> No you don't. You can copy a substring of the input string with
Py_UNICODE_COPY: just pass a smaller length.

Oh, yes, I got your meaning now. I'll follow this.


> You can truncate the input char* on the call to PyUnicode_DecodeUTF8:

Oh, what if the trunked char* cannot be decoded correctly? e.g. a tow-bytes character is divided in the middle? 


> Yes, but I am no more sure that it is the right thing to do.

If I understand correctly(my English ability is limited), your suggestion is to combine, right? I'm afraid that combine may bring us too complicated code to write. The currently 4 steps just divide the process into smaller and simpler pieces. I'm not sure.