What’s New In Python 3.7
Elvis Pranskevichus <email@example.com >
This article explains the new features in Python 3.7, compared to 3.6. Python 3.7 was released on June 27, 2018. For full details, see the changelog.
New syntax features:
PEP 563, postponed evaluation of type annotations.
Backwards incompatible syntax changes:
New library modules:
New built-in features:
Python data model improvements:
PEP 562, customization of access to module attributes.
PEP 560, core support for typing module and generic types.
Significant improvements in the standard library:
CPython implementation improvements:
Avoiding the use of ASCII as a default text encoding:
PEP 552, deterministic .pycs
C API improvements:
PEP 539, new C API for thread-local storage
PEP 545, Python documentation translations
This release features notable performance improvements in many areas. The Optimizations section lists them in detail.
For a list of changes that may affect compatibility with previous Python releases please refer to the Porting to Python 3.7 section.
annotations could only use names which were already available in the current scope, in other words they didn’t support forward references of any kind; and
annotating source code had adverse effects on startup time of Python programs.
Both of these issues are fixed by postponing the evaluation of annotations. Instead of compiling code which executes expressions in annotations at their definition time, the compiler stores the annotation in a string form equivalent to the AST of the expression in question. If needed, annotations can be resolved at runtime using
typing.get_type_hints(). In the common case where this is not required, the annotations are cheaper to store (since short strings are interned by the interpreter) and make startup time faster.
Usability-wise, annotations now support forward references, making the following syntax valid:
class C: @classmethod def from_string(cls, source: str) -> C: ... def validate_b(self, obj: B) -> bool: ... class B: ...
Since this change breaks compatibility, the new behavior needs to be enabled on a per-module basis in Python 3.7 using a
from __future__ import annotations
It will become the default in Python 4.0.
- PEP 563 – Postponed evaluation of annotations
PEP written and implemented by Łukasz Langa.
An ongoing challenge within the Python 3 series has been determining a sensible default strategy for handling the “7-bit ASCII” text encoding assumption currently implied by the use of the default C or POSIX locale on non-Windows platforms.
PEP 538 updates the default interpreter command line interface to automatically coerce that locale to an available UTF-8 based locale as described in the documentation of the new
PYTHONCOERCECLOCALE environment variable. Automatically setting
LC_CTYPE this way means that both the core interpreter and locale-aware C extensions (such as
readline) will assume the use of UTF-8 as the default text encoding, rather than ASCII.
The platform support definition in PEP 11 has also been updated to limit full text handling support to suitably configured non-ASCII based locales.
As part of this change, the default error handler for
stdout is now
surrogateescape (rather than
strict) when using any of the defined coercion target locales (currently
UTF-8). The default error handler for
stderr continues to be
backslashreplace, regardless of locale.
Locale coercion is silent by default, but to assist in debugging potentially locale related integration problems, explicit warnings (emitted directly on
stderr) can be requested by setting
PYTHONCOERCECLOCALE=warn. This setting will also cause the Python runtime to emit a warning if the legacy C locale remains active when the core interpreter is initialized.
While PEP 538 ’s locale coercion has the benefit of also affecting extension modules (such as GNU
readline), as well as child processes (including those running non-Python applications and older versions of Python), it has the downside of requiring that a suitable target locale be present on the running system. To better handle the case where no suitable target locale is available (as occurs on RHEL/CentOS 7, for example), Python 3.7 also implements PEP 540: Forced UTF-8 Runtime Mode.
- PEP 538 – Coercing the legacy C locale to a UTF-8 based locale
PEP written and implemented by Nick Coghlan.
The forced UTF-8 mode can be used to change the text handling behavior in an embedded Python interpreter without changing the locale settings of an embedding application.
While PEP 540 ’s UTF-8 mode has the benefit of working regardless of which locales are available on the running system, it has the downside of having no effect on extension modules (such as GNU
readline), child processes running non-Python applications, and child processes running older versions of Python. To reduce the risk of corrupting text data when communicating with such components, Python 3.7 also implements PEP 540: Forced UTF-8 Runtime Mode).
