This article explains the new features in Python 3.8, compared to 3.7.

For full details, see the :ref:`changelog <changelog>`.

There is new syntax (the "walrus operator", :=) to assign values to variables as part of an expression. Example:

if (n := len(a)) > 10:
    print(f"List is too long ({n} elements, expected <= 10)")

See PEP 572 for a full description.

(Contributed by Emily Morehouse in :issue:`35224`.)

There is new syntax (/) to indicate that some function parameters must be specified positionally (i.e., cannot be used as keyword arguments). This is the same notation as shown by help() for functions implemented in C (produced by Larry Hastings' "Argument Clinic" tool). Example:

def pow(x, y, z=None, /):
    r = x**y
    if z is not None:
        r %= z
    return r

Now pow(2, 10) and pow(2, 10, 17) are valid calls, but pow(x=2, y=10) and pow(2, 10, z=17) are invalid.

See PEP 570 for a full description.

(Contributed by Pablo Galindo in :issue:`36540`.)

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`.)

Python now uses the same ABI whether it built in release or debug mode. On Unix, when Python is built in debug mode, it is now possible to load C extensions built in release mode and C extensions built using the stable ABI.

Release builds and debug builds are now ABI compatible: defining the Py_DEBUG macro no longer implies the Py_TRACE_REFS macro, which introduces the only ABI incompatibility. The Py_TRACE_REFS macro, which adds the :func:`sys.getobjects` function and the :envvar:`PYTHONDUMPREFS` environment variable, can be set using the new ./configure --with-trace-refs build option. (Contributed by Victor Stinner in :issue:`36465`.)

On Unix, C extensions are no longer linked to libpython. It is now possible for a statically linked Python to load a C extension built using a shared library Python. (Contributed by Victor Stinner in :issue:`21536`.)

On Unix, when Python is built in debug mode, import now also looks for C extensions compiled in release mode and for C extensions compiled with the stable ABI. (Contributed by Victor Stinner in :issue:`36722`.)

• 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`.)

• The :class:`int` type now has a new :meth:`~int.as_integer_ratio` method compatible with the existing :meth:`float.as_integer_ratio` method. (Contributed by Lisa Roach in :issue:`33073`.)

• Added support of \N{name} escapes in :mod:`regular expressions <re>`. (Contributed by Jonathan Eunice and Serhiy Storchaka in :issue:`30688`.)

• Dict and dictviews are now iterable in reversed insertion order using :func:`reversed`. (Contributed by Rémi Lapeyre in :issue:`33462`.)

• The syntax allowed for keyword names in function calls was further restricted. In particular, f((keyword)=arg) is no longer allowed. It was never intended to permit more than a bare name on the left-hand side of a keyword argument assignment term. See :issue:`34641`.

• Iterable unpacking is now allowed without parentheses in :keyword:`yield` and :keyword:`return` statements. (Contributed by David Cuthbert and Jordan Chapman in :issue:`32117`.)

• A backslash-character pair that is not a valid escape sequence generates a :exc:`DeprecationWarning` since Python 3.6. In Python 3.8 it generates a :exc:`SyntaxWarning` instead. (Contributed by Serhiy Storchaka in :issue:`32912`.)

• The compiler now produces a :exc:`SyntaxWarning` in some cases when a comma is missed before tuple or list. For example:

  data = [
      (1, 2, 3) # oops, missing comma!
      (4, 5, 6)
  ]

  (Contributed by Serhiy Storchaka in :issue:`15248`.)

• Arithmetic operations between subclasses of :class:`datetime.date` or :class:`datetime.datetime` and :class:`datetime.timedelta` objects now return an instance of the subclass, rather than the base class. This also affects the return type of operations whose implementation (directly or indirectly) uses :class:`datetime.timedelta` arithmetic, such as :meth:`datetime.datetime.astimezone`. (Contributed by Paul Ganssle in :issue:`32417`.)

