| **************************** |
| What's New In Python 3.8 |
| **************************** |
| |
| .. Rules for maintenance: |
| |
| * Anyone can add text to this document. Do not spend very much time |
| on the wording of your changes, because your text will probably |
| get rewritten to some degree. |
| |
| * The maintainer will go through Misc/NEWS periodically and add |
| changes; it's therefore more important to add your changes to |
| Misc/NEWS than to this file. |
| |
| * This is not a complete list of every single change; completeness |
| is the purpose of Misc/NEWS. Some changes I consider too small |
| or esoteric to include. If such a change is added to the text, |
| I'll just remove it. (This is another reason you shouldn't spend |
| too much time on writing your addition.) |
| |
| * If you want to draw your new text to the attention of the |
| maintainer, add 'XXX' to the beginning of the paragraph or |
| section. |
| |
| * It's OK to just add a fragmentary note about a change. For |
| example: "XXX Describe the transmogrify() function added to the |
| socket module." The maintainer will research the change and |
| write the necessary text. |
| |
| * You can comment out your additions if you like, but it's not |
| necessary (especially when a final release is some months away). |
| |
| * Credit the author of a patch or bugfix. Just the name is |
| sufficient; the e-mail address isn't necessary. |
| |
| * It's helpful to add the bug/patch number as a comment: |
| |
| XXX Describe the transmogrify() function added to the socket |
| module. |
| (Contributed by P.Y. Developer in :issue:`12345`.) |
| |
| This saves the maintainer the effort of going through the Mercurial log |
| when researching a change. |
| |
| This article explains the new features in Python 3.8, compared to 3.7. |
| |
| For full details, see the :ref:`changelog <changelog>`. |
| |
| .. note:: |
| |
| Prerelease users should be aware that this document is currently in draft |
| form. It will be updated substantially as Python 3.8 moves towards release, |
| so it's worth checking back even after reading earlier versions. |
| |
| |
| Summary -- Release highlights |
| ============================= |
| |
| .. This section singles out the most important changes in Python 3.8. |
| Brevity is key. |
| |
| |
| .. PEP-sized items next. |
| |
| |
| |
| New Features |
| ============ |
| |
| Parallel filesystem cache for compiled bytecode files |
| ----------------------------------------------------- |
| |
| The new :envvar:`PYTHONPYCACHEPREFIX` setting (also available as |
| :option:`-X` ``pycache_prefix``) configures the implicit bytecode |
| cache to use a separate parallel filesystem tree, rather than |
| the default ``__pycache__`` subdirectories within each source |
| directory. |
| |
| The location of the cache is reported in :data:`sys.pycache_prefix` |
| (:const:`None` indicates the default location in ``__pycache__`` |
| subdirectories). |
| |
| (Contributed by Carl Meyer in :issue:`33499`.) |
| |
| |
| Other Language Changes |
| ====================== |
| |
| * A :keyword:`continue` statement was illegal in the :keyword:`finally` clause |
| due to a problem with the implementation. In Python 3.8 this restriction |
| was lifted. |
| (Contributed by Serhiy Storchaka in :issue:`32489`.) |
| |
| * Added support of ``\N{name}`` escapes in :mod:`regular expressions <re>`. |
| (Contributed by Jonathan Eunice and Serhiy Storchaka in :issue:`30688`.) |
| |
| |
| New Modules |
| =========== |
| |
| * None yet. |
| |
| |
| Improved Modules |
| ================ |
| |
| Optimizations |
| ============= |
| |
