gh-158585: Optimize bytes object creation

Lib/test/test_bytes.py

@@ -410,6 +410,8 @@ def test_concat(self):
         self.assertRaises(TypeError, lambda: b1 + "def")
         self.assertRaises(TypeError, lambda: "abc" + b2)
 
+        self.assertEqual(self.type2test() + memoryview(b''), b'')
+
     def test_repeat(self):
         for b in b"abc", self.type2test(b"abc"):
             self.assertEqual(b * 3, b"abcabcabc")
@@ -1438,6 +1440,25 @@ class SubBytes(bytes):
         self.assertNotEqual(id(s), id(1 * s))
         self.assertNotEqual(id(s), id(s * 2))
 
+    @support.cpython_only
+    def test_concat_cpython(self):
+        # Test optimizations
+        empty = b''
+        s = b"abc"
+        self.assertIs(s + empty, s)
+        self.assertIs(empty + s, s)
+
+        self.assertIs(empty + bytearray(b''), empty)
+        self.assertIs(empty + memoryview(b''), empty)
+
+    @support.cpython_only
+    def test_repeat_cpython(self):
+        # Test optimizations
+        for b in b'', b'x', b"abc":
+            self.assertIs(b * 1, b)
+            self.assertIs(b * 0, b'')
+            self.assertIs(b * -23, b'')
+
 
 class ByteArrayTest(BaseBytesTest, unittest.TestCase):
     type2test = bytearray

Lib/test/test_capi/test_bytes.py

@@ -61,10 +61,12 @@ def test_fromstringandsize(self):
         self.assertEqual(fromstringandsize(b'abc'), b'abc')
         self.assertEqual(fromstringandsize(b'abc', 2), b'ab')
         self.assertEqual(fromstringandsize(b'abc\0def'), b'abc\0def')
-        self.assertEqual(fromstringandsize(b'a'), b'a')
-        self.assertEqual(fromstringandsize(b'a', 1), b'a')
-        self.assertEqual(fromstringandsize(b'', 0), b'')
-        self.assertEqual(fromstringandsize(NULL, 0), b'')
+
+        # Return singleton for 0 or 1 bytes
+        self.assertIs(fromstringandsize(b'', 0), b'')
+        self.assertIs(fromstringandsize(NULL, 0), b'')
+        self.assertIs(fromstringandsize(b'a', 1), b'a')
+
         self.assertEqual(len(fromstringandsize(NULL, 3)), 3)
         self.assertRaises((MemoryError, OverflowError),
                           fromstringandsize, NULL, PY_SSIZE_T_MAX)
@@ -79,7 +81,10 @@ def test_fromstring(self):
         fromstring = _testlimitedcapi.bytes_fromstring
 
         self.assertEqual(fromstring(b'abc\0def'), b'abc')
-        self.assertEqual(fromstring(b''), b'')
+
+        # Return singleton for 0 or 1 bytes
+        self.assertIs(fromstring(b''), b'')
+        self.assertIs(fromstring(b'a'), b'a')
 
         # CRASHES fromstring(NULL)
 

Lib/test/test_tracemalloc.py

@@ -41,6 +41,8 @@ def get_frames(nframe, lineno_delta):
 def allocate_bytes(size):
     nframe = tracemalloc.get_traceback_limit()
     bytes_len = (size - EMPTY_STRING_SIZE)
+    if bytes_len < 0:
+        raise ValueError(f"{size} bytes is too small")
     frames = get_frames(nframe, 1)
     data = b'x' * bytes_len
     return data, tracemalloc.Traceback(frames, min(len(frames), nframe))
@@ -158,7 +160,7 @@ def test_new_reference(self):
         self.assertEqual(traceback, obj_traceback)
 
     def test_set_traceback_limit(self):
-        obj_size = 10
+        obj_size = 123
 
         tracemalloc.stop()
         self.assertRaises(ValueError, tracemalloc.start, -1)