What’s New In Python 3.7

Editor:Elvis Pranskevichus <elvis@magic.io>

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.

Summary – Release Highlights

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.
  • the insertion-order preservation nature of dict objects has been declared to be an official part of the Python language spec.

Significant improvements in the standard library:

CPython implementation improvements:

C API improvements:

  • PEP 539, new C API for thread-local storage

Documentation improvements:

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.

New Features

PEP 563: Postponed Evaluation of Annotations

The advent of type hints in Python uncovered two glaring usability issues with the functionality of annotations added in PEP 3107 and refined further in PEP 526:

  • 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:
    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.

See also

PEP 563 – Postponed evaluation of annotations
PEP written and implemented by Łukasz Langa.

PEP 538: Legacy C Locale Coercion

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.UTF-8, 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.

See also

PEP 538 – Coercing the legacy C locale to a UTF-8 based locale
PEP written and implemented by Nick Coghlan.

PEP 540: Forced UTF-8 Runtime Mode

The new -X utf8 command line option and PYTHONUTF8 environment variable can be used to enable the CPython UTF-8 mode.

When in UTF-8 mode, CPython ignores the locale settings, and uses the UTF-8 encoding by default. The error handlers for sys.stdin and sys.stdout streams are set to surrogateescape.

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 C or 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).

See also

PEP 540 – Add a new UTF-8 mode
PEP written and implemented by Victor Stinner

PEP 553: Built-in breakpoint()

Python 3.7 includes the new built-in breakpoint() function as an easy and consistent way to enter the Python debugger.

Built-in 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 breakpoint().

See also

PEP 553 – Built-in breakpoint()
PEP written and implemented by Barry Warsaw

PEP 539: New C API for Thread-Local Storage

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 int.

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.

See also

PEP 539 – A New C-API for Thread-Local Storage in CPython
PEP written by Erik M. Bray; implementation by Masayuki Yamamoto.

PEP 562: Customization of Access to Module Attributes

Python 3.7 allows defining __getattr__() on modules and will call it whenever a module attribute is otherwise not found. Defining __dir__() on modules is now also allowed.

A typical example of where this may be useful is module attribute deprecation and lazy loading.

See also

PEP 562 – Module __getattr__ and __dir__
PEP written and implemented by Ivan Levkivskyi

PEP 564: New Time Functions With Nanosecond Resolution

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.

Measurements show that on Linux and Windows the resolution of time.time_ns() is approximately 3 times better than that of time.time().

See also

PEP 564 – Add new time functions with nanosecond resolution
PEP written and implemented by Victor Stinner

PEP 565: Show DeprecationWarning in __main__

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: displayed by default only when running tests, intended for cases where a future version upgrade will change the warning category to DeprecationWarning or FutureWarning.

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.

See also

PEP 565 – Show DeprecationWarning in __main__
PEP written and implemented by Nick Coghlan

PEP 560: Core Support for typing module and Generic Types

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.

See also

PEP 560 – Core support for typing module and generic types
PEP written and implemented by Ivan Levkivskyi

PEP 552: Hash-based .pyc Files

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 reproduciblity 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.

Hash-based .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.

See also

PEP 552 – Deterministic pycs
PEP written and implemented by Benjamin Peterson

PEP 545: Python Documentation Translations

PEP 545 describes the process of creating and maintaining Python documentation translations.

Three new translations have been added:

See also

PEP 545 – Python Documentation Translations
PEP written and implemented by Julien Palard, Inada Naoki, and Victor Stinner.

Development Runtime Mode: -X dev

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.

Other Language Changes

  • More than 255 arguments can now be passed to a function, and a function can now have more than 255 parameters. (Contributed by Serhiy Storchaka in bpo-12844 and bpo-18896.)
  • bytes.fromhex() and bytearray.fromhex() now ignore all ASCII whitespace, not only spaces. (Contributed by Robert Xiao in bpo-28927.)
  • str, bytes, and bytearray gained support for the new isascii() method, which can be used to test if a string or bytes contain only the ASCII characters. (Contributed by INADA Naoki in bpo-32677.)
  • ImportError now displays module name and module __file__ path when from ... import ... fails. (Contributed by Matthias Bussonnier in bpo-29546.)
  • 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 str(x) rather than 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_next attribute on tracebacks is now writable. (Contributed by Nathaniel J. Smith in bpo-30579.)
  • When using the -m switch, sys.path[0] is 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 -X importtime option or the PYTHONPROFILEIMPORTTIME environment variable can be used to show the timing of each module import. (Contributed by Victor Stinner in bpo-31415.)

