Merge pull request #33 from Eddy114514/truncate

Add warning for truncate(0)

Author: ltratt

Lib/test/test_py2xwarn.py

@@ -3,6 +3,8 @@
 from test.support.warnings_helper import check_py2x_warnings
 import warnings
 from test import test_support
+import tempfile
+
 
 if not sys.py2x_warning:
     raise unittest.SkipTest('%s must be run with the -2 flag' % __name__)
@@ -35,6 +37,16 @@ def test_next(self):
             self.assertWarning(iterator_marks.next(), w, expected)
             w.reset()
             self.assertNoWarning(iterator_marks.__next__(), w)
+            
+    def test_truncate0(self):
+        expected = "Calling truncate(0) on text stream without seek(0)" + \
+        " may produce inconsistent results. Use seek(0) before truncate(0)"
+        with check_py2x_warnings(("", Py2xWarning)) as w:
+            with tempfile.NamedTemporaryFile("w+", encoding="utf-8", delete=True) as f:
+                f.write("test")
+                f.truncate(0)
+                self.assertWarning((), w, expected)
+            
 
 if __name__ == '__main__':
     unittest.main()

Modules/_io/textio.c

@@ -2846,6 +2846,26 @@ _io_TextIOWrapper_truncate_impl(textio *self, PyObject *pos)
         return NULL;
     Py_DECREF(res);
 
+    if(Py_Py2xWarningFlag){
+        if (pos != Py_None && PyLong_Check(pos)) {
+            if (PyLong_AsLong(pos) == 0) {
+                PyObject *sres = PyObject_CallMethod((PyObject *)self, "seek", "i", 0);
+                if (sres == NULL)
+                    return NULL;
+                Py_DECREF(sres);
+                if (PyErr_WarnEx(PyExc_Py2xWarning,
+                    "Calling truncate(0) on text stream without seek(0)"
+                    " may produce inconsistent results. Use seek(0) before truncate(0)",
+                    2
+                    ) < 0){
+                        return NULL;
+                }
+            }
+            
+        }
+        
+    }
+
     return PyObject_CallMethodOneArg(self->buffer, &_Py_ID(truncate), pos);
 }