There are 5 different usages of the bytes() constructor:

1) bytes(iterable_of_ints) -> bytes
2) bytes(string, encoding[, errors]) -> bytes
3) bytes(bytes_or_buffer) -> immutable copy of bytes_or_buffer
4) bytes(memory_view) -> bytes
5) bytes(int) <=> b'\0' * size

The docstring describes the four first usages, the documentation only (3).
http://docs.python.org/dev/library/functions.html#bytes

Note: bytes(3) accepts 2 other arguments, but I don't think that they are used: bytes(3, 'unused encoding', 'unused errors').