String type problem in python 3

[martinks]

With initializing the pump in python 3.6 it seems there's a problem with the string types.

chain = pumpy.Chain('/dev/tty.usbserial-FTDCM5J7')
pump = pumpy.PHD2000(chain)

gives

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-17-f7ad48ff2e5c> in <module>()
      1 chain = pumpy.Chain('/dev/tty.usbserial-FTDCM5J7')
----> 2 pump = pumpy.PHD2000(chain)
      3 pump.stop()

/Users/kleinschaarsbergmh/miniconda3/lib/python3.6/site-packages/pumpy.py in __init__(self, chain, address, name)
     61         the pump is connected and working."""
     62         try:
---> 63             self.write('VER')
     64             resp = self.read(17)
     65 

/Users/kleinschaarsbergmh/miniconda3/lib/python3.6/site-packages/pumpy.py in write(self, command)
     81 
     82     def write(self,command):
---> 83         self.serialcon.write(self.address + command + '\r')
     84 
     85     def read(self,bytes=5):

/Users/kleinschaarsbergmh/miniconda3/lib/python3.6/site-packages/serial/serialposix.py in write(self, data)
    530         if not self.is_open:
    531             raise portNotOpenError
--> 532         d = to_bytes(data)
    533         tx_len = length = len(d)
    534         timeout = Timeout(self._write_timeout)

/Users/kleinschaarsbergmh/miniconda3/lib/python3.6/site-packages/serial/serialutil.py in to_bytes(seq)
     61         return seq.tobytes()
     62     elif isinstance(seq, unicode):
---> 63         raise TypeError('unicode strings are not supported, please encode to bytes: {!r}'.format(seq))
     64     else:
     65         # handle list of integers and bytes (one or more items) for Python 2 and 3

TypeError: unicode strings are not supported, please encode to bytes: '00VER\r'

[martinks]

When putting in a bytestring (as it would be in Python 2)

chain = pumpy.Chain(b'/dev/tty.usbserial-FTDCM5J7')
pump = pumpy.PHD2000(chain)

this happens;

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-22-aa83e54c9617> in <module>()
----> 1 chain = pumpy.Chain('/dev/tty.usbserial-FTDCM5J7'.encode())
      2 pump = pumpy.PHD2000(chain)
      3 pump.stop()

/Users/kleinschaarsbergmh/miniconda3/lib/python3.6/site-packages/pumpy.py in __init__(self, port)
     31     """
     32     def __init__(self, port):
---> 33         serial.Serial.__init__(self,port=port, stopbits=serial.STOPBITS_TWO, parity=serial.PARITY_NONE, timeout=2)
     34         self.flushOutput()
     35         self.flushInput()

/Users/kleinschaarsbergmh/miniconda3/lib/python3.6/site-packages/serial/serialutil.py in __init__(self, port, baudrate, bytesize, parity, stopbits, timeout, xonxoff, rtscts, write_timeout, dsrdtr, inter_byte_timeout, exclusive, **kwargs)
    216 
    217         # assign values using get/set methods using the properties feature
--> 218         self.port = port
    219         self.baudrate = baudrate
    220         self.bytesize = bytesize

/Users/kleinschaarsbergmh/miniconda3/lib/python3.6/site-packages/serial/serialutil.py in port(self, port)
    262         """
    263         if port is not None and not isinstance(port, basestring):
--> 264             raise ValueError('"port" must be None or a string, not {}'.format(type(port)))
    265         was_open = self.is_open
    266         if was_open:

ValueError: "port" must be None or a string, not <class 'bytes'>

I think the problem is that in python 2 a string is a bytestring, but in python 3 a bytestring is not a string. Serial requires a bytestring, but also requires a string.

[martinks]

Though the errors seem to be in serial only, directly creating a serial.Serial object works:

import serial
serial.Serial(port='COM5',stopbits=serial.STOPBITS_TWO,parity=serial.PARITY_NONE,timeout=2)

gives

Serial<id=0x204cffd65f8, open=True>(port='COM5', baudrate=9600, bytesize=8, parity='N', stopbits=2, timeout=2, xonxoff=False, rtscts=False, dsrdtr=False)

Why does it not work with the subclass implementation of Chain?