Issue #9957: SpooledTemporaryFile.truncate() now accepts an optional size parameter, as other file-like objects.

Patch by Ryan Kelly.

Lib/tempfile.py

@@ -578,8 +578,13 @@ class SpooledTemporaryFile:
     def tell(self):
         return self._file.tell()
 
-    def truncate(self):
-        self._file.truncate()
+    def truncate(self, size=None):
+        if size is None:
+            self._file.truncate()
+        else:
+            if size > self._max_size:
+                self.rollover()
+            self._file.truncate(size)
 
     def write(self, s):
         file = self._file

Lib/test/test_tempfile.py

@@ -846,6 +846,27 @@ class test_SpooledTemporaryFile(TC):
                 pass
         self.assertRaises(ValueError, use_closed)
 
+    def test_truncate_with_size_parameter(self):
+        # A SpooledTemporaryFile can be truncated to zero size
+        f = tempfile.SpooledTemporaryFile(max_size=10)
+        f.write(b'abcdefg\n')
+        f.seek(0)
+        f.truncate()
+        self.assertFalse(f._rolled)
+        self.assertEqual(f._file.getvalue(), b'')
+        # A SpooledTemporaryFile can be truncated to a specific size
+        f = tempfile.SpooledTemporaryFile(max_size=10)
+        f.write(b'abcdefg\n')
+        f.truncate(4)
+        self.assertFalse(f._rolled)
+        self.assertEqual(f._file.getvalue(), b'abcd')
+        # A SpooledTemporaryFile rolls over if truncated to large size
+        f = tempfile.SpooledTemporaryFile(max_size=10)
+        f.write(b'abcdefg\n')
+        f.truncate(20)
+        self.assertTrue(f._rolled)
+        if has_stat:
+            self.assertEqual(os.fstat(f.fileno()).st_size, 20)
 
 test_classes.append(test_SpooledTemporaryFile)
 

Misc/NEWS

@@ -386,10 +386,12 @@ Core and Builtins
 - Issue #12380: The rjust, ljust and center methods of bytes and bytearray
   now accept a bytearray argument.
 
-
 Library
 -------
 
+- Issue #9957: SpooledTemporaryFile.truncate() now accepts an optional size
+  parameter, as other file-like objects.  Patch by Ryan Kelly.
+
 - Issue #13458: Fix a memory leak in the ssl module when decoding a
   certificate with a subjectAltName.  Patch by Robert Xiao.