CoverageData.read and .write are now inverses of each other.

3 files changed, 19 insertions, 10 deletions

diff --git a/CHANGES.txt b/CHANGES.txt
index 47d7bebf..4f7c0fc8 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -18,6 +18,9 @@ Latest
   message.  This is now fixed, and changed some method names in the
   CoverageData interface.  Fixes `issue 399`_.
 
+- `CoverageData.read` and `CoverageData.write` are now properly inverses of
+  each other.
+
 - When using ``report --skip-covered``, a message will now be included in the
   report output indicating how many files were skipped, and if all files are
   skipped, coverage.py won't accidentally scold you for having no data to
diff --git a/coverage/data.py b/coverage/data.py
index 974764b0..c319e65a 100644
--- a/coverage/data.py
+++ b/coverage/data.py
@@ -297,19 +297,14 @@ class CoverageData(object):
     @classmethod
     def _open_for_reading(cls, filename):
         """Open a file appropriately for reading data."""
-        f = open(filename, "r")
-        try:
-            go_away = f.read(len(cls._GO_AWAY))
-            if go_away != cls._GO_AWAY:
-                raise CoverageException("Doesn't seem to be a coverage.py data file")
-        except Exception:
-            f.close()
-            raise
-        return f
+        return open(filename, "r")
 
     @classmethod
     def _read_raw_data(cls, file_obj):
         """Read the raw data from a file object."""
+        go_away = file_obj.read(len(cls._GO_AWAY))
+        if go_away != cls._GO_AWAY:
+            raise CoverageException("Doesn't seem to be a coverage.py data file")
         return json.load(file_obj)
 
     @classmethod
diff --git a/tests/test_data.py b/tests/test_data.py
index 653a401a..ea6b0df0 100644
--- a/tests/test_data.py
+++ b/tests/test_data.py
@@ -11,7 +11,7 @@ import re
 
 import mock
 
-from coverage.backward import iitems
+from coverage.backward import iitems, StringIO
 from coverage.data import CoverageData, CoverageDataFiles, debug_main
 from coverage.files import PathAliases, canonical_filename
 from coverage.misc import CoverageException
@@ -393,6 +393,17 @@ class CoverageDataTest(DataTestHelpers, CoverageTest):
         covdata.touch_file("abc.py")
         self.assertTrue(covdata.has_arcs())
 
+    def test_read_and_write_are_opposites(self):
+        covdata1 = CoverageData()
+        covdata1.add_arcs(ARCS_3)
+        stringio = StringIO()
+        covdata1.write(stringio)
+
+        stringio.seek(0)
+        covdata2 = CoverageData()
+        covdata2.read(stringio)
+        self.assert_arcs3_data(covdata2)
+
 
 class CoverageDataTestInTempDir(DataTestHelpers, CoverageTest):
     """Tests of CoverageData that need a temporary directory to make files."""