gh-146151: memoryview supports 'F' and 'D' format types (complex)

[skirpichev]

• Issue: Support complex (float/double) types in array.array and memoryview #146151

---

📚 Documentation preview 📚: https://cpython-previews--146241.org.readthedocs.build/

[vstinner]

LGTM. The implementation looks complete and well tested.

[vstinner]

I wasn't sure of the behavior on underflow or overflow of the double to float cast, so I tested:

>>>importstruct, sys>>>data=struct.pack('F', 0.0) *4>>>m=memoryview(bytearray(data)).cast('F')
>>>m[0]=math.nextafter(0, 1)
>>>m[1]=sys.float_info.min>>>m[2]=sys.float_info.max>>>m[3]=float("nan")
>>>m.tolist()
[0j, 0j, (inf+0j), (nan+0j)]

I got the values that I expected. I suppose that these conversions are well specified by IEEE 754.

[skirpichev]

The behavior here is same as for 'f' format, just component-wise, see PACK_SINGLE macro.

[vstinner]

Merged, thanks for your contribution.