How to use the icontract._checkers.decorate_with_checker function in icontract

# Collapse preconditions and postconditions from the bases with the the function's own ones
        preconditions = _collapse_preconditions(
            base_preconditions=base_preconditions,
            bases_have_func=bases_have_func,
            preconditions=preconditions,
            func=func)

        snapshots = _collapse_snapshots(base_snapshots=base_snapshots, snapshots=snapshots)

        postconditions = _collapse_postconditions(
            base_postconditions=base_postconditions, postconditions=postconditions)

    if preconditions or postconditions:
        if contract_checker is None:
            contract_checker = icontract._checkers.decorate_with_checker(func=func)

            # Replace the function with the function decorated with contract checks
            if inspect.isfunction(value):
                namespace[key] = contract_checker
            elif isinstance(value, staticmethod):
                namespace[key] = staticmethod(contract_checker)

            elif isinstance(value, classmethod):
                namespace[key] = classmethod(contract_checker)

            else:
                raise NotImplementedError("Unexpected value for a function: {}".format(value))

        # Override the preconditions and postconditions
        contract_checker.__preconditions__ = preconditions
        contract_checker.__postcondition_snapshots__ = snapshots

:param func: function to be wrapped
        :return: contract checker around ``func`` if no contract checker on the decorator stack, or ``func`` otherwise
        """
        if not self.enabled:
            return func

        # Find a contract checker
        contract_checker = icontract._checkers.find_checker(func=func)

        if contract_checker is not None:
            # Do not add an additional wrapper since the function has been already wrapped with a contract checker
            result = func
        else:
            # Wrap the function with a contract checker
            contract_checker = icontract._checkers.decorate_with_checker(func=func)
            result = contract_checker

        # Add the precondition to the list of preconditions stored at the checker
        assert hasattr(contract_checker, "__preconditions__")
        preconditions = getattr(contract_checker, "__preconditions__")
        assert isinstance(preconditions, list)
        assert len(preconditions) <= 1, \
            ("At most a single group of preconditions expected when wrapping with a contract checker. "
             "The preconditions are merged only in the DBC metaclass. "
             "The current number of precondition groups: {}").format(len(preconditions))

        if len(preconditions) == 0:
            # Create the first group if there is no group so far, i.e. this is the first decorator.
            preconditions.append([])

        preconditions[0].append(self._contract)

:param func: function to be wrapped
        :return: contract checker around ``func`` if no contract checker on the decorator stack, or ``func`` otherwise
        """
        if not self.enabled:
            return func

        # Find a contract checker
        contract_checker = icontract._checkers.find_checker(func=func)

        if contract_checker is not None:
            # Do not add an additional wrapper since the function has been already wrapped with a contract checker
            result = func
        else:
            # Wrap the function with a contract checker
            contract_checker = icontract._checkers.decorate_with_checker(func=func)
            result = contract_checker

        # Add the postcondition to the list of postconditions stored at the checker
        assert hasattr(contract_checker, "__postconditions__")
        assert isinstance(getattr(contract_checker, "__postconditions__"), list)
        getattr(contract_checker, "__postconditions__").append(self._contract)

        return result

postconditions = contract_checker.__postconditions__

        preconditions = _collapse_preconditions(
            base_preconditions=base_preconditions,
            bases_have_func=bases_have_func,
            preconditions=preconditions,
            func=func)

        snapshots = _collapse_snapshots(base_snapshots=base_snapshots, snapshots=snapshots)

        postconditions = _collapse_postconditions(
            base_postconditions=base_postconditions, postconditions=postconditions)

        if preconditions or postconditions:
            if contract_checker is None:
                contract_checker = icontract._checkers.decorate_with_checker(func=func)

                # Replace the function with the function decorated with contract checks
                if func == value.fget:
                    fget = contract_checker
                elif func == value.fset:
                    fset = contract_checker
                elif func == value.fdel:
                    fdel = contract_checker
                else:
                    raise NotImplementedError("Unhandled case: func neither fget, fset nor fdel")

            # Override the preconditions and postconditions
            contract_checker.__preconditions__ = preconditions
            contract_checker.__postcondition_snapshots__ = snapshots
            contract_checker.__postconditions__ = postconditions