Incorrect error message when setting element of unmasked array to ma.masked

[gerritholl]

When setting an element of an unmasked array to ma.masked, the error message is ValueError: setting an array element with a sequence.. This error message is confusing or outright wrong. The same error message happens in other contexts, for example, if warning are turned to errors then UserWarning: converting masked element to nan becomes the same error message.

In [301]: A = arange(5)

In [303]: A[0] = ma.masked
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-303-286b16dc7aa0> in <module>()
----> 1 A[0] = ma.masked

ValueError: setting an array element with a sequence.

[eric-wieser]

This is super weird. Ultimately, the problem is that ->f->setitem on numeric types does not know how to handle 0d arrays. The bug doesn't exist for string types, because that setitem handler does know how to deal with it (as of #8903).

It's not clear to me how we end up calling setitem here at all - there's something special going on with masked that I can't reproduce with a plain 0d array

[eric-wieser]

It gets weirder:

>>> a = np.zeros(2)
>>> a[0] = np.ma.masked
ValueError: setting an array element with a sequence.
>>> b = np.zeros((2, 2))
>>> b[0] = np.ma.masked
UserWarning: Warning: converting a masked element to nan.