Referencing `self` In `__old__` In PyContract Constraints

I'm working on writing some constraints for a class method using PyContract (not PyContracts). As a postcondition, I'd like to ensure that the memory address of the instance hasn't

Solution 1:

This seems to fix it:

class Individual:
    def append(self, chrom):
        """
            post[self]:
                __old__.self is self
                len(__old__.self.chromosomes)+1 == len(self.chromosomes)
                self.chromosomes[-1] == chrom
        """
        self.chromosomes.append(chrom)

source