The UTF-8 mode is enabled by default when the locale is
POSIX, and the PEP 538 locale coercion feature fails to change it to a UTF-8 based alternative (whether that failure is due to
PYTHONCOERCECLOCALE=0 being set,
LC_ALL being set, or the lack of a suitable target locale).
- PEP 540 – Add a new UTF-8 mode
PEP written and implemented by Victor Stinner
Python 3.7 includes the new built-in
breakpoint() function as an easy and consistent way to enter the Python debugger.
sys.breakpointhook(). By default, the latter imports
pdb and then calls
pdb.set_trace(), but by binding
sys.breakpointhook() to the function of your choosing,
breakpoint() can enter any debugger. Additionally, the environment variable
PYTHONBREAKPOINT can be set to the callable of your debugger of choice. Set
PYTHONBREAKPOINT=0 to completely disable built-in
- PEP 553 – Built-in breakpoint()
PEP written and implemented by Barry Warsaw
While Python provides a C API for thread-local storage support; the existing Thread Local Storage (TLS) API has used
int to represent TLS keys across all platforms. This has not generally been a problem for officially-support platforms, but that is neither POSIX-compliant, nor portable in any practical sense.
PEP 539 changes this by providing a new Thread Specific Storage (TSS) API to CPython which supersedes use of the existing TLS API within the CPython interpreter, while deprecating the existing API. The TSS API uses a new type
Py_tss_t instead of
int to represent TSS keys–an opaque type the definition of which may depend on the underlying TLS implementation. Therefore, this will allow to build CPython on platforms where the native TLS key is defined in a way that cannot be safely cast to
Note that on platforms where the native TLS key is defined in a way that cannot be safely cast to
int, all functions of the existing TLS API will be no-op and immediately return failure. This indicates clearly that the old API is not supported on platforms where it cannot be used reliably, and that no effort will be made to add such support.
- PEP 539 – A New C-API for Thread-Local Storage in CPython
PEP written by Erik M. Bray; implementation by Masayuki Yamamoto.
A typical example of where this may be useful is module attribute deprecation and lazy loading.
PEP 562 – Module
PEP written and implemented by Ivan Levkivskyi
The resolution of clocks in modern systems can exceed the limited precision of a floating point number returned by the
time.time() function and its variants. To avoid loss of precision, PEP 564 adds six new “nanosecond” variants of the existing timer functions to the
The new functions return the number of nanoseconds as an integer value.
- PEP 564 – Add new time functions with nanosecond resolution
PEP written and implemented by Victor Stinner
The default handling of
DeprecationWarning has been changed such that these warnings are once more shown by default, but only when the code triggering them is running directly in the
__main__ module. As a result, developers of single file scripts and those using Python interactively should once again start seeing deprecation warnings for the APIs they use, but deprecation warnings triggered by imported application, library and framework modules will continue to be hidden by default.
As a result of this change, the standard library now allows developers to choose between three different deprecation warning behaviours:
FutureWarning: always displayed by default, recommended for warnings intended to be seen by application end users (e.g. for deprecated application configuration settings).
DeprecationWarning: displayed by default only in
__main__and when running tests, recommended for warnings intended to be seen by other Python developers where a version upgrade may result in changed behaviour or an error.
PendingDeprecationWarning were only visible when running tests, which meant that developers primarily writing single file scripts or using Python interactively could be surprised by breaking changes in the APIs they used.
PEP 565 – Show DeprecationWarning in
PEP written and implemented by Nick Coghlan
Initially PEP 484 was designed in such way that it would not introduce any changes to the core CPython interpreter. Now type hints and the
typing module are extensively used by the community, so this restriction is removed. The PEP introduces two special methods
__mro_entries__, these methods are now used by most classes and special constructs in
typing. As a result, the speed of various operations with types increased up to 7 times, the generic types can be used without metaclass conflicts, and several long standing bugs in
typing module are fixed.
