# 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.

注解

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

- Added `addModuleCleanup()` and `addClassCleanup()` to unittest to support cleanups for `setUpModule()` and `setUpClass()`. (Contributed by Lisa Roach in bpo-24412.)

### 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

- Deprecated methods `getchildren()` and `getiterator()` in the `ElementTree` module emit now a `DeprecationWarning` instead of `PendingDeprecationWarning`. They will be removed in Python 3.9. (Contributed by Serhiy Storchaka in bpo-29209.)

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

- The `__getitem__()` methods of `xml.dom.pulldom.DOMEventStream`, `wsgiref.util.FileWrapper` and `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 bpo-9372.)

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

- The following functions and methods are deprecated in the `gettext` module: `lgettext()`, `ldgettext()`, `lngettext()` and `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 `bind_textdomain_codeset()`, methods `output_charset()` and `set_output_charset()`, and the codeset parameter of functions `translation()` and `install()` are also deprecated, since they are only used for for the `l*gettext()` functions.

  (Contributed by Serhiy Storchaka in bpo-33710.)

- The `isAlive()` method of `threading.Thread` has been deprecated. (Contributed by Dong-hee Na in bpo-35283.)

## 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.)