webusb: don't create usb context on WebUsbTransport import

trezorlib/device.py

@@ -22,6 +22,7 @@ from .transport_hid import HidTransport
 from .transport_udp import UdpTransport
 from .transport_webusb import WebUsbTransport
 
+
 class TrezorDevice(object):
 
     @classmethod
@@ -44,7 +45,7 @@ class TrezorDevice(object):
 
     @classmethod
     def find_by_path(cls, path):
-        if path == None:
+        if path is None:
             try:
                 return cls.enumerate()[0]
             except IndexError:

trezorlib/transport_webusb.py

@@ -19,8 +19,9 @@
 from __future__ import absolute_import
 
 import time
-import usb1
 import os
+import atexit
+import usb1
 
 from .protocol_v1 import ProtocolV1
 from .protocol_v2 import ProtocolV2
@@ -35,15 +36,6 @@ ENDPOINT = 1
 DEBUG_INTERFACE = 1
 DEBUG_ENDPOINT = 2
 
-context = usb1.USBContext()
-context.open()
-
-import atexit
-
-def exit_handler():
-    context.close()
-
-atexit.register(exit_handler)
 
 class WebUsbHandle(object):
 
@@ -74,6 +66,7 @@ class WebUsbTransport(Transport):
     '''
 
     PATH_PREFIX = 'webusb'
+    context = None
 
     def __init__(self, device, protocol=None, handle=None, debug=False):
         super(WebUsbTransport, self).__init__()
@@ -97,10 +90,14 @@ class WebUsbTransport(Transport):
     def __str__(self):
         return "%s:%s" % (self.PATH_PREFIX, dev_to_str(self.device))
 
-    @staticmethod
-    def enumerate():
+    @classmethod
+    def enumerate(cls):
+        if cls.context is None:
+            cls.context = usb1.USBContext()
+            cls.context.open()
+            atexit.register(cls.context.close)
         devices = []
-        for dev in context.getDeviceIterator(skip_on_error=True):
+        for dev in cls.context.getDeviceIterator(skip_on_error=True):
             if not (is_trezor1(dev) or is_trezor2(dev) or is_trezor2_bl(dev)):
                 continue
             if not is_vendor_class(dev):
@@ -177,10 +174,12 @@ def is_trezor2(dev):
 def is_trezor2_bl(dev):
     return (dev.getVendorID(), dev.getProductID()) == DEV_TREZOR2_BL
 
+
 def is_vendor_class(dev):
     configurationId = 0
     altSettingId = 0
     return dev[configurationId][INTERFACE][altSettingId].getClass() == usb1.libusb1.LIBUSB_CLASS_VENDOR_SPEC
 
+
 def dev_to_str(dev):
     return ':'.join(str(x) for x in ['%03i' % (dev.getBusNumber(), )] + dev.getPortNumberList())