Hooks: Clean up the USB hook logic.

* Normalize logic: however libraries are discovered, they are
accumulated into the 'binaries' list. At the end of the hook, they
are validated and transformed into the structure that the hook engine
requires.

* Put Cygwin logic through the same code path that regular filesystem
library discovery uses, just with a different set of candidate
filenames.

* Update the comments to be more descriptive, add some small grammar
fixes.

* Delete spurious whitespace in response to PR feedback.

PyInstaller/hooks/hook-usb.py

@@ -9,8 +9,6 @@
 
 import ctypes.util
 import os
-import usb.core
-import usb.backend
 
 from PyInstaller.depend.utils import _resolveCtypesImports
 from PyInstaller.compat import is_cygwin, getenv
@@ -23,69 +21,66 @@
 # https://github.com/walac/pyusb/blob/master/docs/faq.rst
 # https://github.com/walac/pyusb/blob/master/docs/tutorial.rst
 
-binaries=[]
+binaries = []
 
-# loading usb.core.find in this script crashes Ubuntu 14.04LTS,
+
+# Running usb.core.find() in this script crashes Ubuntu 14.04LTS,
 # let users circumvent pyusb discovery with an environment variable.
 skip_pyusb_discovery = \
     bool(getenv('PYINSTALLER_USB_HOOK_SKIP_PYUSB_DISCOVERY'))
 
-# first try to use pyusb library locator
+
+# Try to use pyusb's library locator.
 if not skip_pyusb_discovery:
+    import usb.core
+    import usb.backend
     try:
         # get the backend symbols before find
-        pyusb_backend_dir = set(dir(usb.backend))
-
+        backend_contents_before_discovery = set(dir(usb.backend))
         # perform find, which will load a usb library if found
         usb.core.find()
-
         # get the backend symbols which have been added (loaded)
-        backends = set(dir(usb.backend)) - pyusb_backend_dir
-
-        # for each of the loaded backends, see if they have a library
-        binaries = []
+        backends = set(dir(usb.backend)) - backend_contents_before_discovery
+        # gather the libraries from the loaded backends
+        backend_lib_basenames = []
         for usblib in [getattr(usb.backend, be)._lib for be in backends]:
             if usblib is not None:
-                binaries = [(usblib._name, '')]
-
+                # OSX returns the full path, Linux only the filename.
+                # save the basename and reconstruct the path after gathering.
+                backend_lib_basenames.append(os.path.basename(usblib._name))
+        # try to resolve the library names to absolute paths.
+        binaries = _resolveCtypesImports(backend_lib_basenames)
     except (ValueError, usb.core.USBError) as exc:
         logger.warning("%s", exc)
 
 
-# if nothing found, try to use our custom mechanism
+# If pyusb didn't find a backend, manually search for usb libraries.
 if not binaries:
-    # Try to resolve your libusb libraries in the following order:
-    #
-    #   libusb-1.0, libusb-0.1, openusb
-    #
-    # NOTE: Mind updating run-time hook when adding further libs.
-    libusb_candidates = (
-        # libusb10
-        'usb-1.0', 'usb', 'libusb-1.0',
-        # libusb01
-        'usb-0.1', 'libusb0',
-        # openusb
-        'openusb',
-    )
-
+    # NOTE: Update these lists when adding further libs.
+    if is_cygwin:
+        libusb_candidates = ['cygusb-1.0-0.dll', 'cygusb0.dll']
+    else:
+        libusb_candidates = [
+            # libusb10
+            'usb-1.0', 'usb', 'libusb-1.0',
+            # libusb01
+            'usb-0.1', 'libusb0',
+            # openusb
+            'openusb',
+        ]
+
+    backend_library_basenames = []
     for candidate in libusb_candidates:
         libname = ctypes.util.find_library(candidate)
         if libname is not None:
-            break
+            backend_library_basenames.append(os.path.basename(libname))
+    if backend_library_basenames:
+        binaries = _resolveCtypesImports(backend_library_basenames)
 
-    if libname is not None:
-        # Use basename here because Python returns full library path
-        # on Mac OSX when using ctypes.util.find_library.
-        bins = [os.path.basename(libname)]
-        binaries = _resolveCtypesImports(bins)
-    elif is_cygwin:
-        bins = ['cygusb-1.0-0.dll', 'cygusb0.dll']
-        binaries = _resolveCtypesImports(bins)[:1] # use only the first one
-    else:
-        binaries = []
 
-    if binaries:
-        # `_resolveCtypesImports` returns a 3-tuple, but `binaries` are only
-        # 2-tuples, so remove the last element:
-        assert len(binaries[0]) == 3
-        binaries = [(binaries[0][1], '')]
+# Validate and normalize the first found usb library.
+if binaries:
+    # `_resolveCtypesImports` returns a 3-tuple, but `binaries` are only
+    # 2-tuples, so remove the last element:
+    assert len(binaries[0]) == 3
+    binaries = [(binaries[0][1], '')]