code: def is_instance_of(self, some_class):
          
          try:
              if not isinstance(self.val, some_class):
                  if hasattr(self.val, '__name__'):
                      t = self.val.__name__
                  elif hasattr(self.val, '__class__'):
                      t = self.val.__class__.__name__
                  else:
                      t = 'unknown'
                  self._err('Expected <%s:%s> to be instance of class <%s>, but was not.' % (self.val, t, some_class.__name__))
          except TypeError:
              raise TypeError('given arg must be a class')
          return self
docs: Asserts that val is an instance of the given class.