@jcaron I think that this was @tttadam's conclusion.

Never mind I found the problem. It was actually in the line above paramVal = self._device.readfrom_mem(self._address, SI1145_REG_PARAMRD, 1)

Python makes a clear distinction between bytes and strings . Bytes objects contain raw data — a sequence of octets — whereas strings are Unicode sequences . Conversion between these two types is explicit: you encode a string to get bytes, specifying an encoding (which defaults to UTF-8); and you decode bytes to get a string. Clients of these functions should be aware that such conversions may fail, and should consider how failures are handled. We can convert bytes to string using bytes class decode() instance method, So you need to decode the bytes object to produce a string. In Python 3 , the default encoding is "utf-8" , so you can use directly: b"python byte to string".decode("utf-8")