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Same issue for y#:
y# (...) This variant on s# doesn’t accept Unicode objects, only bytes-like objects.
s# (...) The string may contain embedded null bytes.
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y* might mention that it accepts embedded null bytes.
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grep 'PyArg_Parse[^"]\+"[^:;)"]*y[^*]' */*.c finds only usage of y# (no usage of y format):
 - mmap_gfind(), mmap_write_method()
 - oss_write(), oss_writeall()
 - in getsockaddrarg() with s->sock_family==AF_PACKET
 - in sock_setsockopt() if the option name is a string
 - socket_inet_ntoa(), socket_inet_ntop()
These functions have to support embedded null bytes. So I think that y# should specify explicitly that embedded null bytes are accepted.