New Modules


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.

See also

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.


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)"

See also

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.

See also

importlib_resources – a PyPI backport for earlier Python versions.

Improved Modules


The new ArgumentParser.parse_intermixed_args() method allows intermixing options and positional arguments. (Contributed by paul.j3 in bpo-14191.)


The asyncio module has received many new features, usability and performance improvements. Notable changes include:

Several asyncio APIs have been deprecated.


The b2a_uu() function now accepts an optional backtick keyword argument. When it’s true, zeros are represented by '`' instead of spaces. (Contributed by Xiang Zhang in bpo-30103.)


The HTMLCalendar class has new class attributes which ease the customization of CSS classes in the produced HTML calendar. (Contributed by Oz Tiram in bpo-30095.)


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.)


ProcessPoolExecutor and ThreadPoolExecutor now support the new initializer and initargs constructor arguments. (Contributed by Antoine Pitrou in bpo-21423.)

The ProcessPoolExecutor can now take the multiprocessing context via the new mp_context argument. (Contributed by Thomas Moreau in bpo-31540.)


The new nullcontext() is a simpler and faster no-op context manager than ExitStack. (Contributed by Jesse-Bakker in bpo-10049.)

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.)


The cProfile command line now accepts -m module_name as an alternative to script path. (Contributed by Sanyam Khurana in bpo-21862.)


The crypt module now supports the Blowfish hashing method. (Contributed by Serhiy Storchaka in bpo-31664.)

The mksalt() function now allows specifying the number of rounds for hashing. (Contributed by Serhiy Storchaka in bpo-31702.)


The new datetime.fromisoformat() method constructs a datetime object from a string in one of the formats output by datetime.isoformat(). (Contributed by Paul Ganssle in bpo-15873.)

The tzinfo class now supports sub-minute offsets. (Contributed by Alexander Belopolsky in bpo-5288.)


dbm.dumb now supports reading read-only files and no longer writes the index file when it is not changed.


The decimal module now uses context variables to store the decimal context. (Contributed by Yury Selivanov in bpo-32630.)


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.)


The Enum learned the new _ignore_ class property, which allows listing the names of properties which should not become enum members. (Contributed by Ethan Furman in bpo-31801.)

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.)


functools.singledispatch() now supports registering implementations using type annotations. (Contributed by Łukasz Langa in bpo-32227.)


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.)


The hmac module now has an optimized one-shot digest() function, which is up to three times faster than HMAC(). (Contributed by Christian Heimes in bpo-32433.)


HTTPConnection and HTTPSConnection now support the new blocksize argument for improved upload throughput. (Contributed by Nir Soffer in bpo-31945.)


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.)

The new ThreadingHTTPServer class uses threads to handle requests using ThreadingMixin. It is used when http.server is run with -m. (Contributed by Julien Palard in bpo-31639.)

idlelib and IDLE

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).

The changes above have been backported to 3.6 maintenance releases.


The importlib.abc.ResourceReader ABC was introduced to support the loading of resources from packages. See also importlib.resources. (Contributed by Barry Warsaw, Brett Cannon in bpo-32248.)

importlib.reload() now raises ModuleNotFoundError if the module lacks a spec. (Contributed by Garvit Khatri in bpo-29851.)

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.)

The new 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.


The new TextIOWrapper.reconfigure() method can be used to reconfigure the text stream with the new settings. (Contributed by Antoine Pitrou in bpo-30526 and INADA Naoki in bpo-15216.)


The new subnet_of() and 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.)


itertools.islice() now accepts integer-like objects as start, stop, and slice arguments. (Contributed by Will Roberts in bpo-30537.)


The new monetary argument to locale.format_string() can be used to make the conversion use monetary thousands separators and grouping strings. (Contributed by Garvit in bpo-10379.)