• When the Python interpreter is interrupted by Ctrl-C (SIGINT) and the resulting :exc:`KeyboardInterrupt` exception is not caught, the Python process now exits via a SIGINT signal or with the correct exit code such that the calling process can detect that it died due to a Ctrl-C. Shells on POSIX and Windows use this to properly terminate scripts in interactive sessions. (Contributed by Google via Gregory P. Smith in :issue:`1054041`.)

• None yet.

On Windows, the default event loop is now :class:`~asyncio.ProactorEventLoop`.

The :meth:`_asdict()` method for :func:`collections.namedtuple` now returns a :class:`dict` instead of a :class:`collections.OrderedDict`. This works because regular dicts have guaranteed ordering since Python 3.7. If the extra features of :class:`OrderedDict` are required, the suggested remediation is to cast the result to the desired type: OrderedDict(nt._asdict()). (Contributed by Raymond Hettinger in :issue:`35864`.)

On Windows, :class:`~ctypes.CDLL` and subclasses now accept a winmode parameter to specify flags for the underlying LoadLibraryEx call. The default flags are set to only load DLL dependencies from trusted locations, including the path where the DLL is stored (if a full or partial path is used to load the initial DLL) and paths added by :func:`~os.add_dll_directory`.

Added new alternate constructors :meth:`datetime.date.fromisocalendar` and :meth:`datetime.datetime.fromisocalendar`, which construct :class:`date` and :class:`datetime` objects respectively from ISO year, week number and weekday; these are the inverse of each class's isocalendar method. (Contributed by Paul Ganssle in :issue:`36004`.)

Added :func:`~gettext.pgettext` and its variants. (Contributed by Franz Glasner, Éric Araujo, and Cheryl Sabella in :issue:`2504`.)

The :func:`inspect.getdoc` function can now find docstrings for __slots__ if that attribute is a :class:`dict` where the values are docstrings. This provides documentation options similar to what we already have for :func:`property`, :func:`classmethod`, and :func:`staticmethod`:

class AudioClip:
    __slots__ = {'bit_rate': 'expressed in kilohertz to one decimal place',
                 'duration': 'in seconds, rounded up to an integer'}
    def __init__(self, bit_rate, duration):
        self.bit_rate = round(bit_rate / 1000.0, 1)
        self.duration = ceil(duration)

:func:`~gc.get_objects` can now receive an optional generation parameter indicating a generation to get objects from. Contributed in :issue:`36016` by Pablo Galindo.

Added the mtime parameter to :func:`gzip.compress` for reproducible output. (Contributed by Guo Ci Teo in :issue:`34898`.)

Output over N lines (50 by default) is squeezed down to a button. N can be changed in the PyShell section of the General page of the Settings dialog. Fewer, but possibly extra long, lines can be squeezed by right clicking on the output. Squeezed output can be expanded in place by double-clicking the button or into the clipboard or a separate window by right-clicking the button. (Contributed by Tal Einat in :issue:`1529353`.)

The changes above have been backported to 3.7 maintenance releases.

Add option --json-lines to parse every input line as separate JSON object. (Contributed by Weipeng Hong in :issue:`31553`.)

Added new function :func:`math.dist` for computing Euclidean distance between two points. (Contributed by Raymond Hettinger in :issue:`33089`.)

Expanded the :func:`math.hypot` function to handle multiple dimensions. Formerly, it only supported the 2-D case. (Contributed by Raymond Hettinger in :issue:`33089`.)

Added new function, :func:`math.prod`, as analogous function to :func:`sum` that returns the product of a 'start' value (default: 1) times an iterable of numbers. (Contributed by Pablo Galindo in :issue:`35606`)

Added new function :func:`~os.add_dll_directory` on Windows for providing additional search paths for native dependencies when importing extension modules or loading DLLs using :mod:`ctypes`.

