[Numpy-discussion] Misleading/erroneous TypeError message

Peter Creasey p.e.creasey.00 at googlemail.com
Tue Nov 24 20:42:51 EST 2015

> > I just upgraded my numpy and started to received a TypeError from one of
> > my codes that relied on the old, less strict, casting behaviour. The error
> > message, however, left me scratching my head when trying to debug something
> > like this:
> >
> > >>> a = array([0],dtype=uint64)
> > >>> a += array([1],dtype=int64)
> > TypeError: Cannot cast ufunc add output from dtype('float64') to
> > dtype('uint64') with casting rule 'same_kind'
> >
> > Where does the 'float64' come from?!?!
> >
> The combination of uint64 and int64 leads to promotion to float64 as the
> best option for the combination of signed and unsigned. To fix things, you
> can either use `np.add` with an output argument and `casting='unsafe'` or
> just be careful about using unsigned types.

Thanks for the quick response. I understand there are reasons for the
promotion to float64 (although my expectation would usually be that
Numpy is going to follow C conventions), however the I found the error
a little unhelpful. In particular Numpy is complaining about a dtype
(float64) that it silently promoted to, rather than the dtype that the
user provided, which generally seems like a bad idea. Could Numpy
somehow complain about the original dtypes in this case? Or at least
give a warning about the first promotion (e.g. loss of precision)?


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