PyUnicode_FromFormat() is loose in checking for errors in the format string. If it encounter an invalid format specifier, it silently outputs the rest of the format string. And it allows digits between%s:"%123%" is interpreted the same as"%%", i.e. as literal%.

It was not a big problem, because it is expected that the user of the C API do not make mistakes in the format string, and incorrect format string can easily lead to crash. But id did not help either.

Recently it was discussed on Discuss:https://discuss.python.org/t/pyunicode-fromformat-allow-format-with-precision/17873.