bpo-35950: Raise UnsupportedOperation in BufferedReader.truncate() (GH-18586)

The truncate() method of io.BufferedReader() should raise
UnsupportedOperation when it is called on a read-only
io.BufferedReader() instance.





https://bugs.python.org/issue35950



Automerge-Triggered-By: @methane

2 files changed, 14 insertions, 0 deletions

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 4c24146..8eaa114 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -792,6 +792,9 @@ class _BufferedIOMixin(BufferedIOBase):
         return pos
 
     def truncate(self, pos=None):
+        self._checkClosed()
+        self._checkWritable()
+
         # Flush the stream.  We're mixing buffered I/O with lower-level I/O,
         # and a flush may be necessary to synch both views of the current
         # file state.
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 0bfa4d2..c27dfd9 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -1528,6 +1528,13 @@ class BufferedReaderTest(unittest.TestCase, CommonBufferedTests):
         self.assertRaises(ValueError, b.peek)
         self.assertRaises(ValueError, b.read1, 1)
 
+    def test_truncate_on_read_only(self):
+        rawio = self.MockFileIO(b"abc")
+        bufio = self.tp(rawio)
+        self.assertFalse(bufio.writable())
+        self.assertRaises(self.UnsupportedOperation, bufio.truncate)
+        self.assertRaises(self.UnsupportedOperation, bufio.truncate, 0)
+
 
 class CBufferedReaderTest(BufferedReaderTest, SizeofTest):
     tp = io.BufferedReader
@@ -2372,6 +2379,10 @@ class BufferedRandomTest(BufferedReaderTest, BufferedWriterTest):
     # You can't construct a BufferedRandom over a non-seekable stream.
     test_unseekable = None
 
+    # writable() returns True, so there's no point to test it over
+    # a writable stream.
+    test_truncate_on_read_only = None
+
 
 class CBufferedRandomTest(BufferedRandomTest, SizeofTest):
     tp = io.BufferedRandom