Change measured_files to a set

1 files changed, 2 insertions, 2 deletions

diff --git a/coverage/data.py b/coverage/data.py
index f03e90ca..3250196d 100644
--- a/coverage/data.py
+++ b/coverage/data.py
@@ -252,8 +252,8 @@ class CoverageJsonData(object):
         return self._runs
 
     def measured_files(self):
-        """A list of all files that had been measured."""
-        return list(self._arcs or self._lines or {})
+        """A set of all files that had been measured."""
+        return set(self._arcs or self._lines or {})
 
     def __nonzero__(self):
         return bool(self._lines or self._arcs)