1 files changed, 23 insertions, 0 deletions
diff --git a/Lib/lib2to3/fixes/basefix.py b/Lib/lib2to3/fixes/basefix.py
index 38666c1..937f8c1 100644
--- a/Lib/lib2to3/fixes/basefix.py
+++ b/Lib/lib2to3/fixes/basefix.py
@@ -16,6 +16,7 @@ except NameError:
 # Local imports
 from ..patcomp import PatternCompiler
 from .. import pygram
+from .util import does_tree_import
 
 class BaseFix(object):
 
@@ -36,6 +37,8 @@ class BaseFix(object):
     used_names = set() # A set of all used NAMEs
     order = "post" # Does the fixer prefer pre- or post-order traversal
     explicit = False # Is this ignored by refactor.py -f all?
+    run_order = 5   # Fixers will be sorted by run order before execution
+                    # Lower numbers will be run first.
 
     # Shortcut for access to Python grammar symbols
     syms = pygram.python_symbols
@@ -163,3 +166,23 @@ class BaseFix(object):
         filename - the name of the file the tree came from.
         """
         pass
+
+
+class ConditionalFix(BaseFix):
+    """ Base class for fixers which not execute if an import is found. """
+
+    # This is the name of the import which, if found, will cause the test to be skipped
+    skip_on = None
+
+    def start_tree(self, *args):
+        super(ConditionalFix, self).start_tree(*args)
+        self._should_skip = None
+
+    def should_skip(self, node):
+        if self._should_skip is not None:
+            return self._should_skip
+        pkg = self.skip_on.split(".")
+        name = pkg[-1]
+        pkg = ".".join(pkg[:-1])
+        self._should_skip = does_tree_import(pkg, name, node)
+        return self._should_skip