struct.pack() failing with int > 127

Question

Why is

struct.pack("!bbbb", 0x2, r, g, b)

failing in my python code when r, g, or b is > 127?

I know that the "b" means the size of a given value is 1 byte according to the struct docs, but why does it fail with values over 127?

Answer 1

According to the documentation, b stands for:

signed char

which means its valid range is [-128, 127]. And that's what the error messages says explicitly:

>>> struct.pack("!bbbb", 0x2, 127, 127, 128)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
struct.error: byte format requires -128 <= number <= 127

Using B yields no error:

>>> struct.pack("!bbbB", 0x2, 127, 127, 128)
'\x02\x7f\x7f\x80'