- PEP 560 – Core support for typing module and generic types
PEP written and implemented by Ivan Levkivskyi
Python has traditionally checked the up-to-dateness of bytecode cache files (i.e.,
.pyc files) by comparing the source metadata (last-modified timestamp and size) with source metadata saved in the cache file header when it was generated. While effective, this invalidation method has its drawbacks. When filesystem timestamps are too coarse, Python can miss source updates, leading to user confusion. Additionally, having a timestamp in the cache file is problematic for build reproducibility and content-based build systems.
PEP 552 extends the pyc format to allow the hash of the source file to be used for invalidation instead of the source timestamp. Such
.pyc files are called “hash-based”. By default, Python still uses timestamp-based invalidation and does not generate hash-based
.pyc files at runtime. Hash-based
.pyc files may be generated with
.pyc files come in two variants: checked and unchecked. Python validates checked hash-based
.pyc files against the corresponding source files at runtime but doesn’t do so for unchecked hash-based pycs. Unchecked hash-based
.pyc files are a useful performance optimization for environments where a system external to Python (e.g., the build system) is responsible for keeping
.pyc files up-to-date.
See Cached bytecode invalidation for more information.
- PEP 552 – Deterministic pycs
PEP written and implemented by Benjamin Peterson
PEP 545 describes the process of creating and maintaining Python documentation translations.
Three new translations have been added:
- PEP 545 – Python Documentation Translations
PEP written and implemented by Julien Palard, Inada Naoki, and Victor Stinner.
dev command line option or the new
PYTHONDEVMODE environment variable can be used to enable CPython’s development mode. When in development mode, CPython performs additional runtime checks that are too expensive to be enabled by default. See
dev documentation for the full description of the effects of this mode.
awaitexpression and comprehensions containing an
async forclause were illegal in the expressions in formatted string literals due to a problem with the implementation. In Python 3.7 this restriction was lifted.
Circular imports involving absolute imports with binding a submodule to a name are now supported. (Contributed by Serhiy Storchaka in bpo-30024 .)
object.__format__(x, '')is now equivalent to
format(str(self), ''). (Contributed by Serhiy Storchaka in bpo-28974 .)
In order to better support dynamic creation of stack traces,
types.TracebackTypecan now be instantiated from Python code, and the
tb_nextattribute on tracebacks is now writable. (Contributed by Nathaniel J. Smith in bpo-30579 .)
When using the
sys.pathis now eagerly expanded to the full starting directory path, rather than being left as the empty directory (which allows imports from the current working directory at the time when an import occurs) (Contributed by Nick Coghlan in bpo-33053 .)
contextvars module and a set of new C APIs introduce support for context variables. Context variables are conceptually similar to thread-local variables. Unlike TLS, context variables support asynchronous code correctly.
decimal modules have been updated to use and support context variables out of the box. Particularly the active decimal context is now stored in a context variable, which allows decimal operations to work with the correct context in asynchronous code.
- PEP 567 – Context Variables
PEP written and implemented by Yury Selivanov
dataclass() decorator provides a way to declare data classes. A data class describes its attributes using class variable annotations. Its constructor and other magic methods, such as
__hash__() are generated automatically.
@dataclass class Point: x: float y: float z: float = 0.0 p = Point(1.5, 2.5) print(p) # produces "Point(x=1.5, y=2.5, z=0.0)"
- PEP 557 – Data Classes
PEP written and implemented by Eric V. Smith
importlib.resources module provides several new APIs and one new ABC for access to, opening, and reading resources inside packages. Resources are roughly similar to files inside packages, but they needn’t be actual files on the physical file system. Module loaders can provide a
get_resource_reader() function which returns a
importlib.abc.ResourceReader instance to support this new API. Built-in file path loaders and zip file loaders both support this.
Contributed by Barry Warsaw and Brett Cannon in bpo-32248 .
importlib_resources – a PyPI backport for earlier Python versions.
asyncio gained support for
Future.add_done_callback()have a new optional keyword-only context parameter.
