Language: Python
Label: safe
CWE: CWE-193 \u2014 Off-by-one Error
A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value.

Code:
    def allocate_slot(self, n, var_name):
        ret = self._slot
        if self._slot + n >= 2**256:
            raise StorageLayoutException(
                f"Invalid storage slot for var {var_name}, tried to allocate"
                f" slots {self._slot} through {self._slot + n}"
            )
        self._slot += n
        return ret