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Keypoints are not just points! They can carry a size, angle, etc. e.g.:

In [6]: ? cv2.KeyPoint()
Type:        builtin_function_or_method
String form: <built-in function KeyPoint>
Docstring:   KeyPoint([x, y, _size[, _angle[, _response[, _octave[, _class_id]]]]]) -> <KeyPoint object>

Thus, you need to convert your point into a KeyPoint, e.g.:

descriptors = extractor.compute(im, [cv2.KeyPoint(16,16,1)]);

Keypoints are not just points! They can carry a size, angle, etc. e.g.:etc., see also:

In [6]: ? cv2.KeyPoint()
Type:        builtin_function_or_method
String form: <built-in function KeyPoint>
Docstring:   KeyPoint([x, y, _size[, _angle[, _response[, _octave[, _class_id]]]]]) -> <KeyPoint object>

Thus, you need to convert your point into a KeyPoint, e.g.:

descriptors = extractor.compute(im, [cv2.KeyPoint(16,16,1)]);