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 thefinally
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 newas_integer_ratio()
method compatible with the existingfloat.as_integer_ratio()
method. (Contributed by Lisa Roach in bpo-33073.)Added support of
\N{name}
escapes inregular 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
andreturn
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 aSyntaxWarning
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
ordatetime.datetime
anddatetime.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) usesdatetime.timedelta
arithmetic, such asdatetime.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 forcollections.namedtuple()
now returns adict
instead of acollections.OrderedDict
. This works because regular dicts have guaranteed ordering in since Python 3.7. If the extra features ofOrderedDict
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()
andaddClassCleanup()
to unittest to support cleanups forsetUpModule()
andsetUpClass()
. (Contributed by Lisa Roach in bpo-24412.)
venv¶
xml¶
- As mitigation against DTD and external entity retrieval, the
xml.dom.minidom
andxml.sax
modules no longer process external entities by default. (Contributed by Christian Heimes in bpo-17239.)
Optimizations¶
- The
subprocess
module can now use theos.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()
andshutil.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 Windowsshutil.copyfile()
uses a bigger default buffer size (1 MiB instead of 16 KiB) and amemoryview()
-based variant ofshutil.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()
usesos.scandir()
function and all copy functions depending from it use cachedos.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 ofos.stat()
syscalls is reduced by 38% makingshutil.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 fromPyGC_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()
andPyByteArray_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 typeconst char *
rather ofchar *
. (Contributed by Serhiy Storchaka in bpo-33818.)The duality of
Modules/Setup.dist
andModules/Setup
has been removed. Previously, when updating the CPython source tree, one had to manually copyModules/Setup.dist
(inside the source tree) toModules/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()
andgetiterator()
in theElementTree
module emit now aDeprecationWarning
instead ofPendingDeprecationWarning
. 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
toasyncio.loop.set_default_executor()
is deprecated and will be prohibited in Python 3.9. (Contributed by Elvis Pranskevichus in bpo-34075.)The
__getitem__()
methods ofxml.dom.pulldom.DOMEventStream
,wsgiref.util.FileWrapper
andfileinput.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
classesNum
,Str
,Bytes
,NameConstant
andEllipsis
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()
andldngettext()
. 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()
, methodsoutput_charset()
andset_output_charset()
, and the codeset parameter of functionstranslation()
andinstall()
are also deprecated, since they are only used for for thel*gettext()
functions.(Contributed by Serhiy Storchaka in bpo-33710.)
The
isAlive()
method ofthreading.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: useos.popen()
instead. - The
pyvenv
script has been removed in favor ofpython3.8 -m venv
to help eliminate confusion as to what Python interpreter thepyvenv
script is tied to. (Contributed by Brett Cannon in bpo-25427.) parse_qs
,parse_qsl
, andescape
are removed fromcgi
module. They are deprecated from Python 3.2 or older.filemode
function is removed fromtarfile
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 ofXMLParser
. (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
andyield from
clauses) are now disallowed in comprehensions and generator expressions (aside from the iterable expression in the leftmostfor
clause). (Contributed by Serhiy Storchaka in bpo-10544.) - The compiler now produces a
SyntaxWarning
when identity checks (is
andis 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: useos.popen()
instead. - The
selection()
method of thetkinter.ttk.Treeview
class no longer takes arguments. Using it with arguments for changing the selection was deprecated in Python 3.6. Use specialized methods likeselection_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 ofXMLParser
will no longer be called and will cause emitting aRuntimeWarning
instead of aDeprecationWarning
. Define thedoctype()
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 totype.__new__
. ADeprecationWarning
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()
andshutil.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 opaquePyInterpreterState
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 toasyncio.create_task()
or thecreate_task()
event loop method, or by calling theset_name()
method on the task object. The task name is visible in therepr()
output ofasyncio.Task
and can also be retrieved using theget_name()
method. - The
mmap.flush()
method now returnsNone
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
andxml.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
ordbm.ndbm
) raiseserror
(dbm.dumb.error
,dbm.gnu.error
ordbm.ndbm.error
) instead ofKeyError
. (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
andreturn
.Removed opcodes
BREAK_LOOP
,CONTINUE_LOOP
,SETUP_LOOP
andSETUP_EXCEPT
. Added new opcodesROT_FOUR
,BEGIN_FINALLY
,CALL_FINALLY
andPOP_FINALLY
. Changed the behavior ofEND_FINALLY
andWITH_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 anasync for
loop. (Contributed by Serhiy Storchaka in bpo-33041.)