Tasksnow track their context automatically. See PEP 567 for more details. (Contributed by Yury Selivanov in bpo-32436 .)
asyncio.current_task()function returns the currently running
Taskinstance, and the new
asyncio.all_tasks()function returns a set of all existing
Taskinstances in a given loop. The
Task.all_tasks()methods have been deprecated. (Contributed by Andrew Svetlov in bpo-32250 .)
asyncio.get_running_loop()function returns the currently running loop, and raises a
RuntimeErrorif no loop is running. This is in contrast with
asyncio.get_event_loop(), which will create a new event loop if none is running. (Contributed by Yury Selivanov in bpo-32269 .)
StreamWriter.wait_closed()coroutine method allows waiting until the stream writer is closed. The new
StreamWriter.is_closing()method can be used to determine if the writer is closing. (Contributed by Andrew Svetlov in bpo-32391 .)
Task.get_loop()methods return the instance of the loop on which a task or a future were created.
Server.get_loop()allows doing the same for
asyncio.Serverobjects. (Contributed by Yury Selivanov in bpo-32415 and Srinivas Reddy Thatiparthy in bpo-32418 .)
It is now possible to control how instances of
asyncio.Serverbegin serving. Previously, the server would start serving immediately when created. The new start_serving keyword argument to
loop.create_unix_server(), as well as
Server.serve_forever()can be used to decouple server instantiation and serving. The new
Trueif the server is serving.
Serverobjects are now asynchronous context managers:
srv = await loop.create_server(...) async with srv: # some code # At this point, srv is closed and no longer accepts new connections.
(Contributed by Yury Selivanov in bpo-32662 .)
loop.create_accepted_socket()methods and their corresponding UNIX socket variants now accept the ssl_handshake_timeout keyword argument. (Contributed by Neil Aspinall in bpo-29970 .)
ReadTransport.is_reading()method can be used to determine the reading state of the transport. Additionally, calls to
ReadTransport.pause_reading()are now idempotent. (Contributed by Yury Selivanov in bpo-32356 .)
Exceptions occurring in cancelled tasks are no longer logged. (Contributed by Yury Selivanov in bpo-30508 .)
WindowsProactorEventLoopPolicyclasses. (Contributed by Yury Selivanov in bpo-33792 .)
asyncio APIs have been deprecated.
collections.namedtuple() now supports default values. (Contributed by Raymond Hettinger in bpo-32320 .)
compileall.compile_dir() learned the new invalidation_mode parameter, which can be used to enable hash-based .pyc invalidation. The invalidation mode can also be specified on the command line using the new
--invalidation-mode argument. (Contributed by Benjamin Peterson in bpo-31650 .)
AsyncExitStack have been added to complement their synchronous counterparts. (Contributed by Jelle Zijlstra in bpo-29679 and bpo-30241 , and by Alexander Mohr and Ilya Kulakov in bpo-29302 .)
dbm.dumb now supports reading read-only files and no longer writes the index file when it is not changed.
dis() function is now able to disassemble nested code objects (the code of comprehensions, generator expressions and nested functions, and the code used for building nested classes). The maximum depth of disassembly recursion is controlled by the new depth parameter. (Contributed by Serhiy Storchaka in bpo-11822 .)
README.rst is now included in the list of distutils standard READMEs and therefore included in source distributions. (Contributed by Ryan Gonzalez in bpo-11913 .)
In Python 3.8, attempting to check for non-Enum objects in
Enum classes will raise a
1 in Color); similarly, attempting to check for non-Flag objects in a
Flag member will raise
1 in Perm.RW); currently, both operations return
False instead and are deprecated. (Contributed by Ethan Furman in bpo-33217 .)
gc.freeze() function allows freezing all objects tracked by the garbage collector and excluding them from future collections. This can be used before a POSIX
fork() call to make the GC copy-on-write friendly or to speed up collection. The new
gc.unfreeze() functions reverses this operation. Additionally,
gc.get_freeze_count() can be used to obtain the number of frozen objects. (Contributed by Li Zekun in bpo-31558 .)
SimpleHTTPRequestHandler now supports the HTTP
If-Modified-Since header. The server returns the 304 response status if the target file was not modified after the time specified in the header. (Contributed by Pierre Quentel in bpo-29654 .)
