Following example copies the entire buffer object while copying only smart part is needed:

    m = memoryview(b'x'*10**6)
    b'%.100b' % m

I don't know whether this is important use case that is worth an optimization. The workaround is using slicing rather than truncating in format:

    b'%b' % m[:100]

Or in the case of general buffer object:

    b'%b' % memoryview(m).cast('B')[:100]

But in that case it is not hard to add an explicit conversion to bytes.

    b'%b' % bytes(memoryview(m).cast('B')[:100])