Fix writing I2C messages when I2C addr >= 0x80

Since strings in python3 are UTF-8, calling chr() on an I2C address
>= 0x80 would create an invalid UTF-8 character (0x80 to 0xFF are
illegal). Then later, when encode() was called in add_string(), a
UnicodeEncodeError exception would be thrown because of this.

This adds a new add_bytes() method to the Telegram class for adding
bytes objects. Then, this new method is used in ls_write() so that
we can pass arbitrary I2C addresses without error.

nxt/direct.py

@@ -156,7 +156,7 @@ def ls_write(opcode, port, tx_data, rx_bytes):
     tgram.add_u8(port)
     tgram.add_u8(len(tx_data))
     tgram.add_u8(rx_bytes)
-    tgram.add_string(len(tx_data), tx_data)
+    tgram.add_bytes(tx_data)
     return tgram
 
 def ls_read(opcode, port):

nxt/sensor/digital.py

@@ -95,7 +95,7 @@ def _i2c_command(self, address, value, format):
         a tuple of values corresponding to the given format.
         """
         value = struct.pack(format, *value)
-        msg = chr(self.I2C_DEV) + chr(address) + value
+        msg = bytes((self.I2C_DEV, address)) + value
         now = time()
         if self.last_poll+self.poll_delay > now:
             diff = now - self.last_poll
@@ -109,7 +109,7 @@ def _i2c_query(self, address, format):
         module. See http://docs.python.org/library/struct.html#format-strings
         """
         n_bytes = struct.calcsize(format)
-        msg = chr(self.I2C_DEV) + chr(address)
+        msg = bytes((self.I2C_DEV, address))
         now = time()
         if self.last_poll+self.poll_delay > now:
             diff = now - self.last_poll

nxt/telegram.py

@@ -64,6 +64,9 @@ def bytes(self):
     def is_reply(self):
         return self.typ == Telegram.TYPE_REPLY
 
+    def add_bytes(self, b):
+        self.pkt.write(pack('%ds' % len(b), b))
+
     def add_string(self, n_bytes, v):
         self.pkt.write(pack('%ds' % n_bytes, v.encode('windows-1252')))