:mod:`os.path` functions that return a boolean result like :func:`~os.path.exists`, :func:`~os.path.lexists`, :func:`~os.path.isdir`, :func:`~os.path.isfile`, :func:`~os.path.islink`, and :func:`~os.path.ismount` now return False instead of raising :exc:`ValueError` or its subclasses :exc:`UnicodeEncodeError` and :exc:`UnicodeDecodeError` for paths that contain characters or bytes unrepresentable at the OS level. (Contributed by Serhiy Storchaka in :issue:`33721`.)

:func:`~os.path.expanduser` on Windows now prefers the :envvar:`USERPROFILE` environment variable and does not use :envvar:`HOME`, which is not normally set for regular user accounts.

Added a new variable holding structured version information for the underlying ncurses library: :data:`~curses.ncurses_version`. (Contributed by Serhiy Storchaka in :issue:`31680`.)

:mod:`pathlib.Path` methods that return a boolean result like :meth:`~pathlib.Path.exists()`, :meth:`~pathlib.Path.is_dir()`, :meth:`~pathlib.Path.is_file()`, :meth:`~pathlib.Path.is_mount()`, :meth:`~pathlib.Path.is_symlink()`, :meth:`~pathlib.Path.is_block_device()`, :meth:`~pathlib.Path.is_char_device()`, :meth:`~pathlib.Path.is_fifo()`, :meth:`~pathlib.Path.is_socket()` now return False instead of raising :exc:`ValueError` or its subclass :exc:`UnicodeEncodeError` for paths that contain characters unrepresentable at the OS level. (Contributed by Serhiy Storchaka in :issue:`33721`.)

Added :meth:`pathlib.Path.link_to()` which creates a hard link pointing to a path. (Contributed by Joannah Nanjekye in :issue:`26978`)

Added :meth:`~socket.create_server()` and :meth:`~socket.has_dualstack_ipv6()` convenience functions to automate the necessary tasks usually involved when creating a server socket, including accepting both IPv4 and IPv6 connections on the same socket. (Contributed by Giampaolo Rodola in :issue:`17561`.)

:func:`shutil.copytree` now accepts a new dirs_exist_ok keyword argument. (Contributed by Josh Bronson in :issue:`20849`.)

:func:`shutil.make_archive` now defaults to the modern pax (POSIX.1-2001) format for new archives to improve portability and standards conformance, inherited from the corresponding change to the :mod:`tarfile` module. (Contributed by C.A.M. Gerlach in :issue:`30661`.)

Added :attr:`SSLContext.post_handshake_auth` to enable and :meth:`ssl.SSLSocket.verify_client_post_handshake` to initiate TLS 1.3 post-handshake authentication. (Contributed by Christian Heimes in :issue:`34670`.)

Added :func:`statistics.fmean` as a faster, floating point variant of :func:`statistics.mean()`. (Contributed by Raymond Hettinger and Steven D'Aprano in :issue:`35904`.)

Added :func:`statistics.geometric_mean()` (Contributed by Raymond Hettinger in :issue:`27181`.)

Added :func:`statistics.multimode` that returns a list of the most common values. (Contributed by Raymond Hettinger in :issue:`35892`.)

Added :func:`statistics.quantiles` that divides data or a distribution in to equiprobable intervals (e.g. quartiles, deciles, or percentiles). (Contributed by Raymond Hettinger in :issue:`36546`.)

Added :class:`statistics.NormalDist`, a tool for creating and manipulating normal distributions of a random variable. (Contributed by Raymond Hettinger in :issue:`36018`.)

>>> temperature_feb = NormalDist.from_samples([4, 12, -3, 2, 7, 14])
>>> temperature_feb.mean
6.0
>>> temperature_feb.stdev
6.356099432828281

>>> temperature_feb.cdf(3)            # Chance of being under 3 degrees
0.3184678262814532
>>> # Relative chance of being 7 degrees versus 10 degrees
>>> temperature_feb.pdf(7) / temperature_feb.pdf(10)
1.2039930378537762

>>> el_niño = NormalDist(4, 2.5)
>>> temperature_feb += el_niño        # Add in a climate effect
>>> temperature_feb
NormalDist(mu=10.0, sigma=6.830080526611674)

>>> temperature_feb * (9/5) + 32      # Convert to Fahrenheit
NormalDist(mu=50.0, sigma=12.294144947901014)
>>> temperature_feb.samples(3)        # Generate random samples
[7.672102882379219, 12.000027119750287, 4.647488369766392]

The :mod:`tarfile` module now defaults to the modern pax (POSIX.1-2001) format for new archives, instead of the previous GNU-specific one. This improves cross-platform portability with a consistent encoding (UTF-8) in a standardized and extensible format, and offers several other benefits. (Contributed by C.A.M. Gerlach in :issue:`36268`.)

