Warn for truncate

Author: nanjekyejoannah

Lib/test/test_py3kwarn.py

@@ -308,6 +308,14 @@ def test_file_xreadlines(self):
             with check_py3k_warnings() as w:
                 self.assertWarning(f.xreadlines(), w, expected)
 
+    def test_bytesio_truncate(self):
+        from io import BytesIO
+        x = BytesIO(b'AAAAAA')
+        expected = "BytesIO.truncate() does not shift the file pointer: use seek(0) before doing truncate(0)"
+        self.assertWarning(x.truncate(0), w, expected)
+        w.reset()
+        self.assertNoWarning(x.truncate(-1), w)
+
     def test_file_open(self):
         expected = ("The builtin 'file()'/'open()' function is not supported in 3.x, "
                        "use the 'io.open()' function instead with the encoding keyword argument")

Modules/_io/bytesio.c

@@ -447,6 +447,10 @@ bytesio_truncate(bytesio *self, PyObject *args)
         size = PyNumber_AsSsize_t(arg, PyExc_OverflowError);
         if (size == -1 && PyErr_Occurred())
             return NULL;
+        if (size == 0 && PyErr_WarnPy3k_WithFix("BytesIO.truncate() does not shift the file pointer",
+                       "use seek(0) before doing truncate(0)", 1) < 0){
+            return NULL;
+        }
     }
     else if (arg == Py_None) {
         /* Truncate to current position if no argument is passed. */