I cannot convert String to Bytes.

I try to convert a string (`'55 55 00 00'`) in hex format to bytes and send those bytes out via Pyserial, but i found i have error on string to bytes conversion. My code is:

```
msg = '55 55 00 00'

bytes.fromhex(msg)

```

I expected the result should be `b'\0x55\0x55\0x00\0x00'`, why is the result  

`b'UU\x00\x00'`.

What is `UU`?