[Numpy-discussion] round / set_printoptions discrepancy

[Stefano Miccoli]

In my opinion the problem is that numpy floats break the Liskov substitution principle,

>>> pyfloat = 16.055
>>> npfloat = np.float64(pyfloat)
>>> isinstance(npfloat, float)
True
>>> round(pyfloat, 2)
16.05
>>> round(npfloat, 2)
16.06

Since numpy.float64 is a subclass of builtins.float I would expect that

>>> round(x, j) == round(np.float64(x), j)

is an invariant, but unfortunately this is not the case. 

Moreover the python3 semantics of the round function require that when the number of digits is None,
the return value should be of integral type:

>>> round(pyfloat)
16
>>> round(pyfloat, None)
16
>>> round(pyfloat, 0)
16.0
>>> round(npfloat)
16.0
>>> round(npfloat, None)
16.0
>>> round(npfloat, 0)
16.0

see also https://github.com/numpy/numpy/issues/11810

Stefano