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 538, legacy C locale coercion
- PEP 540, forced UTF-8 runtime mode
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 future import:
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 stdin and stdout is now
surrogateescape (rather than
strict ) when using any of the defined coercion target locales (currently
C.utf8 , and
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.
breakpoint() calls 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 time module:
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.
Previously both DeprecationWarning and 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 class_getitem() and
__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 py_compile or compileall.
.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.
The new -X
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 -X
dev documentation for the full description of the effects of this mode.
An await expression and comprehensions containing an async for clause 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.TracebackType can 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 -m switch,
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.)
The new 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.
The asyncio and 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
The new 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 repr(), eq(), and 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
The new 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 contextvars. loop.call_soon(), loop.call_soon_threadsafe(), loop.call_later(), loop.call_at(), and Future.add_done_callback() have a new optional keyword-only context parameter. Tasks now track their context automatically. See PEP 567 for more details. (Contributed by Yury Selivanov in bpo-32436.)
The new asyncio.current_task() function returns the currently running Task instance, and the new asyncio.all_tasks() function returns a set of all existing
Taskinstances in a given loop. The Task.current_task() and Task.all_tasks() methods have been deprecated. (Contributed by Andrew Svetlov in bpo-32250.)
The new asyncio.get_running_loop() function returns the currently running loop, and raises a RuntimeError if 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.)
The new 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.)
The new Future.get_loop() and
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.Server objects. (Contributed by Yury Selivanov in bpo-32415 and Srinivas Reddy Thatiparthy in bpo-32418.)
It is now possible to control how instances of asyncio.Server begin serving. Previously, the server would start serving immediately when created. The new start_serving keyword argument to loop.create_server() and loop.create_unix_server(), as well as Server.start_serving(), and Server.serve_forever() can be used to decouple server instantiation and serving. The new Server.is_serving() method returns
Trueif the server is serving. Server objects 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.)
The asyncio.open_connection(), asyncio.start_server() functions, loop.create_connection(), loop.create_server(), 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.)
The new ReadTransport.is_reading() method can be used to determine the reading state of the transport. Additionally, calls to ReadTransport.resume_reading() and 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.)
The new asynccontextmanager(), AbstractAsyncContextManager, and 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.
The 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 TypeError (e.g.
1 in Color ); similarly, attempting to check for non-Flag objects in a
Flag member will raise TypeError (e.g.
1 in Perm.RW ); currently, both operations return False instead and are deprecated. (Contributed by Ethan Furman in bpo-33217.)
The new 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.IPv6Network and 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.)
The new 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.)
Added os.preadv() (combine the functionality of os.readv() and os.pread()) and os.pwritev() functions (combine the functionality of os.writev() and 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.)
The new 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.)
The 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.
The 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)
SSLSocket and SSLObject no longer have a public constructor. Direct instantiation was never a documented and supported feature. Instances must be created with SSLContext methods wrap_socket() and 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.minimum_version and SSLContext.maximum_version. Supported protocols are indicated by several new flags, such as HAS_TLSv1_1. (Contributed by Christian Heimes in bpo-32609.)
The 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 KeyboardInterrupt during subprocess.call(), 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.)
The new sys.get_coroutine_origin_tracking_depth() function returns the current coroutine origin tracking depth, as set by the new sys.set_coroutine_origin_tracking_depth(). 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.
The new 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.)
The new WrapperDescriptorType, MethodWrapperType, MethodDescriptorType, and 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.)
The new 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:
BytesWarningfilter for -b (or
any filters specified with the -W option
any filters specified with the PYTHONWARNINGS environment variable
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.)
Function 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.)
Subdirectories in archives created by
ZipFile are now stored in alphabetical order. (Contributed by Bernhard M. Wiedemann in bpo-30693.)
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 PyMemberDef, PyGetSetDef, PyStructSequence_Field, PyStructSequence_Desc, and
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_FromOffset() and 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.)
The 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.)
The 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.)
The 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.)
