What's New In Python 3.8

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

For full details, see the changelog.

備註

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

New Features

Parallel filesystem cache for compiled bytecode files

The new PYTHONPYCACHEPREFIX setting (also available as -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 sys.pycache_prefix (None indicates the default location in __pycache__ subdirectories).

(Contributed by Carl Meyer in bpo-33499.)

Other Language Changes

  • A continue statement was illegal in the finally clause due to a problem with the implementation. In Python 3.8 this restriction was lifted. (Contributed by Serhiy Storchaka in bpo-32489.)

  • The int type now has a new as_integer_ratio() method compatible with the existing float.as_integer_ratio() method. (Contributed by Lisa Roach in bpo-33073.)

  • Added support of \N{name} escapes in regular expressions. (Contributed by Jonathan Eunice and Serhiy Storchaka in bpo-30688.)

  • Dict and dictviews are now iterable in reversed insertion order using reversed(). (Contributed by Rémi Lapeyre in bpo-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 bpo-34641.

  • Iterable unpacking is now allowed without parentheses in yield and return statements. (Contributed by David Cuthbert and Jordan Chapman in bpo-32117.)

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

  • The compiler now produces a 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 bpo-15248.)

  • Arithmetic operations between subclasses of datetime.date or datetime.datetime and 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 datetime.timedelta arithmetic, such as datetime.datetime.astimezone(). (Contributed by Paul Ganssle in bpo-32417.)

  • When the Python interpreter is interrupted by Ctrl-C (SIGINT) and the resulting 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 bpo-1054041.)

New Modules

  • None yet.

Improved Modules

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

asyncio

On Windows, the default event loop is now ProactorEventLoop.

gettext

Added pgettext() and its variants. (Contributed by Franz Glasner, Éric Araujo, and Cheryl Sabella in bpo-2504.)

gc

get_objects() can now receive an optional generation parameter indicating a generation to get objects from. Contributed in bpo-36016 by Pablo Galindo.

gzip

Added the mtime parameter to gzip.compress() for reproducible output. (Contributed by Guo Ci Teo in bpo-34898.)

idlelib and IDLE

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 bpo-1529353.)

The changes above have been backported to 3.7 maintenance releases.

json.tool

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

math

Added new function math.dist() for computing Euclidean distance between two points. (Contributed by Raymond Hettinger in bpo-33089.)

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

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

os.path

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

ncurses

Added a new variable holding structured version information for the underlying ncurses library: ncurses_version. (Contributed by Serhiy Storchaka in bpo-31680.)

pathlib

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

shutil

shutil.copytree() now accepts a new dirs_exist_ok keyword argument. (Contributed by Josh Bronson in bpo-20849.)

ssl

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

statistics

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

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

>>> temperature_feb = NormalDist.from_samples([4, 12, -3, 2, 7, 14])
>>> temperature_feb
NormalDist(mu=6.0, sigma=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_nino = NormalDist(4, 2.5)
>>> temperature_feb += el_nino        # 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]

tokenize

The 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 bpo-33899.)

tkinter

Added methods selection_from(), selection_present(), selection_range() and selection_to() in the tkinter.Spinbox class. (Contributed by Juliette Monsel in bpo-34829.)

Added method moveto() in the tkinter.Canvas class. (Contributed by Juliette Monsel in bpo-23831.)

time

Added new clock CLOCK_UPTIME_RAW for macOS 10.12. (Contributed by Joannah Nanjekye in bpo-35702.)

unicodedata

  • New function is_normalized() can be used to verify a string is in a specific normal form. (Contributed by Max Belanger and David Euresti in bpo-32285).

unittest

venv

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

xml

  • As mitigation against DTD and external entity retrieval, the xml.dom.minidom and xml.sax modules no longer process external entities by default. (Contributed by Christian Heimes in bpo-17239.)

Optimizations

  • The subprocess module can now use the 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.
  • shutil.copyfile(), shutil.copy(), shutil.copy2(), shutil.copytree() and 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 shutil.copyfile() uses a bigger default buffer size (1 MiB instead of 16 KiB) and a memoryview()-based variant of 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 Platform-dependent efficient copy operations section. (Contributed by Giampaolo Rodola' in bpo-33671.)
  • shutil.copytree() uses os.scandir() function and all copy functions depending from it use cached 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 os.stat() syscalls is reduced by 38% making shutil.copytree() especially faster on network filesystems. (Contributed by Giampaolo Rodola' in bpo-33695.)
  • The default protocol in the 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 bpo-33597)
  • uuid.UUID now uses __slots__ to reduce its memory footprint.
  • Improved performance of 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 bpo-35664.)
  • Sped-up field lookups in 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 bpo-32492.)
  • The 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 bpo-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 bpo-36012.)