The locale.getpreferredencoding() function now always returns 'UTF-8' on Android or when in the forced UTF-8 mode.


Logger instances can now be pickled. (Contributed by Vinay Sajip in bpo-30520.)

The new StreamHandler.setStream() method can be used to replace the logger stream after handler creation. (Contributed by Vinay Sajip in bpo-30522.)

It is now possible to specify keyword arguments to handler constructors in configuration passed to logging.config.fileConfig(). (Contributed by Preston Landers in bpo-31080.)


The new math.remainder() function implements the IEEE 754-style remainder operation. (Contributed by Mark Dickinson in bpo-29962.)


The MIME type of .bmp has been changed from 'image/x-ms-bmp' to 'image/bmp'. (Contributed by Nitish Chandra in bpo-22589.)


The new Database.Close() method can be used to close the MSI database. (Contributed by Berker Peksag in bpo-20486.)


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.)

The new Process.kill() method can be used to terminate the process using the SIGKILL signal on Unix. (Contributed by Vitor Pereira in bpo-30794.)

Non-daemonic threads created by Process are now joined on process exit. (Contributed by Antoine Pitrou in bpo-18966.)


os.fwalk() now accepts the path argument as bytes. (Contributed by Serhiy Storchaka in bpo-28682.)

os.scandir() gained support for file descriptors. (Contributed by Serhiy Storchaka in bpo-25996.)

The new register_at_fork() function allows registering Python callbacks to be executed at process fork. (Contributed by Antoine Pitrou in bpo-16500.)

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.)

The mode argument of os.makedirs() no longer affects the file permission bits of newly-created intermediate-level directories. (Contributed by Serhiy Storchaka in bpo-19930.)

os.dup2() now returns the new file descriptor. Previously, None was always returned. (Contributed by Benjamin Peterson in bpo-32441.)

The structure returned by os.stat() now contains the st_fstype attribute on Solaris and its derivatives. (Contributed by Jesús Cea Avión in bpo-32659.)


The new Path.is_mount() method is now available on POSIX systems and can be used to determine whether a path is a mount point. (Contributed by Cooper Ry Lees in bpo-30897.)


pdb.set_trace() now takes an optional header keyword-only argument. If given, it is printed to the console just before debugging begins. (Contributed by Barry Warsaw in bpo-31389.)

pdb command line now accepts -m module_name as an alternative to script file. (Contributed by Mario Corchero in bpo-32206.)


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.)


The new SimpleQueue class is an unbounded FIFO queue. (Contributed by Antoine Pitrou in bpo-14976.)


The flags re.ASCII, re.LOCALE and re.UNICODE can be set within the scope of a group. (Contributed by Serhiy Storchaka in bpo-31690.)

re.split() now supports splitting on a pattern like r'\b', '^$' or (?=-) that matches an empty string. (Contributed by Serhiy Storchaka in bpo-25054.)

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.)

Compiled regular expression and match objects can now be copied using copy.copy() and copy.deepcopy(). (Contributed by Serhiy Storchaka in bpo-10076.)


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.getblocking() method returns True if the socket is in blocking mode and False otherwise. (Contributed by Yury Selivanov in bpo-32373.)

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.)

Support for socket.AF_VSOCK sockets has been added to allow communication between virtual machines and their hosts. (Contributed by Cathy Avery in bpo-27584.)

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.

Add a new socketserver.ForkingMixIn.block_on_close class attribute to socketserver.ForkingMixIn and socketserver.ThreadingMixIn classes. Set the class attribute to False to get the pre-3.7 behaviour.


sqlite3.Connection now exposes the backup() method when the underlying SQLite library is at version 3.6.11 or higher. (Contributed by Lele Gaifax in bpo-27645.)

The database argument of sqlite3.connect() now accepts any path-like object, instead of just a string. (Contributed by Anders Lorentsen in bpo-31843.)


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.host_flags. (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.

The ssl module no longer sends IP addresses in SNI TLS extension. (Contributed by Christian Heimes in bpo-32185.)

match_hostname() no longer supports partial wildcards like www*.example.org. SSLContext.host_flags has partial wildcard matching disabled by default. (Contributed by Mandeep Singh in bpo-23033 and Christian Heimes in bpo-31399.)

