updates from mlz repo

- asynconn: raise * from *
- asynconn: correct handling for timeout in AsynSerial
- add new py35compat
- target unit $

Change-Id: I052185ad3ebb3e3d1e3374f7ece9c7df06223951

secop/lib/asynconn.py

@@ -48,6 +48,7 @@ class ConnectionClosed(ConnectionError):
 
 class AsynConn:
     timeout = 1  # inter byte timeout
+    scheme = None
     SCHEME_MAP = {}
     connection = None  # is not None, if connected
     defaultport = None
@@ -62,11 +63,11 @@ class AsynConn:
             except (ValueError, TypeError, AssertionError):
                 if 'COM' in uri:
                     raise ValueError("the correct uri for a COM port is: "
-                                     "'serial://COM<i>[?<option>=<value>[+<option>=value ...]]'")
+                                     "'serial://COM<i>[?<option>=<value>[+<option>=value ...]]'") from None
                 if '/dev' in uri:
                     raise ValueError("the correct uri for a serial port is: "
-                                     "'serial:///dev/<tty>[?<option>=<value>[+<option>=value ...]]'")
-                raise ValueError('invalid uri: %s' % uri)
+                                     "'serial:///dev/<tty>[?<option>=<value>[+<option>=value ...]]'") from None
+                raise ValueError('invalid uri: %s' % uri) from None
             iocls = cls.SCHEME_MAP['tcp']
             uri = 'tcp://%s:%d' % host_port
         return object.__new__(iocls)
@@ -80,7 +81,9 @@ class AsynConn:
 
     @classmethod
     def __init_subclass__(cls):
-        cls.SCHEME_MAP[cls.scheme] = cls
+        """register subclass to scheme, if available"""
+        if cls.scheme:
+            cls.SCHEME_MAP[cls.scheme] = cls
 
     def disconnect(self):
         raise NotImplementedError
@@ -166,7 +169,7 @@ class AsynTcp(AsynConn):
             self.connection = tcpSocket(uri, self.defaultport, self.timeout)
         except (ConnectionRefusedError, socket.gaierror) as e:
             # indicate that retrying might make sense
-            raise CommunicationFailedError(str(e))
+            raise CommunicationFailedError(str(e)) from None
 
     def disconnect(self):
         if self.connection:
@@ -237,8 +240,8 @@ class AsynSerial(AsynConn):
             options = dict((kv.split('=') for kv in uri[1].split('+')))
         except IndexError:  # no uri[1], no options
             options = {}
-        except ValueError:
-            raise ConfigError('illegal serial options')
+        except ValueError as e:
+            raise ConfigError('illegal serial options') from e
         parity = options.pop('parity', None)  # only parity is to be treated as text
         for k, v in options.items():
             try:
@@ -251,14 +254,12 @@ class AsynSerial(AsynConn):
             if not fullname.startswith(name):
                 raise ConfigError('illegal parity: %s' % parity)
             options['parity'] = name[0]
-        if 'timeout' in options:
-            options['timeout'] = float(self.timeout)
-        else:
+        if 'timeout' not in options:
             options['timeout'] = self.timeout
         try:
             self.connection = Serial(dev, **options)
         except ValueError as e:
-            raise ConfigError(e)
+            raise ConfigError(e) from None
         # TODO: turn exceptions into ConnectionFailedError, where a retry makes sense
 
     def disconnect(self):