SimpleHTTPRequestHandler accepts the new directory argument, in addition to the new
--directory command line argument. With this parameter, the server serves the specified directory, by default it uses the current working directory. (Contributed by Stéphane Wirtel and Julien Palard in bpo-28707 .)
Multiple fixes for autocompletion. (Contributed by Louie Lu in bpo-15786 .)
Module Browser (on the File menu, formerly called Class Browser), now displays nested functions and classes in addition to top-level functions and classes. (Contributed by Guilherme Polo, Cheryl Sabella, and Terry Jan Reedy in bpo-1612262 .)
The Settings dialog (Options, Configure IDLE) has been partly rewritten to improve both appearance and function. (Contributed by Cheryl Sabella and Terry Jan Reedy in multiple issues.)
The font sample now includes a selection of non-Latin characters so that users can better see the effect of selecting a particular font. (Contributed by Terry Jan Reedy in bpo-13802 .) The sample can be edited to include other characters. (Contributed by Serhiy Storchaka in bpo-31860 .)
The IDLE features formerly implemented as extensions have been reimplemented as normal features. Their settings have been moved from the Extensions tab to other dialog tabs. (Contributed by Charles Wohlganger and Terry Jan Reedy in bpo-27099 .)
Editor code context option revised. Box displays all context lines up to maxlines. Clicking on a context line jumps the editor to that line. Context colors for custom themes is added to Highlights tab of Settings dialog. (Contributed by Cheryl Sabella and Terry Jan Reedy in bpo-33642 , bpo-33768 , and bpo-33679 .)
On Windows, a new API call tells Windows that tk scales for DPI. On Windows 8.1+ or 10, with DPI compatibility properties of the Python binary unchanged, and a monitor resolution greater than 96 DPI, this should make text and lines sharper. It should otherwise have no effect. (Contributed by Terry Jan Reedy in bpo-33656 .)
New in 3.7.1:
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 3.6 maintenance releases.
NEW in 3.7.4:
Add “Run Customized” to the Run menu to run a module with customized settings. Any command line arguments entered are added to sys.argv. They re-appear in the box for the next customized run. One can also suppress the normal Shell main module restart. (Contributed by Cheryl Sabella, Terry Jan Reedy, and others in bpo-5680 and bpo-37627 .)
New in 3.7.5:
Add optional line numbers for IDLE editor windows. Windows open without line numbers unless set otherwise in the General tab of the configuration dialog. Line numbers for an existing window are shown and hidden in the Options menu. (Contributed by Tal Einat and Saimadhav Heblikar in bpo-17535 .)
importlib.find_spec() now raises
ModuleNotFoundError instead of
AttributeError if the specified parent module is not a package (i.e. lacks a
__path__ attribute). (Contributed by Milan Oberkirch in bpo-30436 .)
importlib.source_hash() can be used to compute the hash of the passed source. A hash-based .pyc file embeds the value returned by this function.
supernet_of() methods of
ipaddress.IPv4Network can be used for network containment tests. (Contributed by Michel Albert and Cheryl Sabella in bpo-20825 .)
The MIME type of .bmp has been changed from
'image/bmp'. (Contributed by Nitish Chandra in bpo-22589 .)
Process.close() method explicitly closes the process object and releases all resources associated with it.
ValueError is raised if the underlying process is still running. (Contributed by Antoine Pitrou in bpo-30596 .)
os.preadv() (combine the functionality of
os.pwritev() functions (combine the functionality of
os.pwrite()). (Contributed by Pablo Galindo in bpo-31368 .)
py_compile.compile() – and by extension,
compileall – now respects the
SOURCE_DATE_EPOCH environment variable by unconditionally creating
.pyc files for hash-based validation. This allows for guaranteeing reproducible builds of
.pyc files when they are created eagerly. (Contributed by Bernhard M. Wiedemann in bpo-29708 .)
The pydoc server can now bind to an arbitrary hostname specified by the new
-n command-line argument. (Contributed by Feanil Patel in bpo-31128 .)
