Issue #15381: Optimized io.BytesIO to make less allocations and copyings.

Doc/whatsnew/3.5.rst

@@ -362,6 +362,9 @@ The following performance enhancements have been added:
   The speed up can range from 3x to 15x.
   (:issue:`21486`, :issue:`21487`, :issue:`20826`)
 
+* Many operations on :class:`io.BytesIO` are now 50% to 100% faster.
+  (Contributed by Serhiy Storchaka in :issue:`15381`.)
+
 
 Build and C API Changes
 =======================

Lib/test/test_memoryio.py

@@ -718,12 +718,11 @@ class CBytesIOTest(PyBytesIOTest):
 
     @support.cpython_only
     def test_sizeof(self):
-        basesize = support.calcobjsize('P2nN2PnP')
+        basesize = support.calcobjsize('P2n2Pn')
         check = self.check_sizeof
         self.assertEqual(object.__sizeof__(io.BytesIO()), basesize)
         check(io.BytesIO(), basesize )
-        check(io.BytesIO(b'a'), basesize + 1 )
-        check(io.BytesIO(b'a' * 1000), basesize + 1000)
+        check(io.BytesIO(b'a' * 1000), basesize + sys.getsizeof(b'a' * 1000))
 
     # Various tests of copy-on-write behaviour for BytesIO.
 

Misc/NEWS

@@ -232,6 +232,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #15381: Optimized io.BytesIO to make less allocations and copyings.
+
 - Issue #22818: Splitting on a pattern that could match an empty string now
   raises a warning.  Patterns that can only match empty strings are now
   rejected.