The :mod:`tokenize` module now implicitly emits a NEWLINE token when provided with input that does not have a trailing new line. This behavior now matches what the C tokenizer does internally. (Contributed by Ammar Askar in :issue:`33899`.)

Added methods :meth:`~tkinter.Spinbox.selection_from`, :meth:`~tkinter.Spinbox.selection_present`, :meth:`~tkinter.Spinbox.selection_range` and :meth:`~tkinter.Spinbox.selection_to` in the :class:`tkinter.Spinbox` class. (Contributed by Juliette Monsel in :issue:`34829`.)

Added method :meth:`~tkinter.Canvas.moveto` in the :class:`tkinter.Canvas` class. (Contributed by Juliette Monsel in :issue:`23831`.)

The :class:`tkinter.PhotoImage` class now has :meth:`~tkinter.PhotoImage.transparency_get` and :meth:`~tkinter.PhotoImage.transparency_set` methods. (Contributed by Zackery Spytz in :issue:`25451`.)

Added new clock :data:`~time.CLOCK_UPTIME_RAW` for macOS 10.12. (Contributed by Joannah Nanjekye in :issue:`35702`.)

• The :mod:`unicodedata` module has been upgraded to use the Unicode 12.0.0 release.
• New function :func:`~unicodedata.is_normalized` can be used to verify a string is in a specific normal form. (Contributed by Max Belanger and David Euresti in :issue:`32285`).

• XXX Added :class:`AsyncMock` to support an asynchronous version of :class:`Mock`. Appropriate new assert functions for testing have been added as well. (Contributed by Lisa Roach in :issue:`26467`).
• Added :func:`~unittest.addModuleCleanup()` and :meth:`~unittest.TestCase.addClassCleanup()` to unittest to support cleanups for :func:`~unittest.setUpModule()` and :meth:`~unittest.TestCase.setUpClass()`. (Contributed by Lisa Roach in :issue:`24412`.)

• :mod:`venv` now includes an Activate.ps1 script on all platforms for activating virtual environments under PowerShell Core 6.1. (Contributed by Brett Cannon in :issue:`32718`.)

• The proxy objects returned by :func:`weakref.proxy` now support the matrix multiplication operators @ and @= in addition to the other numeric operators. (Contributed by Mark Dickinson in :issue:`36669`.)

• As mitigation against DTD and external entity retrieval, the :mod:`xml.dom.minidom` and :mod:`xml.sax` modules no longer process external entities by default. (Contributed by Christian Heimes in :issue:`17239`.)
• The .find*() methods in the :mod:`xml.etree.ElementTree` module support wildcard searches like {*}tag which ignores the namespace and {namespace}* which returns all tags in the given namespace. (Contributed by Stefan Behnel in :issue:`28238`.)
• The :mod:`xml.etree.ElementTree` module provides a new function :func:`–xml.etree.ElementTree.canonicalize()` that implements C14N 2.0. (Contributed by Stefan Behnel in :issue:`13611`.)

