<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _validate(self, key, cls=None):
 """Verify the manifest schema."""

        if key not in self.manifest:
            raise ValueError("Manifest %s requires '%s'."
                             % (self.manifest_path, key))
        if cls:
            if not isinstance(self.manifest[key], cls):
                raise TypeError("Manifest value '%s' should be %s, not %s"
                                % (key, cls, type(self.manifest[key])))