Why do I get AttributeError: 'NoneType' object has no attribute 'something'?

Question

I keep getting an error that says

AttributeError: \'NoneType\' object has no attribute \'something\'

Answer 1

When building a estimator (sklearn), if you forget to return self in the fit function, you get the same error.

class ImputeLags(BaseEstimator, TransformerMixin):
    def __init__(self, columns):
        self.columns = columns

    def fit(self, x, y=None):
        """ do something """

    def transfrom(self, x):
        return x

AttributeError: 'NoneType' object has no attribute 'transform'?

Adding return self to the fit function fixes the error.