
and if they are of the right type (i.e. I must recode the validation  
process inside validation_partial()!).

Since all this stuff doesn't make too much sense to me, I guess there  
should be a different approach to this kind of problem. Unfortunately  
I can't see it. What's the right approach to check "coupled" fields?  
Is there any way to get multiple chained validators to fails without  
re-invent the validation-wheel?

Suggestions and comments are always welcome.

Cheers,
  Andrea

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