Adding PyDict_GetItemStringWithError makes is possible to migrate existing code to a design that does not ignore errors.  I have a significant number of calls to PyDict_GetItemString that cannot be switch toed PyDict_GetItem without increasing the number of lines of C code I have to write (or in other words, if PyDict_GetItemString wouldn't exist I'd have invented this myself for use in extensions I write).

Another reason for adding the function is consistency in the API. 

BTW. I've added a function with the same signature as the proposed PyDict_GetItemStringWithError to my exension's code base, which is something I'd have to do anyway for backward compatibility with older CPython versions.

W.r.t API design: I prefer the interface of _PyObject_LookupAttr to that of PyDict_GetItemWithError, even if the additional cost of a call to PyErr_Occurred() is insignificant in most of my code (esp. when guarded with a test for a NULL return value from PyDict_GetItemWithError).