| * :func:`shutil.copyfile`, :func:`shutil.copy`, :func:`shutil.copy2`, |
| :func:`shutil.copytree` and :func:`shutil.move` use platform-specific |
| "fast-copy" syscalls on Linux, macOS and Solaris in order to copy the file |
| more efficiently. |
| "fast-copy" means that the copying operation occurs within the kernel, |
| avoiding the use of userspace buffers in Python as in |
| "``outfd.write(infd.read())``". |
| On Windows :func:`shutil.copyfile` uses a bigger default buffer size (1 MiB |
| instead of 16 KiB) and a :func:`memoryview`-based variant of |
| :func:`shutil.copyfileobj` is used. |
| The speedup for copying a 512 MiB file within the same partition is about |
| +26% on Linux, +50% on macOS and +40% on Windows. Also, much less CPU cycles |
| are consumed. |
| See :ref:`shutil-platform-dependent-efficient-copy-operations` section. |
| (Contributed by Giampaolo Rodola' in :issue:`25427`.) |
| |
| * The default protocol in the :mod:`pickle` module is now Protocol 4, |
| first introduced in Python 3.4. It offers better performance and smaller |
| size compared to Protocol 3 available since Python 3.0. |
| |
| * Removed one ``Py_ssize_t`` member from ``PyGC_Head``. All GC tracked |
| objects (e.g. tuple, list, dict) size is reduced 4 or 8 bytes. |
| (Contributed by Inada Naoki in :issue:`33597`) |
| |
| |
| Build and C API Changes |
| ======================= |
| |
| * The result of :c:func:`PyExceptionClass_Name` is now of type |
| ``const char *`` rather of ``char *``. |
| (Contributed by Serhiy Storchaka in :issue:`33818`.) |
| |
| * The duality of ``Modules/Setup.dist`` and ``Modules/Setup`` has been |
| removed. Previously, when updating the CPython source tree, one had |
| to manually copy ``Modules/Setup.dist`` (inside the source tree) to |
| ``Modules/Setup`` (inside the build tree) in order to reflect any changes |
| upstream. This was of a small benefit to packagers at the expense of |
| a frequent annoyance to developers following CPython development, as |
| forgetting to copy the file could produce build failures. |
| |
| Now the build system always reads from ``Modules/Setup`` inside the source |
| tree. People who want to customize that file are encouraged to maintain |
| their changes in a git fork of CPython or as patch files, as they would do |
| for any other change to the source tree. |
| |
| (Contributed by Antoine Pitrou in :issue:`32430`.) |
| |
| |
| Deprecated |
| ========== |
| |
| * Deprecated methods ``getchildren()`` and ``getiterator()`` in |
| the :mod:`~xml.etree.ElementTree` module emit now a |
| :exc:`DeprecationWarning` instead of :exc:`PendingDeprecationWarning`. |
| They will be removed in Python 3.9. |
| (Contributed by Serhiy Storchaka in :issue:`29209`.) |
| |
| |
| Removed |
| ======= |
| |
| * The ``pyvenv`` script has been removed in favor of ``python3.8 -m venv`` |
| to help eliminate confusion as to what Python interpreter the ``pyvenv`` |
| script is tied to. (Contributed by Brett Cannon in :issue:`25427`.) |
| |
| * ``parse_qs``, ``parse_qsl``, and ``escape`` are removed from :mod:`cgi` |
| module. They are deprecated from Python 3.2 or older. |
| |
| * ``filemode`` function is removed from :mod:`tarfile` module. |
| It is not documented and deprecated since Python 3.3. |
| |
| * The :class:`~xml.etree.ElementTree.XMLParser` constructor no longer accepts |
| the *html* argument. It never had effect and was deprecated in Python 3.4. |
| All other parameters are now :ref:`keyword-only <keyword-only_parameter>`. |
| (Contributed by Serhiy Storchaka in :issue:`29209`.) |
| |
| * Removed the ``doctype()`` method of :class:`~xml.etree.ElementTree.XMLParser`. |