The modify() methods of classes selectors.EpollSelector, selectors.PollSelector and 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 isinstance() and 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 improvements to alternate constructors for datetime.date and datetime.datetime by using fast-path constructors when not constructing subclasses. (Contributed by Paul Ganssle in bpo-32403)
The speed of comparison of array.array instances has been improved considerably in certain cases. It is now from 10x to 70x faster when comparing arrays holding values of the same integer type. (Contributed by Adrian Wielgosik in bpo-24700.)
Trace hooks may now opt out of receiving the
lineand opt into receiving the
opcodeevents from the interpreter by setting the corresponding new
f_trace_opcodesattributes on the frame being traced. (Contributed by Nick Coghlan in bpo-31344.)
Fixed some consistency problems with namespace package module attributes. Namespace module objects now have an
__file__that is set to
None(previously unset), and their
__spec__.originis also set to
None(previously the string
"namespace"). See bpo-32305. Also, the namespace module object’s
__spec__.loaderis set to the same value as
__loader__(previously, the former was set to
None). See bpo-32303.
uploadcommand no longer tries to change CR end-of-line characters to CRLF. This fixes a corruption issue with sdists that ended with a byte equivalent to CR. (Contributed by Bo Bayles in bpo-32304.)
Yield expressions (both
yield from clauses) are now deprecated in comprehensions and generator expressions (aside from the iterable expression in the leftmost
for clause). This ensures that comprehensions always immediately return a container of the appropriate type (rather than potentially returning a generator iterator object), while generator expressions won’t attempt to interleave their implicit output with the output from any explicit yield expressions. In Python 3.7, such expressions emit DeprecationWarning when compiled, in Python 3.8 this will be a SyntaxError. (Contributed by Serhiy Storchaka in bpo-10544.)
Returning a subclass of complex from object.complex() is deprecated and will be an error in future Python versions. This makes
__complex__() consistent with object.int() and object.float(). (Contributed by Serhiy Storchaka in bpo-28894.)
Support for directly
await -ing instances of asyncio.Lock and other asyncio synchronization primitives has been deprecated. An asynchronous context manager must be used in order to acquire and release the synchronization resource. (Contributed by Andrew Svetlov in bpo-32253.)
In Python 3.8, the abstract base classes in collections.abc will no longer be exposed in the regular collections module. This will help create a clearer distinction between the concrete classes and the abstract base classes. (Contributed by Serhiy Storchaka in bpo-25988.)
dbm.dumb now supports reading read-only files and no longer writes the index file when it is not changed. A deprecation warning is now emitted if the index file is missing and recreated in the
'w' modes (this will be an error in future Python releases). (Contributed by Serhiy Storchaka in bpo-28847.)
In Python 3.8, attempting to check for non-Enum objects in
Enum classes will raise a TypeError (e.g.
1 in Color ); similarly, attempting to check for non-Flag objects in a
Flag member will raise TypeError (e.g.
1 in Perm.RW ); currently, both operations return False instead. (Contributed by Ethan Furman in bpo-33217.)
Methods MetaPathFinder.find_module() (replaced by MetaPathFinder.find_spec()) and PathEntryFinder.find_loader() (replaced by PathEntryFinder.find_spec()) both deprecated in Python 3.4 now emit DeprecationWarning. (Contributed by Matthias Bussonnier in bpo-29576)
macpath is now deprecated and will be removed in Python 3.8. (Contributed by Chi Hsuan Yen in bpo-9850.)
The silent argument value truncation in socket.htons() and socket.ntohs() has been deprecated. In future versions of Python, if the passed argument is larger than 16 bits, an exception will be raised. (Contributed by Oren Milman in bpo-28332.)
sys.callstats() function has been deprecated and will be removed in a future Python version. (Contributed by Victor Stinner in bpo-28799.)
Function PySlice_GetIndicesEx() is deprecated and replaced with a macro if
Py_LIMITED_API is not set or set to a value in the range between
0x03060000 (not inclusive), or is
0x03060100 or higher. (Contributed by Serhiy Storchaka in bpo-27867.)
FreeBSD 9 and older are no longer officially supported.