• The :mod:`subprocess` module can now use the :func:`os.posix_spawn` function in some cases for better performance. Currently, it is only used on macOS and Linux (using glibc 2.24 or newer) if all these conditions are met:
  • close_fds is false;
  • preexec_fn, pass_fds, cwd and start_new_session parameters are not set;
  • the executable path contains a directory.
• :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:`33671`.)
• :func:`shutil.copytree` uses :func:`os.scandir` function and all copy functions depending from it use cached :func:`os.stat` values. The speedup for copying a directory with 8000 files is around +9% on Linux, +20% on Windows and +30% on a Windows SMB share. Also the number of :func:`os.stat` syscalls is reduced by 38% making :func:`shutil.copytree` especially faster on network filesystems. (Contributed by Giampaolo Rodola' in :issue:`33695`.)
• 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`)
• :class:`uuid.UUID` now uses __slots__ to reduce its memory footprint.
• Improved performance of :func:`operator.itemgetter` by 33%. Optimized argument handling and added a fast path for the common case of a single non-negative integer index into a tuple (which is the typical use case in the standard library). (Contributed by Raymond Hettinger in :issue:`35664`.)
• Sped-up field lookups in :func:`collections.namedtuple`. They are now more than two times faster, making them the fastest form of instance variable lookup in Python. (Contributed by Raymond Hettinger, Pablo Galindo, and Joe Jevnik, Serhiy Storchaka in :issue:`32492`.)
• The :class:`list` constructor does not overallocate the internal item buffer if the input iterable has a known length (the input implements __len__). This makes the created list 12% smaller on average. (Contributed by Raymond Hettinger and Pablo Galindo in :issue:`33234`.)
• Doubled the speed of class variable writes. When a non-dunder attribute was updated, there was an unnecessary call to update slots. (Contributed by Stefan Behnel, Pablo Galindo Salgado, Raymond Hettinger, Neil Schemenauer, and Serhiy Storchaka in :issue:`36012`.)
• Reduced an overhead of converting arguments passed to many builtin functions and methods. This sped up calling some simple builtin functions and methods up to 20--50%. (Contributed by Serhiy Storchaka in :issue:`23867`, :issue:`35582` and :issue:`36127`.)

• The :c:func:`PyByteArray_Init` and :c:func:`PyByteArray_Fini` functions have been removed. They did nothing since Python 2.7.4 and Python 3.2.0, were excluded from the limited API (stable ABI), and were not documented.

• 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`.)

• Functions that convert Python number to C integer like :c:func:`PyLong_AsLong` and argument parsing functions like :c:func:`PyArg_ParseTuple` with integer converting format units like 'i' will now use the :meth:`~object.__index__` special method instead of :meth:`~object.__int__`, if available. The deprecation warning will be emitted for objects with the __int__() method but without the __index__() method (like :class:`~decimal.Decimal` and :class:`~fractions.Fraction`). :c:func:`PyNumber_Check` will now return 1 for objects implementing __index__(). (Contributed by Serhiy Storchaka in :issue:`36048`.)

• Heap-allocated type objects will now increase their reference count in :c:func:`PyObject_Init` (and its parallel macro PyObject_INIT) instead of in :c:func:`PyType_GenericAlloc`. Types that modify instance allocation or deallocation may need to be adjusted. (Contributed by Eddie Elizondo in :issue:`35810`.)

• 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`.)

• Passing an object that is not an instance of :class:`concurrent.futures.ThreadPoolExecutor` to :meth:`asyncio.loop.set_default_executor()` is deprecated and will be prohibited in Python 3.9. (Contributed by Elvis Pranskevichus in :issue:`34075`.)

• The :meth:`__getitem__` methods of :class:`xml.dom.pulldom.DOMEventStream`, :class:`wsgiref.util.FileWrapper` and :class:`fileinput.FileInput` have been deprecated.

  Implementations of these methods have been ignoring their index parameter, and returning the next item instead.

  (Contributed by Berker Peksag in :issue:`9372`.)

• The :class:`typing.NamedTuple` class has deprecated the _field_types attribute in favor of the __annotations__ attribute which has the same information. (Contributed by Raymond Hettinger in :issue:`36320`.)

• :mod:`ast` classes Num, Str, Bytes, NameConstant and Ellipsis are considered deprecated and will be removed in future Python versions. :class:`~ast.Constant` should be used instead. (Contributed by Serhiy Storchaka in :issue:`32892`.)