Build and C API Changes

  • The PyByteArray_Init() and 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 PyExceptionClass_Name() is now of type const char * rather of char *. (Contributed by Serhiy Storchaka in bpo-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 bpo-32430.)

Deprecated

API and Feature Removals

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

  • The macpath module, deprecated in Python 3.7, has been removed. (Contributed by Victor Stinner in bpo-35471.)
  • The function platform.popen() has been removed, it was deprecated since Python 3.3: use 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 bpo-25427.)
  • parse_qs, parse_qsl, and escape are removed from cgi module. They are deprecated from Python 3.2 or older.
  • filemode function is removed from tarfile module. It is not documented and deprecated since Python 3.3.
  • The XMLParser constructor no longer accepts the html argument. It never had effect and was deprecated in Python 3.4. All other parameters are now keyword-only. (Contributed by Serhiy Storchaka in bpo-29209.)
  • Removed the doctype() method of XMLParser. (Contributed by Serhiy Storchaka in bpo-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 for clause). (Contributed by Serhiy Storchaka in bpo-10544.)
  • The compiler now produces a 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 bpo-34850.)

Changes in the Python API

  • The function platform.popen() has been removed, it was deprecated since Python 3.3: use os.popen() instead.
  • The selection() method of the 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 selection_set() for changing the selection. (Contributed by Serhiy Storchaka in bpo-31508.)
  • A dbm.dumb database opened with flags 'r' is now read-only. dbm.dumb.open() with flags 'r' and 'w' no longer creates a database if it does not exist. (Contributed by Serhiy Storchaka in bpo-32749.)
  • The doctype() method defined in a subclass of XMLParser will no longer be called and will cause emitting a RuntimeWarning instead of a DeprecationWarning. Define the doctype() method on a target for handling an XML doctype declaration. (Contributed by Serhiy Storchaka in bpo-29209.)
  • A RuntimeError is now raised when the custom metaclass doesn't provide the __classcell__ entry in the namespace passed to type.__new__. A DeprecationWarning was emitted in Python 3.6--3.7. (Contributed by Serhiy Storchaka in bpo-23722.)
  • The cProfile.Profile class can now be used as a context manager. (Contributed by Scott Sanderson in bpo-29235.)
  • shutil.copyfile(), shutil.copy(), shutil.copy2(), shutil.copytree() and shutil.move() use platform-specific "fast-copy" syscalls (see Platform-dependent efficient copy operations section).
  • 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 bpo-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 bpo-35886.)
  • Asyncio tasks can now be named, either by passing the name keyword argument to asyncio.create_task() or the create_task() event loop method, or by calling the set_name() method on the task object. The task name is visible in the repr() output of asyncio.Task and can also be retrieved using the get_name() method.
  • The 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 bpo-2122.)
  • The function math.factorial() no longer accepts arguments that are not int-like. (Contributed by Pablo Galindo in bpo-33083.)
  • xml.dom.minidom and xml.sax modules no longer process external entities by default. (Contributed by Christian Heimes in bpo-17239.)
  • Deleting a key from a read-only dbm database (dbm.dumb, dbm.gnu or dbm.ndbm) raises error (dbm.dumb.error, dbm.gnu.error or dbm.ndbm.error) instead of KeyError. (Contributed by Xiang Zhang in bpo-33106.)

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 break, continue and return.

    Removed opcodes BREAK_LOOP, CONTINUE_LOOP, SETUP_LOOP and SETUP_EXCEPT. Added new opcodes ROT_FOUR, BEGIN_FINALLY, CALL_FINALLY and POP_FINALLY. Changed the behavior of END_FINALLY and WITH_CLEANUP_START.

    (Contributed by Mark Shannon, Antoine Pitrou and Serhiy Storchaka in bpo-17611.)

  • Added new opcode END_ASYNC_FOR for handling exceptions raised when awaiting a next item in an async for loop. (Contributed by Serhiy Storchaka in bpo-33041.)

Demos and Tools

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