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.)

The 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 serveral new flags, such as HAS_TLSv1_1. (Contributed by Christian Heimes in bpo-32609.)


string.Template now lets you to optionally modify the regular expression pattern for braced placeholders and non-braced placeholders separately. (Contributed by Barry Warsaw in bpo-1198569.)


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.)

The subprocess.run function and the subprocess.Popen constructor now accept the text keyword argument as an alias to universal_newlines. (Contributed by Andrew Clegg in bpo-31756.)

On Windows the default for close_fds was changed from False to 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.breakpointhook() hook function is called by the built-in breakpoint(). (Contributed by Barry Warsaw in bpo-31353.)

On Android, the new sys.getandroidapilevel() returns the build-time Android API version. (Contributed by Victor Stinner in bpo-28740.)

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.)


PEP 564 adds six new functions with nanosecond resolution to the time module:

New clock identifiers have been added:

  • time.CLOCK_BOOTTIME (Linux): Identical to time.CLOCK_MONOTONIC, except it also includes any time that the system is suspended.
  • 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.thread_time() and time.thread_time_ns() functions can be used to get per-thread CPU time measurements. (Contributed by Antoine Pitrou in bpo-32025.)

The new time.pthread_getcpuclockid() function returns the clock ID of the thread-specific CPU-time clock.


The new tkinter.ttk.Spinbox class is now available. (Contributed by Alan Moore in bpo-32585.)


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.)

The new types.resolve_bases() function resolves MRO entries dynamically as specified by PEP 560. (Contributed by Ivan Levkivskyi in bpo-32717.)


The internal unicodedata database has been upgraded to use Unicode 11. (Contributed by Benjamin Peterson.)


The new -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 foo_tests.SomeTest.test_something, bar_tests.SomeTest.test_foo, but not bar_tests.FooTest.test_something. (Contributed by Jonas Haag in bpo-32071.)


The sentinel attributes now preserve their identity when they are copied or pickled. (Contributed by Serhiy Storchaka in bpo-20804.)

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.)


The uu.encode() function now accepts an optional backtick keyword argument. When it’s true, zeros are represented by '`' instead of spaces. (Contributed by Xiang Zhang in bpo-30103.)


The new UUID.is_safe attribute relays information from the platform about whether generated UUIDs are generated with a multiprocessing-safe method. (Contributed by Barry Warsaw in bpo-22807.)

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 enabled via command line options (including those for -b and the new CPython-specific -X dev option) are always passed to the warnings machinery via the sys.warnoptions attribute.

  • warnings filters enabled via the command line or the environment now have the following order of precedence:

    • the BytesWarning filter for -b (or -bb)
    • any filters specified with the -W option
    • any filters specified with the PYTHONWARNINGS environment variable
    • any other CPython specific filters (e.g. the default filter added for the new -X dev mode)
    • 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)

(Contributed by Nick Coghlan and Victor Stinner in bpo-20361, bpo-32043, and bpo-32230.)

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 filter argument to allow the user to select which files should be included in the archive. (Contributed by Irmen de Jong in bpo-31072.)

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.)


ZipFile now accepts the new compresslevel parameter to control the compression level. (Contributed by Bo Bayles in bpo-21417.)

Subdirectories in archives created by ZipFile are now stored in alphabetical order. (Contributed by Bernhard M. Wiedemann in bpo-30693.)

C API Changes

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.)

The new context variables functionality exposes a number of new C APIs.

The new PyImport_GetModule() function returns the previously imported module with the given name. (Contributed by Eric Snow in bpo-28411.)

The new Py_RETURN_RICHCOMPARE macro eases writing rich comparison functions. (Contributed by Petr Victorin in bpo-23699.)

The new Py_UNREACHABLE macro can be used to mark unreachable code paths. (Contributed by Barry Warsaw in bpo-31338.)

The tracemalloc now exposes a C API through the new PyTraceMalloc_Track() and PyTraceMalloc_Untrack() functions. (Contributed by Victor Stinner in bpo-30054.)

The new import__find__load__start() and import__find__load__done() static markers can be used to trace module imports. (Contributed by Christian Heimes in bpo-31574.)

The fields name and 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.)