• The following functions and methods are deprecated in the :mod:`gettext` module: :func:`~gettext.lgettext`, :func:`~gettext.ldgettext`, :func:`~gettext.lngettext` and :func:`~gettext.ldngettext`. They return encoded bytes, and it's possible that you will get unexpected Unicode-related exceptions if there are encoding problems with the translated strings. It's much better to use alternatives which return Unicode strings in Python 3. These functions have been broken for a long time.

  Function :func:`~gettext.bind_textdomain_codeset`, methods :meth:`~gettext.NullTranslations.output_charset` and :meth:`~gettext.NullTranslations.set_output_charset`, and the codeset parameter of functions :func:`~gettext.translation` and :func:`~gettext.install` are also deprecated, since they are only used for for the l*gettext() functions.

  (Contributed by Serhiy Storchaka in :issue:`33710`.)

• The :meth:`~threading.Thread.isAlive()` method of :class:`threading.Thread` has been deprecated. (Contributed by Dong-hee Na in :issue:`35283`.)

• Many builtin and extension functions that take integer arguments will now emit a deprecation warning for :class:`~decimal.Decimal`s, :class:`~fractions.Fraction`s and any other objects that can be converted to integers only with a loss (e.g. that have the :meth:`~object.__int__` method but do not have the :meth:`~object.__index__` method). In future version they will be errors. (Contributed by Serhiy Storchaka in :issue:`36048`.)

• Deprecated passing the following arguments as keyword arguments:

  • func in :func:`functools.partialmethod`, :func:`weakref.finalize`, :meth:`profile.Profile.runcall`, :meth:`cProfile.Profile.runcall`, :meth:`bdb.Bdb.runcall`, :meth:`trace.Trace.runfunc` and :func:`curses.wrapper`.
  • function in :func:`unittest.addModuleCleanup` and :meth:`unittest.TestCase.addCleanup`.
  • fn in the :meth:`~concurrent.futures.Executor.submit` method of :class:`concurrent.futures.ThreadPoolExecutor` and :class:`concurrent.futures.ProcessPoolExecutor`.
  • callback in :meth:`contextlib.ExitStack.callback`, :meth:`contextlib.AsyncExitStack.callback` and :meth:`contextlib.AsyncExitStack.push_async_callback`.
  • c and typeid in the :meth:`~multiprocessing.managers.Server.create` method of :class:`multiprocessing.managers.Server` and :class:`multiprocessing.managers.SharedMemoryServer`.
  • obj in :func:`weakref.finalize`.

  In future releases of Python they will be :ref:`positional-only <positional-only_parameter>`. (Contributed by Serhiy Storchaka in :issue:`36492`.)

• The function :func:`~inspect.getfullargspec` in the :mod:`inspect` module is deprecated in favor of the :func:`inspect.signature` API. (Contributed by Pablo Galindo in :issue:`36751`.)

The following features and APIs have been removed from Python 3.8:

• The :mod:`macpath` module, deprecated in Python 3.7, has been removed. (Contributed by Victor Stinner in :issue:`35471`.)
• The function :func:`platform.popen` has been removed, it was deprecated since Python 3.3: use :func:`os.popen` instead.
• 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`.)
• "unicode_internal" codec is removed. (Contributed by Inada Naoki in :issue:`36297`.)

This section lists previously described changes and other bugfixes that may require changes to your code.

