["Prevent copying X when possible"](https://github.com/scikit-learn/scikit-learn/blob/edb2ff7be3dd266fd363fb008b13341ef899d940/sklearn/utils/validation.py#L34-L36)

Also, Travis failed with this line. I could replace it with `X = X.copy() if copy and not isinstance(X, list) else X` but it doesn't ensure that only one copy will be done.