MAINT: Better error message for norm

[larsoner]

While testing cupy/cupy#2355 I found I was getting this error:

E     File "/home/larsoner/python/cupy/tests/cupyx_tests/scipy_tests/fft_tests/test_fft.py", line 76, in test_ifft_overwrite
E       **overwrite_kw)
E     File "<__array_function__ internals>", line 6, in ifft
E     File "/home/larsoner/python/numpy/numpy/fft/pocketfft.py", line 274, in ifft
E       fct = 1/n
E   ZeroDivisionError: division by zero

This fixes it by ensuring the correct, more informative error message is emitted.

[charris]

Thanks @larsoner .

[charris]

Is it assured that an error message is still raised when n == 0? Is it checked elsewhere?

[charris]

Why not raise an error when n == 0 instead of setting it to 1?

[charris]

I may have merged too soon...

[larsoner]

Why not raise an error when n == 0 instead of setting it to 1?

Because just a little bit downstream an informative error is already going to be raised. This just delays it (presumably) a few microseconds.

Is it assured that an error message is still raised when n == 0? Is it checked elsewhere?=

Yes indeed, the test checks this explicitly with pytest.raises(...).