- Issue #6939: Fix file I/O objects in the `io` module to keep the original

  file position when calling `truncate()`.  It would previously change the
  file position to the given argument, which goes against the tradition of
  ftruncate() and other truncation APIs.  Patch by Pascal Chambon.

1 files changed, 17 insertions, 3 deletions

diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 6f136f242a..b91c0a128e 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -234,6 +234,11 @@ class IOTest(unittest.TestCase):
 
     def write_ops(self, f):
         self.assertEqual(f.write(b"blah."), 5)
+        f.truncate(0)
+        self.assertEqual(f.tell(), 5)
+        f.seek(0)
+
+        self.assertEqual(f.write(b"blah."), 5)
         self.assertEqual(f.seek(0), 0)
         self.assertEqual(f.write(b"Hello."), 6)
         self.assertEqual(f.tell(), 6)
@@ -244,8 +249,9 @@ class IOTest(unittest.TestCase):
         self.assertEqual(f.write(b"h"), 1)
         self.assertEqual(f.seek(-1, 2), 13)
         self.assertEqual(f.tell(), 13)
+
         self.assertEqual(f.truncate(12), 12)
-        self.assertEqual(f.tell(), 12)
+        self.assertEqual(f.tell(), 13)
         self.assertRaises(TypeError, f.seek, 0.0)
 
     def read_ops(self, f, buffered=False):
@@ -290,7 +296,7 @@ class IOTest(unittest.TestCase):
         self.assertEqual(f.tell(), self.LARGE + 2)
         self.assertEqual(f.seek(0, 2), self.LARGE + 2)
         self.assertEqual(f.truncate(self.LARGE + 1), self.LARGE + 1)
-        self.assertEqual(f.tell(), self.LARGE + 1)
+        self.assertEqual(f.tell(), self.LARGE + 2)
         self.assertEqual(f.seek(0, 2), self.LARGE + 1)
         self.assertEqual(f.seek(-1, 2), self.LARGE)
         self.assertEqual(f.read(2), b"x")
@@ -988,7 +994,7 @@ class BufferedWriterTest(unittest.TestCase, CommonBufferedTests):
             bufio = self.tp(raw, 8)
             bufio.write(b"abcdef")
             self.assertEqual(bufio.truncate(3), 3)
-            self.assertEqual(bufio.tell(), 3)
+            self.assertEqual(bufio.tell(), 6)
         with self.open(support.TESTFN, "rb", buffering=0) as f:
             self.assertEqual(f.read(), b"abc")
 
@@ -1374,6 +1380,14 @@ class BufferedRandomTest(BufferedReaderTest, BufferedWriterTest):
             self.assertEqual(s,
                 b"A" + b"B" * overwrite_size + b"A" * (9 - overwrite_size))
 
+    def test_truncate_after_read_or_write(self):
+        raw = self.BytesIO(b"A" * 10)
+        bufio = self.tp(raw, 100)
+        self.assertEqual(bufio.read(2), b"AA") # the read buffer gets filled
+        self.assertEqual(bufio.truncate(), 2)
+        self.assertEqual(bufio.write(b"BB"), 2) # the write buffer increases
+        self.assertEqual(bufio.truncate(), 4)
+
     def test_misbehaved_io(self):
         BufferedReaderTest.test_misbehaved_io(self)
         BufferedWriterTest.test_misbehaved_io(self)