For full Unicode support, including within extension modules, *nix platforms are now expected to provide at least one of
C.utf8(full locale) or
LC_CTYPE-only locale) as an alternative to the legacy
OpenSSL 0.9.8 and 1.0.1 are no longer supported, which means building CPython 3.7 with SSL/TLS support on older platforms still using these versions requires custom build options that link to a more recent version of OpenSSL.
Notably, this issue affects the Debian 8 (aka “jessie”) and Ubuntu 14.04 (aka “Trusty”) LTS Linux distributions, as they still use OpenSSL 1.0.1 by default.
Debian 9 (“stretch”) and Ubuntu 16.04 (“xenial”), as well as recent releases of other LTS Linux releases (e.g. RHEL/CentOS 7.5, SLES 12-SP3), use OpenSSL 1.0.2 or later, and remain supported in the default build configuration.
CPython’s own CI configuration file provides an example of using the SSL compatibility testing infrastructure in CPython’s test suite to build and link against OpenSSL 1.1.0 rather than an outdated system provided OpenSSL.
The following features and APIs have been removed from Python 3.7:
os.stat_float_times()function has been removed. It was introduced in Python 2.3 for backward compatibility with Python 2.2, and was deprecated since Python 3.1.
Unknown escapes consisting of
'\'and an ASCII letter in replacement templates for re.sub() were deprecated in Python 3.5, and will now cause an error.
Removed support of the exclude argument in tarfile.TarFile.add(). It was deprecated in Python 2.7 and 3.2. Use the filter argument instead.
splitunc()function in the
ntpathmodule was deprecated in Python 3.1, and has now been removed. Use the splitdrive() function instead.
collections.namedtuple() no longer supports the verbose parameter or
_sourceattribute which showed the generated source code for the named tuple class. This was part of an optimization designed to speed-up class creation. (Contributed by Jelle Zijlstra with further improvements by INADA Naoki, Serhiy Storchaka, and Raymond Hettinger in bpo-28638.)
Removed previously deprecated in Python 2.4 classes
_InternalDictin the plistlib module. Dict values in the result of functions readPlist() and readPlistFromBytes() are now normal dicts. You no longer can use attribute access to access items of these dictionaries.
asyncio.windows_utils.socketpair()function has been removed. Use the socket.socketpair() function instead, it is available on all platforms since Python 3.5.
asyncio.windows_utils.socketpairwas just an alias to
socket.socketpairon Python 3.5 and newer.
Direct instantiation of ssl.SSLSocket and ssl.SSLObject objects is now prohibited. The constructors were never documented, tested, or designed as public constructors. Users were supposed to use ssl.wrap_socket() or ssl.SSLContext. (Contributed by Christian Heimes in bpo-32951.)
fpectl module has been removed. It was never enabled by default, never worked correctly on x86-64, and it changed the Python ABI in ways that caused unexpected breakage of C extensions. (Contributed by Nathaniel J. Smith in bpo-29137.)
The python launcher, (py.exe), can accept 32 & 64 bit specifiers without having to specify a minor version as well. So
py -3-32 and
py -3-64 become valid as well as
py -3.7-32 , also the - m -64 and - m.n -64 forms are now accepted to force 64 bit python even if 32 bit would have otherwise been used. If the specified version is not available py.exe will error exit. (Contributed by Steve Barnes in bpo-30291.)
The launcher can be run as
py -0 to produce a list of the installed pythons, with default marked with an asterisk . Running
py -0p will include the paths. If py is run with a version specifier that cannot be matched it will also print the short form list of available specifiers. (Contributed by Steve Barnes in bpo-30362.)
This section lists previously described changes and other bugfixes that may require changes to your code.
PEP 479 is enabled for all code in Python 3.7, meaning that StopIteration exceptions raised directly or indirectly in coroutines and generators are transformed into RuntimeError exceptions. (Contributed by Yury Selivanov in bpo-32670.)
Due to an oversight, earlier Python versions erroneously accepted the following syntax:
f(1 for x in ,) class C(1 for x in ): pass
Python 3.7 now correctly raises a SyntaxError, as a generator expression always needs to be directly inside a set of parentheses and cannot have a comma on either side, and the duplication of the parentheses can be omitted only on calls. (Contributed by Serhiy Storchaka in bpo-32012 and bpo-32023.)