• 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`.)
• The compiler now produces a :exc:`SyntaxWarning` when identity checks (is and is not) are used with certain types of literals (e.g. strings, ints). These can often work by accident in CPython, but are not guaranteed by the language spec. The warning advises users to use equality tests (== and !=) instead. (Contributed by Serhiy Storchaka in :issue:`34850`.)
• The CPython interpreter can swallow exceptions in some circumstances. In Python 3.8 this happens in less cases. In particular, exceptions raised when getting the attribute from the type dictionary are no longer ignored. (Contributed by Serhiy Storchaka in :issue:`35459`.)
• Removed __str__ implementations from builtin types :class:`bool`, :class:`int`, :class:`float`, :class:`complex` and few classes from the standard library. They now inherit __str__() from :class:`object`. As result, defining the __repr__() method in the subclass of these classes will affect they string representation. (Contributed by Serhiy Storchaka in :issue:`36793`.)
• On AIX, :attr:`sys.platform` doesn't contain the major version anymore. It is always 'aix', instead of 'aix3' .. 'aix7'. Since older Python versions include the version number, it is recommended to always use the sys.platform.startswith('aix'). (Contributed by M. Felt in :issue:`36588`.)
• :c:func:`PyEval_AcquireLock` and :c:func:`PyEval_AcquireThread` now terminate the current thread if called while the interpreter is finalizing, making them consistent with :c:func:`PyEval_RestoreThread`, :c:func:`Py_END_ALLOW_THREADS`, and :c:func:`PyGILState_Ensure`. If this behaviour is not desired, guard the call by checking :c:func:`_Py_IsFinalizing` or :c:func:`sys.is_finalizing`.

• :class:`subprocess.Popen` can now use :func:`os.posix_spawn` in some cases for better performance. On Windows Subsystem for Linux and QEMU User Emulation, Popen constructor using :func:`os.posix_spawn` no longer raise an exception on errors like missing program, but the child process fails with a non-zero :attr:`~Popen.returncode`.
• The :meth:`imap.IMAP4.logout` method no longer ignores silently arbitrary exceptions.
• The function :func:`platform.popen` has been removed, it was deprecated since Python 3.3: use :func:`os.popen` instead.
• The :func:`statistics.mode` function no longer raises an exception when given multimodal data. Instead, it returns the first mode encountered in the input data. (Contributed by Raymond Hettinger in :issue:`35892`.)
• 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`.)
• The :meth:`writexml`, :meth:`toxml` and :meth:`toprettyxml` methods of the :mod:`xml.dom.minidom` module, and :mod:`xml.etree` now preserve the attribute order specified by the user. (Contributed by Diego Rojas and Raymond Hettinger in :issue:`34160`.)
• 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`)
• The PyInterpreterState struct has been moved into the "internal" header files (specifically Include/internal/pycore_pystate.h). An opaque PyInterpreterState is still available as part of the public API (and stable ABI). The docs indicate that none of the struct's fields are public, so we hope no one has been using them. However, if you do rely on one or more of those private fields and have no alternative then please open a BPO issue. We'll work on helping you adjust (possibly including adding accessor functions to the public API). (See :issue:`35886`.)
• Asyncio tasks can now be named, either by passing the name keyword argument to :func:`asyncio.create_task` or the :meth:`~asyncio.loop.create_task` event loop method, or by calling the :meth:`~asyncio.Task.set_name` method on the task object. The task name is visible in the repr() output of :class:`asyncio.Task` and can also be retrieved using the :meth:`~asyncio.Task.get_name` method.
• The :meth:`mmap.flush() <mmap.mmap.flush>` method now returns None on success and raises an exception on error under all platforms. Previously, its behavior was platform-depended: a nonzero value was returned on success; zero was returned on error under Windows. A zero value was returned on success; an exception was raised on error under Unix. (Contributed by Berker Peksag in :issue:`2122`.)
• The function :func:`math.factorial` no longer accepts arguments that are not int-like. (Contributed by Pablo Galindo in :issue:`33083`.)
• :mod:`xml.dom.minidom` and :mod:`xml.sax` modules no longer process external entities by default. (Contributed by Christian Heimes in :issue:`17239`.)
• Deleting a key from a read-only :mod:`dbm` database (:mod:`dbm.dumb`, :mod:`dbm.gnu` or :mod:`dbm.ndbm`) raises :attr:`error` (:exc:`dbm.dumb.error`, :exc:`dbm.gnu.error` or :exc:`dbm.ndbm.error`) instead of :exc:`KeyError`. (Contributed by Xiang Zhang in :issue:`33106`.)
• :func:`~os.path.expanduser` on Windows now prefers the :envvar:`USERPROFILE` environment variable and does not use :envvar:`HOME`, which is not normally set for regular user accounts.

