from sklearn.base import BaseEstimator
class A(BaseEstimator):
    def _more_tags(self):
        return {'allow_nan': True,
                'multioutput': False}

class B(A):
    def _more_tags(self):
        return {'multioutput': True}

class C(B):
    def _more_tags(self):
        return {'allow_nan': False}

class D(C):
    # No _more_tags(), yet self._more_tags() exists and it's C's method
    pass

# "correctly" resolves to C's allow_nan only because C has _more_tags().
print(D()._get_tags()['allow_nan'])  # False, OK

# resolves to A's multioutput instead of B's since C's _more_tags does not
# specifies multioutput, and because we're going up the mro instead of down the
# mro.
print(D()._get_tags()['multioutput'])  # False, KO