Conditional import of adblocker.

tools/android/loading/content_classification_lens.py

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Labels requests according to the type of content they represent."""
 
-import adblockparser # Available on PyPI, through pip.
 import collections
+import logging
 import os
 
 import loading_trace
 import request_track
 
 
 class ContentClassificationLens(object):
   """Associates requests and frames with the type of content they represent."""
   def __init__(self, trace, ad_rules, tracking_rules):
     """Initializes an instance of ContentClassificationLens.
@@ skipping 67 matching lines @@
       'Script': 'script', 'Stylesheet': 'stylesheet', 'Image': 'image',
       'XHR': 'xmlhttprequest'}
   def __init__(self, rules, no_whitelist):
     """Initializes an instance of _RulesMatcher.
 
     Args:
       rules: ([str]) list of rules.
       no_whitelist: (bool) Whether the whitelisting rules should be ignored.
     """
     self._rules = self._FilterRules(rules, no_whitelist)
-    self._matcher = adblockparser.AdblockRules(self._rules)
+    if self._rules:

[blundell, 2016/01/28 11:55:36]

Maybe a comment above this like "Only try to include adblockparser if the user
has actually supplied rules to avoid requiring that all users install
adblockparser."

+      try:
+        import adblockparser
+        self._matcher = adblockparser.AdblockRules(self._rules)
+      except ImportError:
+        logging.critical('Likely you need to install adblockparser. Try:\n'
+                         ' pip install --user adblockparser\n'
+                         'For 10-100x better performance, also try:\n'
+                         " pip install --user 're2 >= 0.2.21'")
+        raise
+    else:
+      self._matcher = None
 
   def Matches(self, request):
     """Returns whether a request matches one of the rules."""
+    if self._matcher is None:
+      return False
     url = request.url
     return self._matcher.should_block(url, self._GetOptions(request))
 
   @classmethod
   def _GetOptions(cls, request):
     options = {}
     resource_type = request.resource_type
     option = cls._RESOURCE_TYPE_TO_OPTIONS_KEY.get(resource_type)
     if option:
       options[option] = True
     return options
 
   @classmethod
   def _FilterRules(cls, rules, no_whitelist):
     if not no_whitelist:
       return rules
     else:
       return [rule for rule in rules
               if not rule.startswith(cls._WHITELIST_PREFIX)]