• DLL dependencies for extension modules and DLLs loaded with :mod:`ctypes` on Windows are now resolved more securely. Only the system paths, the directory containing the DLL or PYD file, and directories added with :func:`~os.add_dll_directory` are searched for load-time dependencies. Specifically, :envvar:`PATH` and the current working directory are no longer used, and modifications to these will no longer have any effect on normal DLL resolution. If your application relies on these mechanisms, you should check for :func:`~os.add_dll_directory` and if it exists, use it to add your DLLs directory while loading your library. Note that Windows 7 users will need to ensure that Windows Update KB2533625 has been installed (this is also verified by the installer). (See :issue:`36085`.)
• The header files and functions related to pgen have been removed after its replacement by a pure Python implementation. (Contributed by Pablo Galindo in :issue:`36623`.)

• On Unix, C extensions are no longer linked to libpython except on Android. When Python is embedded, libpython must not be loaded with RTLD_LOCAL, but RTLD_GLOBAL instead. Previously, using RTLD_LOCAL, it was already not possible to load C extensions which were not linked to libpython, like C extensions of the standard library built by the *shared* section of Modules/Setup.

• Use of # variants of formats in parsing or building value (e.g. :c:func:`PyArg_ParseTuple`, :c:func:`Py_BuildValue`, :c:func:`PyObject_CallFunction`, etc.) without PY_SSIZE_T_CLEAN defined raises DeprecationWarning now. It will be removed in 3.10 or 4.0. Read :ref:`arg-parsing` for detail. (Contributed by Inada Naoki in :issue:`36381`.)

• Instances of heap-allocated types (such as those created with :c:func:`PyType_FromSpec`) hold a reference to their type object. Increasing the reference count of these type objects has been moved from :c:func:`PyType_GenericAlloc` to the more low-level functions, :c:func:`PyObject_Init` and :c:func:`PyObject_INIT`. This makes types created through :c:func:`PyType_FromSpec` behave like other classes in managed code.

  Statically allocated types are not affected.

  For the vast majority of cases, there should be no side effect. However, types that manually increase the reference count after allocating an instance (perhaps to work around the bug) may now become immortal. To avoid this, these classes need to call Py_DECREF on the type object during instance deallocation.

  To correctly port these types into 3.8, please apply the following changes:

  • Remove :c:macro:`Py_INCREF` on the type object after allocating an instance - if any. This may happen after calling :c:func:`PyObject_New`, :c:func:`PyObject_NewVar`, :c:func:`PyObject_GC_New`, :c:func:`PyObject_GC_NewVar`, or any other custom allocator that uses :c:func:`PyObject_Init` or :c:func:`PyObject_INIT`.

    Example:

    static foo_struct *
    foo_new(PyObject *type) {
        foo_struct *foo = PyObject_GC_New(foo_struct, (PyTypeObject *) type);
        if (foo == NULL)
            return NULL;
    #if PY_VERSION_HEX < 0x03080000
        // Workaround for Python issue 35810; no longer necessary in Python 3.8
        PY_INCREF(type)
    #endif
        return foo;
    }

  • Ensure that all custom tp_dealloc functions of heap-allocated types decrease the type's reference count.

    Example:

    static void
    foo_dealloc(foo_struct *instance) {
        PyObject *type = Py_TYPE(instance);
        PyObject_GC_Del(instance);
    #if PY_VERSION_HEX >= 0x03080000
        // This was not needed before Python 3.8 (Python issue 35810)
        Py_DECREF(type);
    #endif
    }

  (Contributed by Eddie Elizondo in :issue:`35810`.)

• 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`.)

• Added a benchmark script for timing various ways to access variables: Tools/scripts/var_access_benchmark.py. (Contributed by Raymond Hettinger in :issue:`35884`.)