Small style fixes following #382

Tested with `tox`.

libagent/device/jade.py

@@ -1,15 +1,12 @@
 """Jade-related code (see https://www.keepkey.com/)."""
 
-import ecdsa
 import logging
+
+import ecdsa
 import semver
-from serial.tools import list_ports
-
-from jadepy import JadeAPI
 
+from .. import formats, util
 from . import interface
-from .. import formats
-from .. import util
 
 log = logging.getLogger(__name__)
 
@@ -23,6 +20,7 @@ def _verify_support(identity, ecdh):
 
 class BlockstreamJade(interface.Device):
     """Connection to Blockstream Jade device."""
+
     MIN_SUPPORTED_FW_VERSION = semver.VersionInfo(0, 1, 33)
     DEVICE_IDS = [(0x10c4, 0xea60), (0x1a86, 0x55d4)]
     connection = None
@@ -33,6 +31,11 @@ class BlockstreamJade(interface.Device):
         return 'jade-agent'
 
     def connect(self):
+        """Connect to the first matching device."""
+        # pylint: disable=import-error
+        from jadepy import JadeAPI
+        from serial.tools import list_ports
+
         # Return the existing connection if we have one
         if BlockstreamJade.connection is not None:
             return BlockstreamJade.connection
@@ -41,7 +44,6 @@ class BlockstreamJade(interface.Device):
         # Scan com ports looking for the relevant vid and pid, and connect to the
         # first matching device.  Then call 'auth_user' - this usually requires network
         # access in order to unlock the device with a PIN and the remote blind pinserver.
-        devices = []
         for devinfo in list_ports.comports():
             device_product_key = (devinfo.vid, devinfo.pid)
             if device_product_key in self.DEVICE_IDS:
@@ -59,7 +61,7 @@ class BlockstreamJade(interface.Device):
                     fwversion = semver.VersionInfo.parse(verinfo['JADE_VERSION'])
                     if self.MIN_SUPPORTED_FW_VERSION > fwversion.finalize_version():
                         msg = ('Outdated {} firmware for device. Please update using'
-                              ' a Blockstream Green companion app')
+                               ' a Blockstream Green companion app')
                         raise ValueError(msg.format(fwversion))
 
                     # Authenticate the user (unlock with pin)
@@ -75,13 +77,14 @@ class BlockstreamJade(interface.Device):
                 except Exception as e:
                     raise interface.NotFoundError(
                         '{} not connected: "{}"'.format(self, e))
+        return None
 
     @staticmethod
-    def get_identity_string(identity):
+    def _get_identity_string(identity):
         return interface.identity_to_string(identity.identity_dict)
 
     @staticmethod
-    def load_uncompressed_pubkey(pubkey, curve_name):
+    def _load_uncompressed_pubkey(pubkey, curve_name):
         assert curve_name == formats.CURVE_NIST256
         assert len(pubkey) == 65 and pubkey[0] == 0x04
         curve = ecdsa.NIST256p
@@ -94,7 +97,7 @@ class BlockstreamJade(interface.Device):
     def pubkey(self, identity, ecdh=False):
         """Get PublicKey object for specified BIP32 address and elliptic curve."""
         _verify_support(identity, ecdh)
-        identity_string = self.get_identity_string(identity)
+        identity_string = self._get_identity_string(identity)
         curve_name = identity.get_curve_name(ecdh=ecdh)
         key_type = 'slip-0017' if ecdh else 'slip-0013'
 
@@ -104,13 +107,15 @@ class BlockstreamJade(interface.Device):
         log.debug('result: %s', result)
 
         assert len(result) == 33 or len(result) == 65
-        convert_pubkey = formats.decompress_pubkey if len(result) == 33 else self.load_uncompressed_pubkey
+        convert_pubkey = (formats.decompress_pubkey
+                          if len(result) == 33 else
+                          self._load_uncompressed_pubkey)
         return convert_pubkey(pubkey=result, curve_name=curve_name)
 
     def sign(self, identity, blob):
         """Sign given blob and return the signature (as bytes)."""
         _verify_support(identity, ecdh=False)
-        identity_string = self.get_identity_string(identity)
+        identity_string = self._get_identity_string(identity)
         curve_name = identity.get_curve_name(ecdh=False)
 
         log.debug('"%s" signing %r (%s) on %s',
@@ -127,7 +132,7 @@ class BlockstreamJade(interface.Device):
     def ecdh(self, identity, pubkey):
         """Get shared session key using Elliptic Curve Diffie-Hellman."""
         _verify_support(identity, ecdh=True)
-        identity_string = self.get_identity_string(identity)
+        identity_string = self._get_identity_string(identity)
         curve_name = identity.get_curve_name(ecdh=True)
 
         log.debug('"%s" shared session key (%s) for %r from %s',