- When using the -m switch, the initial working directory is now added to sys.path, rather than an empty string (which dynamically denoted the current working directory at the time of each import). Any programs that are checking for the empty string, or otherwise relying on the previous behaviour, will need to be updated accordingly (e.g. by also checking for
os.path.dirname(__main__.__file__), depending on why the code was checking for the empty string in the first place).
socketserver.ThreadingMixIn.server_close()now waits until all non-daemon threads complete. Set the new
socketserver.ThreadingMixIn.block_on_closeclass attribute to
Falseto get the pre-3.7 behaviour. (Contributed by Victor Stinner in bpo-31233 and bpo-33540.)
socketserver.ForkingMixIn.server_close()now waits until all child processes complete. Set the new
socketserver.ForkingMixIn.block_on_closeclass attribute to
Falseto get the pre-3.7 behaviour. (Contributed by Victor Stinner in bpo-31151 and bpo-33540.)
Attributes key, value and coded_value of class http.cookies.Morsel are now read-only. Assigning to them was deprecated in Python 3.5. Use the set() method for setting them. (Contributed by Serhiy Storchaka in bpo-29192.)
The mode argument of os.makedirs() no longer affects the file permission bits of newly-created intermediate-level directories. To set their file permission bits you can set the umask before invoking
makedirs(). (Contributed by Serhiy Storchaka in bpo-19930.)
parse_multipart() now accepts the encoding and errors arguments and returns the same results as
FieldStorage: for non-file fields, the value associated to a key is a list of strings, not bytes. (Contributed by Pierre Quentel in bpo-29979.)
Because shutil.rmtree() is now implemented using the os.scandir() function, the user specified handler onerror is now called with the first argument
os.listdirwhen listing the directory is failed.
Support for nested sets and set operations in regular expressions as in Unicode Technical Standard #18 might be added in the future. This would change the syntax. To facilitate this future change a FutureWarning will be raised in ambiguous cases for the time being. That include sets starting with a literal
'['or containing literal character sequences
'||'. To avoid a warning, escape them with a backslash. (Contributed by Serhiy Storchaka in bpo-30349.)
The result of splitting a string on a regular expression that could match an empty string has been changed. For example splitting on
r'\s*'will now split not only on whitespaces as it did previously, but also on empty strings before all non-whitespace characters and just before the end of the string. The previous behavior can be restored by changing the pattern to
r'\s+'. A FutureWarning was emitted for such patterns since Python 3.5.
For patterns that match both empty and non-empty strings, the result of searching for all matches may also be changed in other cases. For example in the string
'a\n\n' , the pattern
r'(?m)^\s*?$' will not only match empty strings at positions 2 and 3, but also the string
'\n' at positions 2–3. To match only blank lines, the pattern should be rewritten as
re.sub() now replaces empty matches adjacent to a previous non-empty match. For example
re.sub('x*', '-', 'abxd') returns now
'-a-b--d-' instead of
'-a-b-d-' (the first minus between ‘b’ and ‘d’ replaces ‘x’, and the second minus replaces an empty string between ‘x’ and ‘d’).
On OSes that support socket.SOCK_NONBLOCK or socket.SOCK_CLOEXEC bit flags, the socket.type no longer has them applied. Therefore, checks like
if sock.type == socket.SOCK_STREAMwork as expected on all platforms. (Contributed by Yury Selivanov in bpo-32331.)
On Windows the default for the close_fds argument of subprocess.Popen was changed from False to True when redirecting the standard handles. If you previously depended on handles being inherited when using subprocess.Popen with standard io redirection, you will have to pass
close_fds=Falseto preserve the previous behaviour, or use STARTUPINFO.lpAttributeList.
importlib.machinery.PathFinder.invalidate_caches() – which implicitly affects importlib.invalidate_caches() – now deletes entries in sys.path_importer_cache which are set to
None. (Contributed by Brett Cannon in bpo-33169.)
In asyncio, loop.sock_recv(), loop.sock_sendall(), loop.sock_accept(), loop.getaddrinfo(), loop.getnameinfo() have been changed to be proper coroutine methods to match their documentation. Previously, these methods returned asyncio.Future instances. (Contributed by Yury Selivanov in bpo-32327.)
