I use a custom schema that accepts as sub-trees pandas.DataFrame and pandas.Series instances, classes that have overridden the eq() method.

In case a ValidationError involves such an object, the generated exception is thereafter unusable, since the following check within it unicode() method fails:

 if _unset in ( self.validator, self.validator_value, self.instance, self.schema, ): 

The reason for this is that _unset == self.instance (what the compiler generates for XX in YY expressions) returns multiple boolean values when the self.instance is a pandas.DataFrame.

Since _unset is a sentinel instance, i believe that a more proper approach would be to use the is operator instead, which it cannot be overridden.

The following code snippet would suffice:

if any(m is _unset for m in ( self.validator, self.validator_value, self.instance, self.schema )): 

I would be glad to provide such a patch.