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167 lines
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****************************
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What's New In Python 3.8
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****************************
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.. Rules for maintenance:
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* Anyone can add text to this document. Do not spend very much time
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on the wording of your changes, because your text will probably
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get rewritten to some degree.
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* The maintainer will go through Misc/NEWS periodically and add
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changes; it's therefore more important to add your changes to
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Misc/NEWS than to this file.
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* This is not a complete list of every single change; completeness
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is the purpose of Misc/NEWS. Some changes I consider too small
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or esoteric to include. If such a change is added to the text,
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I'll just remove it. (This is another reason you shouldn't spend
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too much time on writing your addition.)
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* If you want to draw your new text to the attention of the
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maintainer, add 'XXX' to the beginning of the paragraph or
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section.
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* It's OK to just add a fragmentary note about a change. For
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example: "XXX Describe the transmogrify() function added to the
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socket module." The maintainer will research the change and
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write the necessary text.
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* You can comment out your additions if you like, but it's not
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necessary (especially when a final release is some months away).
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* Credit the author of a patch or bugfix. Just the name is
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sufficient; the e-mail address isn't necessary.
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* It's helpful to add the bug/patch number as a comment:
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XXX Describe the transmogrify() function added to the socket
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module.
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(Contributed by P.Y. Developer in :issue:`12345`.)
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This saves the maintainer the effort of going through the Mercurial log
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when researching a change.
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This article explains the new features in Python 3.8, compared to 3.7.
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For full details, see the :ref:`changelog <changelog>`.
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.. note::
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Prerelease users should be aware that this document is currently in draft
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form. It will be updated substantially as Python 3.8 moves towards release,
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so it's worth checking back even after reading earlier versions.
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Summary -- Release highlights
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=============================
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.. This section singles out the most important changes in Python 3.8.
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Brevity is key.
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.. PEP-sized items next.
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New Features
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============
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Other Language Changes
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======================
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* A :keyword:`continue` statement was illegal in the :keyword:`finally` clause
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due to a problem with the implementation. In Python 3.8 this restriction
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was lifted.
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(Contributed by Serhiy Storchaka in :issue:`32489`.)
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* Added support of ``\N{name}`` escapes in :mod:`regular expressions <re>`.
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(Contributed by Jonathan Eunice and Serhiy Storchaka in :issue:`30688`.)
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New Modules
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===========
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* None yet.
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Improved Modules
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================
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Optimizations
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=============
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* The default protocol in the :mod:`pickle` module is now Protocol 4,
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first introduced in Python 3.4. It offers better performance and smaller
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size compared to Protocol 3 available since Python 3.0.
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Build and C API Changes
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=======================
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Deprecated
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==========
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Removed
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=======
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Porting to Python 3.8
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=====================
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This section lists previously described changes and other bugfixes
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that may require changes to your code.
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Changes in Python behavior
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--------------------------
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* Yield expressions (both ``yield`` and ``yield from`` clauses) are now disallowed
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in comprehensions and generator expressions (aside from the iterable expression
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in the leftmost :keyword:`for` clause).
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(Contributed by Serhiy Storchaka in :issue:`10544`.)
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Changes in the Python API
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-------------------------
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* The :meth:`~tkinter.ttk.Treeview.selection` method of the
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:class:`tkinter.ttk.Treeview` class no longer takes arguments. Using it with
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arguments for changing the selection was deprecated in Python 3.6. Use
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specialized methods like :meth:`~tkinter.ttk.Treeview.selection_set` for
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changing the selection. (Contributed by Serhiy Storchaka in :issue:`31508`.)
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* A :mod:`dbm.dumb` database opened with flags ``'r'`` is now read-only.
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:func:`dbm.dumb.open` with flags ``'r'`` and ``'w'`` no longer creates
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a database if it does not exist.
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(Contributed by Serhiy Storchaka in :issue:`32749`.)
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CPython bytecode changes
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------------------------
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* The interpreter loop has been simplified by moving the logic of unrolling
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the stack of blocks into the compiler. The compiler emits now explicit
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instructions for adjusting the stack of values and calling the cleaning
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up code for :keyword:`break`, :keyword:`continue` and :keyword:`return`.
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Removed opcodes :opcode:`BREAK_LOOP`, :opcode:`CONTINUE_LOOP`,
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:opcode:`SETUP_LOOP` and :opcode:`SETUP_EXCEPT`. Added new opcodes
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:opcode:`ROT_FOUR`, :opcode:`BEGIN_FINALLY`, :opcode:`CALL_FINALLY` and
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:opcode:`POP_FINALLY`. Changed the behavior of :opcode:`END_FINALLY`
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and :opcode:`WITH_CLEANUP_START`.
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(Contributed by Mark Shannon, Antoine Pitrou and Serhiy Storchaka in
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:issue:`17611`.)
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* Added new opcode :opcode:`END_ASYNC_FOR` for handling exceptions raised
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when awaiting a next item in an :keyword:`async for` loop.
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(Contributed by Serhiy Storchaka in :issue:`33041`.)
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