Calendar.itermonthdates will now consistently raise an exception when a date falls outside of the
9999-12-31range. To support applications that cannot tolerate such exceptions, the new Calendar.itermonthdays3 and Calendar.itermonthdays4 can be used. The new methods return tuples and are not restricted by the range supported by datetime.date. (Contributed by Alexander Belopolsky in bpo-28292.)
submit()method of concurrent.futures.ThreadPoolExecutor and concurrent.futures.ProcessPoolExecutor now raises a RuntimeError if called during interpreter shutdown. (Contributed by Mark Nemec in bpo-33097.)
The configparser.ConfigParser constructor now uses
read_dict()to process the default values, making its behavior consistent with the rest of the parser. Non-string keys and values in the defaults dictionary are now being implicitly converted to strings. (Contributed by James Tocknell in bpo-23835.)
Several undocumented internal imports were removed. One example is that
os.errnois no longer available; use
import errnodirectly instead. Note that such undocumented internal imports may be removed any time without notice, even in micro version releases.
The function PySlice_GetIndicesEx() is considered unsafe for resizable sequences. If the slice indices are not instances of int, but objects that implement the
__index__() method, the sequence can be resized after passing its length to
PySlice_GetIndicesEx() . This can lead to returning indices out of the length of the sequence. For avoiding possible problems use new functions PySlice_Unpack() and PySlice_AdjustIndices(). (Contributed by Serhiy Storchaka in bpo-27867.)
STORE_ANNOTATION opcode has been removed. (Contributed by Mark Shannon in bpo-32550.)
In preparation for potential future changes to the public CPython runtime initialization API (see PEP 432 for an initial, but somewhat outdated, draft), CPython’s internal startup and configuration management logic has been significantly refactored. While these updates are intended to be entirely transparent to both embedding applications and users of the regular CPython CLI, they’re being mentioned here as the refactoring changes the internal order of various operations during interpreter startup, and hence may uncover previously latent defects, either in embedding applications, or in CPython itself. (Initially contributed by Nick Coghlan and Eric Snow as part of bpo-22257, and further updated by Nick, Eric, and Victor Stinner in a number of other issues). Some known details affected:
warnings filters added by an embedding application with PySys_AddWarnOption() should now more consistently take precedence over the default filters set by the interpreter
Due to changes in the way the default warnings filters are configured, setting Py_BytesWarningFlag to a value greater than one is no longer sufficient to both emit BytesWarning messages and have them converted to exceptions. Instead, the flag must be set (to cause the warnings to be emitted in the first place), and an explicit
error::BytesWarning warnings filter added to convert them to exceptions.
Due to a change in the way docstrings are handled by the compiler, the implicit
return None in a function body consisting solely of a docstring is now marked as occurring on the same line as the docstring, not on the function’s header line.
The current exception state has been moved from the frame object to the co-routine. This simplified the interpreter and fixed a couple of obscure bugs caused by having swap exception state when entering or exiting a generator. (Contributed by Mark Shannon in bpo-25612.)
Starting in 3.7.1, Py_Initialize() now consistently reads and respects all of the same environment settings as Py_Main() (in earlier Python versions, it respected an ill-defined subset of those environment variables, while in Python 3.7.0 it didn’t read any of them due to bpo-34247). If this behavior is unwanted, set Py_IgnoreEnvironmentFlag to 1 before calling Py_Initialize().
In 3.7.1 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.)
In 3.7.2, venv on Windows no longer copies the original binaries, but creates redirector scripts named
pythonw.exe instead. This resolves a long standing issue where all virtual environments would have to be upgraded or recreated with each Python update. However, note that this release will still require recreation of virtual environments in order to get the new scripts.
Due to significant security concerns, the reuse_address parameter of asyncio.loop.create_datagram_endpoint() is no longer supported. This is because of the behavior of the socket option
SO_REUSEADDR in UDP. For more details, see the documentation for
loop.create_datagram_endpoint() . (Contributed by Kyle Stanley, Antoine Pitrou, and Yury Selivanov in bpo-37228.)