Regular expressions compiled with the
re.LOCALE flag no longer depend on the locale at compile time. Locale settings are applied only when the compiled regular expression is used. (Contributed by Serhiy Storchaka in bpo-30215 .)
FutureWarning is now emitted if a regular expression contains character set constructs that will change semantically in the future, such as nested sets and set operations. (Contributed by Serhiy Storchaka in bpo-30349 .)
The new warn_on_full_buffer argument to the
signal.set_wakeup_fd() function makes it possible to specify whether Python prints a warning on stderr when the wakeup buffer overflows. (Contributed by Nathaniel J. Smith in bpo-30050 .)
socket.close() function closes the passed socket file descriptor. This function should be used instead of
os.close() for better compatibility across platforms. (Contributed by Christian Heimes in bpo-32454 .)
socket module now exposes the
socket.TCP_CONGESTION (Linux 2.6.13),
socket.TCP_USER_TIMEOUT (Linux 2.6.37), and
socket.TCP_NOTSENT_LOWAT (Linux 3.12) constants. (Contributed by Omar Sandoval in bpo-26273 and Nathaniel J. Smith in bpo-29728 .)
Sockets now auto-detect family, type and protocol from file descriptor by default. (Contributed by Christian Heimes in bpo-28134 .)
socketserver.ThreadingMixIn.server_close() now waits until all non-daemon threads complete.
socketserver.ForkingMixIn.server_close() now waits until all child processes complete.
ssl module now uses OpenSSL’s builtin API instead of
match_hostname() to check a host name or an IP address. Values are validated during TLS handshake. Any certificate validation error including failing the host name check now raises
SSLCertVerificationError and aborts the handshake with a proper TLS Alert message. The new exception contains additional information. Host name validation can be customized with
SSLContext.hostname_checks_common_name. (Contributed by Christian Heimes in bpo-31399 .)
The improved host name check requires a libssl implementation compatible with OpenSSL 1.0.2 or 1.1. Consequently, OpenSSL 0.9.8 and 1.0.1 are no longer supported (see Platform Support Removals for more details). The ssl module is mostly compatible with LibreSSL 2.7.2 and newer.
ssl module no longer sends IP addresses in SNI TLS extension. (Contributed by Christian Heimes in bpo-32185 .)
The default cipher suite selection of the
ssl module now uses a blacklist approach rather than a hard-coded whitelist. Python no longer re-enables ciphers that have been blocked by OpenSSL security updates. Default cipher suite selection can be configured at compile time. (Contributed by Christian Heimes in bpo-31429 .)
Validation of server certificates containing internationalized domain names (IDNs) is now supported. As part of this change, the
SSLSocket.server_hostname attribute now stores the expected hostname in A-label form (
"xn--pythn-mua.org"), rather than the U-label form (
"pythön.org"). (Contributed by Nathaniel J. Smith and Christian Heimes in bpo-28414 .)
ssl module has preliminary and experimental support for TLS 1.3 and OpenSSL 1.1.1. At the time of Python 3.7.0 release, OpenSSL 1.1.1 is still under development and TLS 1.3 hasn’t been finalized yet. The TLS 1.3 handshake and protocol behaves slightly differently than TLS 1.2 and earlier, see TLS 1.3. (Contributed by Christian Heimes in bpo-32947 , bpo-20995 , bpo-29136 , bpo-30622 and bpo-33618 )
SSLObject no longer have a public constructor. Direct instantiation was never a documented and supported feature. Instances must be created with
wrap_bio(). (Contributed by Christian Heimes in bpo-32951 )
OpenSSL 1.1 APIs for setting the minimum and maximum TLS protocol version are available as
SSLContext.maximum_version. Supported protocols are indicated by several new flags, such as
HAS_TLSv1_1. (Contributed by Christian Heimes in bpo-32609 .)
subprocess.run() function accepts the new capture_output keyword argument. When true, stdout and stderr will be captured. This is equivalent to passing
subprocess.PIPE as stdout and stderr arguments. (Contributed by Bo Bayles in bpo-32102 .)
