transport: fix all_transports when required modules are missing (#232)

This lets the library work without libusb or hidapi (`--disable-libusb`, `--disable-hidapi`).

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist =
-    py27,
+    py33,
     py34,
     py35,
     py36,
@@ -9,6 +9,7 @@ envlist =
 deps =
     -rrequirements.txt
     pytest
+    mock
 commands =
     python -m compileall trezorlib/
     python trezorctl --help

trezorlib/tests/unit_tests/test_transport.py

@@ -0,0 +1,13 @@
+import mock
+
+from trezorlib.transport import all_transports
+
+
+def test_all_transports_without_hid():
+    # import all transports, assume this doesn't fail
+    transports_ref = all_transports()
+    # also shouldn't fail when bridge transport is missing
+    with mock.patch.dict('sys.modules', {'trezorlib.transport.bridge': None}):
+        transports = all_transports()
+        # there should now be less transports
+        assert len(transports_ref) > len(transports)

trezorlib/transport/__init__.py

@@ -64,11 +64,32 @@ class Transport(object):
 
 
 def all_transports():
-    from .bridge import BridgeTransport
-    from .hid import HidTransport
-    from .udp import UdpTransport
-    from .webusb import WebUsbTransport
-    return (BridgeTransport, HidTransport, UdpTransport, WebUsbTransport)
+    transports = []
+    try:
+        from .bridge import BridgeTransport
+        transports.append(BridgeTransport)
+    except:
+        pass
+
+    try:
+        from .hid import HidTransport
+        transports.append(HidTransport)
+    except:
+        pass
+
+    try:
+        from .udp import UdpTransport
+        transports.append(UdpTransport)
+    except:
+        pass
+
+    try:
+        from .webusb import WebUsbTransport
+        transports.append(WebUsbTransport)
+    except:
+        pass
+
+    return transports
 
 
 def enumerate_devices():