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The C code often uses %.<number><format> in PyUnicode_FromFormat(). %.200s protects from unlimited output when broken pointer points on random non-null-terminated data. %.200R is used to limit the size of human-readable messages.
In all these case formatted string can look well-formed with short data, but mis-formed (not closed quote, truncated backslash escaping or � decoded from truncated UTF-8 sequence) with long data.
I propose to make truncating in PyUnicode_FromFormat() more smart.
1. Truncated %R should keep at least one end character (the quote or ">").
2. Truncated output should include "..." or "[...]" as truncating sign.
3. \c, \OOO, \xXX, \uXXXX, and \UXXXXXXXX should not be truncated. It is better to omit these sequences at all (cut the string before them) that output them truncated.
4. Doesn't truncate UTF-8 sequence inside a character for %s.