On Windows the default for close_fds was changed from
True when redirecting the standard handles. It’s now possible to set close_fds to true when redirecting the standard handles. See
subprocess.Popen. This means that close_fds now defaults to
True on all supported platforms. (Contributed by Segev Finer in bpo-19764 .)
The subprocess module is now more graceful when handling
subprocess.run(), or in a
Popen context manager. It now waits a short amount of time for the child to exit, before continuing the handling of the
KeyboardInterrupt exception. (Contributed by Gregory P. Smith in bpo-25942 .)
sys.get_coroutine_origin_tracking_depth() function returns the current coroutine origin tracking depth, as set by the new
asyncio has been converted to use this new API instead of the deprecated
sys.set_coroutine_wrapper(). (Contributed by Nathaniel J. Smith in bpo-32591 .)
New clock identifiers have been added:
time.CLOCK_PROF(FreeBSD, NetBSD and OpenBSD): High-resolution per-process CPU timer.
time.CLOCK_UPTIME(FreeBSD, OpenBSD): Time whose absolute value is the time the system has been running and not suspended, providing accurate uptime measurement.
time.pthread_getcpuclockid() function returns the clock ID of the thread-specific CPU-time clock.
tracemalloc.Traceback behaves more like regular tracebacks, sorting the frames from oldest to most recent.
Traceback.format() now accepts negative limit, truncating the result to the
abs(limit) oldest frames. To get the old behaviour, use the new most_recent_first argument to
Traceback.format(). (Contributed by Jesse Bakker in bpo-32121 .)
ClassMethodDescriptorType classes are now available. (Contributed by Manuel Krebber and Guido van Rossum in bpo-29377 , and Serhiy Storchaka in bpo-32265 .)
-k command-line option allows filtering tests by a name substring or a Unix shell-like pattern. For example,
python -m unittest -k foo runs
bar_tests.SomeTest.test_foo, but not
bar_tests.FooTest.test_something. (Contributed by Jonas Haag in bpo-32071 .)
seal() function allows sealing
Mock instances, which will disallow further creation of attribute mocks. The seal is applied recursively to all attributes that are themselves mocks. (Contributed by Mario Corchero in bpo-30541 .)
urllib.parse.quote() has been updated from RFC 2396 to RFC 3986 , adding
~ to the set of characters that are never quoted by default. (Contributed by Christian Theune and Ratnadeep Debnath in bpo-16285 .)
uuid.getnode() now prefers universally administered MAC addresses over locally administered MAC addresses. This makes a better guarantee for global uniqueness of UUIDs returned from
uuid.uuid1(). If only locally administered MAC addresses are available, the first such one found is returned. (Contributed by Barry Warsaw in bpo-32107 .)
The initialization of the default warnings filters has changed as follows:
warnings filters enabled via the command line or the environment now have the following order of precedence:
any filters specified with the
any filters specified with the
any other CPython specific filters (e.g. the
defaultfilter added for the new
any implicit filters defined directly by the warnings machinery
in CPython debug builds, all warnings are now displayed by default (the implicit filter list is empty)
Deprecation warnings are once again shown by default in single-file scripts and at the interactive prompt. See PEP 565: Show DeprecationWarning in __main__ for details. (Contributed by Nick Coghlan in bpo-31975 .)
ElementPath predicates in the
find() methods can now compare text of the current node with
[. = "text"], not only text in children. Predicates also allow adding spaces for better readability. (Contributed by Stefan Behnel in bpo-31648 .)
SimpleXMLRPCDispatcher.register_function can now be used as a decorator. (Contributed by Xiang Zhang in bpo-7769 .)
create_archive() now accepts an optional compressed argument to generate a compressed archive. A command line option
--compress has also been added to support compression. (Contributed by Zhiming Wang in bpo-31638 .)
A new API for thread-local storage has been implemented. See PEP 539: New C API for Thread-Local Storage for an overview and Thread Specific Storage (TSS) API for a complete reference. (Contributed by Masayuki Yamamoto in bpo-25658 .)
doc of structures
wrapperbase are now of type
const char * rather of
char *. (Contributed by Serhiy Storchaka in bpo-28761 .)