| (Contributed by Serhiy Storchaka in :issue:`29209`.) |
| |
| |
| Porting to Python 3.8 |
| ===================== |
| |
| This section lists previously described changes and other bugfixes |
| that may require changes to your code. |
| |
| |
| Changes in Python behavior |
| -------------------------- |
| |
| * Yield expressions (both ``yield`` and ``yield from`` clauses) are now disallowed |
| in comprehensions and generator expressions (aside from the iterable expression |
| in the leftmost :keyword:`for` clause). |
| (Contributed by Serhiy Storchaka in :issue:`10544`.) |
| |
| |
| Changes in the Python API |
| ------------------------- |
| |
| * The :meth:`~tkinter.ttk.Treeview.selection` method of the |
| :class:`tkinter.ttk.Treeview` class no longer takes arguments. Using it with |
| arguments for changing the selection was deprecated in Python 3.6. Use |
| specialized methods like :meth:`~tkinter.ttk.Treeview.selection_set` for |
| changing the selection. (Contributed by Serhiy Storchaka in :issue:`31508`.) |
| |
| * A :mod:`dbm.dumb` database opened with flags ``'r'`` is now read-only. |
| :func:`dbm.dumb.open` with flags ``'r'`` and ``'w'`` no longer creates |
| a database if it does not exist. |
| (Contributed by Serhiy Storchaka in :issue:`32749`.) |
| |
| * The ``doctype()`` method defined in a subclass of |
| :class:`~xml.etree.ElementTree.XMLParser` will no longer be called and will |
| cause emitting a :exc:`RuntimeWarning` instead of a :exc:`DeprecationWarning`. |
| Define the :meth:`doctype() <xml.etree.ElementTree.TreeBuilder.doctype>` |
| method on a target for handling an XML doctype declaration. |
| (Contributed by Serhiy Storchaka in :issue:`29209`.) |
| |
| * A :exc:`RuntimeError` is now raised when the custom metaclass doesn't |
| provide the ``__classcell__`` entry in the namespace passed to |
| ``type.__new__``. A :exc:`DeprecationWarning` was emitted in Python |
| 3.6--3.7. (Contributed by Serhiy Storchaka in :issue:`23722`.) |
| |
| * The :class:`cProfile.Profile` class can now be used as a context |
| manager. (Contributed by Scott Sanderson in :issue:`29235`.) |
| |
| * :func:`shutil.copyfile`, :func:`shutil.copy`, :func:`shutil.copy2`, |
| :func:`shutil.copytree` and :func:`shutil.move` use platform-specific |
| "fast-copy" syscalls (see |
| :ref:`shutil-platform-dependent-efficient-copy-operations` section). |
| |
| * :func:`shutil.copyfile` default buffer size on Windows was changed from |
| 16 KiB to 1 MiB. |
| |
| * ``PyGC_Head`` struct is changed completely. All code touched the |
| struct member should be rewritten. (See :issue:`33597`) |
| |
| |
| CPython bytecode changes |
| ------------------------ |
| |
| * The interpreter loop has been simplified by moving the logic of unrolling |
| the stack of blocks into the compiler. The compiler emits now explicit |
| instructions for adjusting the stack of values and calling the cleaning |
| up code for :keyword:`break`, :keyword:`continue` and :keyword:`return`. |
| |
| Removed opcodes :opcode:`BREAK_LOOP`, :opcode:`CONTINUE_LOOP`, |
| :opcode:`SETUP_LOOP` and :opcode:`SETUP_EXCEPT`. Added new opcodes |
| :opcode:`ROT_FOUR`, :opcode:`BEGIN_FINALLY`, :opcode:`CALL_FINALLY` and |
| :opcode:`POP_FINALLY`. Changed the behavior of :opcode:`END_FINALLY` |
| and :opcode:`WITH_CLEANUP_START`. |
| |
| (Contributed by Mark Shannon, Antoine Pitrou and Serhiy Storchaka in |
| :issue:`17611`.) |
| |
| * Added new opcode :opcode:`END_ASYNC_FOR` for handling exceptions raised |
| when awaiting a next item in an :keyword:`async for` loop. |
| (Contributed by Serhiy Storchaka in :issue:`33041`.) |