from collections import UserString

class ustr2(UserString):
    def __mod__(self, other):
        print('ustr2.__mod__ was called')
        return super().__mod__(other)


class ustr3(ustr2):
    def __mod__(self, other):
        print('ustr3.__mod__ was called')
        return super().__mod__(other)
    
    def __rmod__(self, other):
        print('ustr3.__rmod__ was called')
        return super().__rmod__(other)


fmt2 = ustr2('value is %s')
fmt3 = ustr3('value is %s')
str3 = ustr3('TEST')

print('Trying simple case of ustr3 instance % ustr3 instance...')
# This will call ustr3.__mod__, which will call ustr2.__mod__, which will
# call UserString.__mod__, which will return 'value is TEST' without error
print(fmt3 % str3)

print('Now trying case of ustr2 instance % ustr3 instance...')
# Here, the % operator allows the subclass to override the method, and
# thus delegates to ustr3.__rmod__. This calls UserString.__rmod__, which
# throws the error.
print(fmt2 % str3)