bpo-16580: [doc] Add examples to int.to_bytes and int.from_bytes (GH-27760)

* added code equivs. for to_bytes and from_bytes

Based on woparry's patch[1] from the relevant issue thread[2].

[1]: https://bugs.python.org/file30372/issue16580.patch
[2]: https://bugs.python.org/issue16580

Co-authored-by: Mark Dickinson <dickinsm@gmail.com>

1 files changed, 42 insertions, 0 deletions

diff --git a/Lib/test/test_long.py b/Lib/test/test_long.py
index c97842b..1de75bf 100644
--- a/Lib/test/test_long.py
+++ b/Lib/test/test_long.py
@@ -1103,6 +1103,13 @@ class LongTest(unittest.TestCase):
 
     def test_to_bytes(self):
         def check(tests, byteorder, signed=False):
+            def equivalent_python(n, length, byteorder, signed=False):
+                if byteorder == 'little':
+                    order = range(length)
+                elif byteorder == 'big':
+                    order = reversed(range(length))
+                return bytes((n >> i*8) & 0xff for i in order)
+
             for test, expected in tests.items():
                 try:
                     self.assertEqual(
@@ -1113,6 +1120,18 @@ class LongTest(unittest.TestCase):
                         "failed to convert {0} with byteorder={1} and signed={2}"
                         .format(test, byteorder, signed)) from err
 
+                try:
+                    self.assertEqual(
+                        equivalent_python(
+                            test, len(expected), byteorder, signed=signed),
+                        expected
+                    )
+                except Exception as err:
+                    raise AssertionError(
+                        "Code equivalent from docs is not equivalent for "
+                        "conversion of {0} with byteorder byteorder={1} and "
+                        "signed={2}".format(test, byteorder, signed)) from err
+
         # Convert integers to signed big-endian byte arrays.
         tests1 = {
             0: b'\x00',
@@ -1202,6 +1221,18 @@ class LongTest(unittest.TestCase):
 
     def test_from_bytes(self):
         def check(tests, byteorder, signed=False):
+            def equivalent_python(byte_array, byteorder, signed=False):
+                if byteorder == 'little':
+                    little_ordered = list(byte_array)
+                elif byteorder == 'big':
+                    little_ordered = list(reversed(byte_array))
+
+                n = sum(b << i*8 for i, b in enumerate(little_ordered))
+                if signed and little_ordered and (little_ordered[-1] & 0x80):
+                    n -= 1 << 8*len(little_ordered)
+
+                return n
+
             for test, expected in tests.items():
                 try:
                     self.assertEqual(
@@ -1212,6 +1243,17 @@ class LongTest(unittest.TestCase):
                         "failed to convert {0} with byteorder={1!r} and signed={2}"
                         .format(test, byteorder, signed)) from err
 
+                try:
+                    self.assertEqual(
+                        equivalent_python(test, byteorder, signed=signed),
+                        expected
+                    )
+                except Exception as err:
+                    raise AssertionError(
+                        "Code equivalent from docs is not equivalent for "
+                        "conversion of {0} with byteorder={1!r} and signed={2}"
+                        .format(test, byteorder, signed)) from err
+
         # Convert signed big-endian byte arrays to integers.
         tests1 = {
             b'': 0,