PyExc_RecursionErrorInst singleton that was part of the public API has been removed as its members being never cleared may cause a segfault during finalization of the interpreter. Contributed by Xavier de Gaye in bpo-22898 and bpo-30697 .
Added C API support for timezones with timezone constructors
PyTimeZone_FromOffsetAndName(), and access to the UTC singleton with
PyDateTime_TimeZone_UTC. Contributed by Paul Ganssle in bpo-10381 .
The type of results of
PyThread_get_thread_ident(), and the id parameter of
PyThreadState_SetAsyncExc() changed from
unsigned long. (Contributed by Serhiy Storchaka in bpo-6532 .)
Changes to the startup sequence and the management of dynamic memory allocators mean that the long documented requirement to call
Py_Initialize() before calling most C API functions is now relied on more heavily, and failing to abide by it may lead to segfaults in embedding applications. See the Porting to Python 3.7 section in this document and the Before Python Initialization section in the C API documentation for more details.
A full copy of libffi is no longer bundled for use when building the
_ctypes module on non-OSX UNIX platforms. An installed copy of libffi is now required when building
_ctypes on such platforms. (Contributed by Zachary Ware in bpo-27979 .)
The Windows build process no longer depends on Subversion to pull in external sources, a Python script is used to download zipfiles from GitHub instead. If Python 3.6 is not found on the system (via
py -3.6), NuGet is used to download a copy of 32-bit Python for this purpose. (Contributed by Zachary Ware in bpo-30450 .)
ssl module requires OpenSSL 1.0.2 or 1.1 compatible libssl. OpenSSL 1.0.1 has reached end of lifetime on 2016-12-31 and is no longer supported. LibreSSL is temporarily not supported as well. LibreSSL releases up to version 2.6.4 are missing required OpenSSL 1.0.2 APIs.
The overhead of calling many methods of various standard library classes implemented in C has been significantly reduced by porting more code to use the
METH_FASTCALL convention. (Contributed by Victor Stinner in bpo-29300 , bpo-29507 , bpo-29452 , and bpo-29286 .)
Method calls are now up to 20% faster due to the bytecode changes which avoid creating bound method instances. (Contributed by Yury Selivanov and INADA Naoki in bpo-26110 .)
asyncio module received a number of notable optimizations for commonly used functions:
The performance overhead of asyncio debug mode has been reduced. (Contributed by Antoine Pitrou in bpo-31970 .)
Searching for certain Unicode characters (like Ukrainian capital “Є”) in a string was up to 25 times slower than searching for other characters. It is now only 3 times slower in the worst case. (Contributed by Serhiy Storchaka in bpo-24821 .)
collections.namedtuple() factory has been reimplemented to make the creation of named tuples 4 to 6 times faster. (Contributed by Jelle Zijlstra with further improvements by INADA Naoki, Serhiy Storchaka, and Raymond Hettinger in bpo-28638 .)
date.fromtimestamp() are now up to 30% faster in the common case. (Contributed by Paul Ganssle in bpo-32403 .)
re.compile() now converts
flags parameter to int object if it is
RegexFlag. It is now as fast as Python 3.5, and faster than Python 3.6 by about 10% depending on the pattern. (Contributed by INADA Naoki in bpo-31671 .)
modify() methods of classes
selectors.DevpollSelector may be around 10% faster under heavy loads. (Contributed by Giampaolo Rodola’ in bpo-30014 )
Constant folding has been moved from the peephole optimizer to the new AST optimizer, which is able perform optimizations more consistently. (Contributed by Eugene Toder and INADA Naoki in bpo-29469 and bpo-11549 .)
Most functions and methods in
abc have been rewritten in C. This makes creation of abstract base classes, and calling
issubclass() on them 1.5x faster. This also reduces Python start-up time by up to 10%. (Contributed by Ivan Levkivskyi and INADA Naoki in bpo-31333 )
Significant speed improveme