Currently PyUnicode_FromFormat doesn't check an argument for %c and can raise SystemError only due maxchar check in PyUnicode_New. On 2.7 an error doesn't raised, but %c argument can be silently wrapped (on narrow build) or illegal Unicode string can be created (on wide build). The proposed patch adds explicit range check for %c argument.

Both patches look good to me. Do you feel motivated to check if all formating methods have a test? Here is a list of format methods:

PyUnicode_Format():
- 3.3, 3.4: formatchar() raises OverflowError if x > MAX_UNICODE
- 2.7: formatchar() raises OverflowError if x > 0x10ffff (or x > 0xffff, in narrow mode)

PyUnicode_FromFromatV():
- 2.7: no check, *s++ = va_arg(vargs, int); => BUG
- 3.3: indirect check, maxchar = Py_MAX(maxchar, ordinal); and then PyUnicode_New(n, maxchar); should fail => (ok)
- 3.4: raise ValueError if ordinal > MAX_UNICODE => OK

int.__format__('c'):
- 3.3, 3.4: format_long_internal() raises OverflowError if x > 0x10ffff => OK
- 2.7: format_int_or_long_internal() raises OverflowError if x > 0x10ffff (or x > 0xffff in narrow mode) => OK

IMO a ValueError would be better than OverflowError, it's not really an overflow (limitation of the C language, or a C type). It is maybe too late to change this.

No, I don't feel motivated right now (and this is a case when even never is better than right now).

I checked other format methods and found a bug only in PyUnicode_FromFormatV(). I had added test for 3.3+ because 3.3+ already have a test for PyUnicode_FromFormatV(). I hadn't add test for 2.7 because there is no test for PyUnicode_FromFormatV() on 2.7.

New changeset f8ede55cf92b by Serhiy Storchaka in branch '3.3':
Issue #18184: PyUnicode_FromFormat() and PyUnicode_FromFormatV() now raise
http://hg.python.org/cpython/rev/f8ede55cf92b

New changeset 42def600210e by Serhiy Storchaka in branch 'default':
Issue #18184: PyUnicode_FromFormat() and PyUnicode_FromFormatV() now raise
http://hg.python.org/cpython/rev/42def600210e

New changeset 2f1e8b7fa534 by Serhiy Storchaka in branch '2.7':
Issue #18184: PyUnicode_FromFormat() and PyUnicode_FromFormatV() now raise
http://hg.python.org/cpython/rev/2f1e8b7fa534

I agree, a ValueError may be better than OverflowError, but all other formattings raise OverflowError, and we should support them consistent.