diff --git a/autopep8.py b/autopep8.py
index e5b73c11..fd0d3823 100755
--- a/autopep8.py
+++ b/autopep8.py
@@ -834,16 +834,19 @@ def fix_e265(self, result):
 
         self.source[result['line'] - 1] = fixed
 
+    def _get_e266_ignored_line_numbers(self):
+        # Optimisation: avoid unnecessary recomputation
+        if not hasattr(self, '_e266_ignored_line_numbers'):
+            source = ''.join(self.source)
+            self._e266_ignored_line_numbers = set.union(
+                multiline_string_lines(source, include_docstrings=True),
+                commented_out_code_lines(source))
+        return self._e266_ignored_line_numbers
+
     def fix_e266(self, result):
         """Fix too many block comment hashes."""
 
-        source = ''.join(self.source)
-        # TODO: don't recompute this every time!
-        ignored_line_numbers = multiline_string_lines(
-            source,
-            include_docstrings=True) | set(commented_out_code_lines(source))
-
-        if result['line'] in ignored_line_numbers:
+        if result['line'] in self._get_e266_ignored_line_numbers():
             return
 
         # Leave stylistic outlined blocks alone.