struct.pack("B", -1) generates an OverflowError
exception since the B format corresponds to the
"unsigned char" type which can have values between 0
and 255.

But struct.pack("I", -1) and struct.pack("L", -1) do
not generate these errors, even if struct.pack("I",
-1L) and struct.pack("L", -1L) do (notice the final L).

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Attaching patch, raises struct.error. Neal, please look over it.

Note that I find struct's error handling confusing: it's not
clear from the docs in which cases OverflowError is raised,
and in which struct.error.

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Also it appears that there is some kind of interference
between range checking and byte-order chars:

  import struct; struct.pack("H", 70000)  #Exception

  import struct; struct.pack(">H", 70000) # no exception

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Assigning to Bob since he rewrote/extended the struct module
for 2.5.

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The test suite now covers this issue and it's fixed in revision 46320.

Note that this is definitely a backwards incompatible change and we might have 
to change it to a warning or something stupid like that.