The result of PyUnicode_AsUTF8AndSize() and PyUnicode_AsUTF8() is now of type const char * rather of char *. (Contributed by Serhiy Storchaka in bpo-28769.)

The result of PyMapping_Keys(), PyMapping_Values() and PyMapping_Items() is now always a list, rather than a list or a tuple. (Contributed by Oren Milman in bpo-28280.)

Added functions PySlice_Unpack() and PySlice_AdjustIndices(). (Contributed by Serhiy Storchaka in bpo-27867.)

PyOS_AfterFork() is deprecated in favour of the new functions PyOS_BeforeFork(), PyOS_AfterFork_Parent() and PyOS_AfterFork_Child(). (Contributed by Antoine Pitrou in bpo-16500.)

The 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_start_new_thread() and PyThread_get_thread_ident(), and the id parameter of PyThreadState_SetAsyncExc() changed from long to unsigned long. (Contributed by Serhiy Storchaka in bpo-6532.)

PyUnicode_AsWideCharString() now raises a ValueError if the second argument is NULL and the wchar_t* string contains null characters. (Contributed by Serhiy Storchaka in bpo-30708.)

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.

The new PyInterpreterState_GetID() returns the unique ID for a given interpreter. (Contributed by Eric Snow in bpo-29102.)

Py_DecodeLocale(), Py_EncodeLocale() now use the UTF-8 encoding when the UTF-8 mode is enabled. (Contributed by Victor Stinner in bpo-29240.)

PyUnicode_DecodeLocaleAndSize() and PyUnicode_EncodeLocale() now use the current locale encoding for surrogateescape error handler. (Contributed by Victor Stinner in bpo-29240.)

The start and end parameters of PyUnicode_FindChar() are now adjusted to behave like string slices. (Contributed by Xiang Zhang in bpo-28822.)

Build Changes

Support for building --without-threads has been removed. The threading module is now always available. (Contributed by Antoine Pitrou in bpo-31370.).

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.)

Various optimizations have reduced Python startup time by 10% on Linux and up to 30% on macOS. (Contributed by Victor Stinner, INADA Naoki in bpo-29585, and Ivan Levkivskyi in bpo-31333.)

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 asyncio.get_event_loop() function has been reimplemented in C to make it up to 15 times faster. (Contributed by Yury Selivanov in bpo-32296.)
  • asyncio.Future callback management has been optimized. (Contributed by Yury Selivanov in bpo-32348.)
  • asyncio.gather() is now up to 15% faster. (Contributed by Yury Selivanov in bpo-32355.)
  • asyncio.sleep() is now up to 2 times faster when the delay argument is zero or negative. (Contributed by Andrew Svetlov in bpo-32351.)
  • The performance overhead of asyncio debug mode has been reduced. (Contributed by Antoine Pitrou in bpo-31970.)

As a result of PEP 560 work, the import time of typing has been reduced by a factor of 7, and many typing operations are now faster. (Contributed by Ivan Levkivskyi in bpo-32226.)

sorted() and list.sort() have been optimized for common cases to be up to 40-75% faster. (Contributed by Elliot Gorokhovsky in bpo-28685.)

dict.copy() is now up to 5.5 times faster. (Contributed by Yury Selivanov in bpo-31179.)

hasattr() and getattr() are now about 4 times faster when name is not found and obj does not override object.__getattr__() or object.__getattribute__(). (Contributed by INADA Naoki in bpo-32544.)

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.fromordinal() and date.fromtimestamp() are now up to 30% faster in the common case. (Contributed by Paul Ganssle in bpo-32403.)

The os.fwalk() function is now up to 2 times faster thanks to the use of os.scandir(). (Contributed by Serhiy Storchaka in bpo-25996.)

The speed of the shutil.rmtree() function has been improved by 20–40% thanks to the use of the os.scandir() function. (Contributed by Serhiy Storchaka in bpo-28564.)

Optimized case-insensitive matching and searching of regular expressions. Searching some patterns can now be up to 20 times faster. (Contributed by Serhiy Storchaka in bpo-30285.)

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.)

The math.erf() and math.erfc() functions now use the (faster) C library implementation on most platforms. (Contributed by Serhiy Storchaka in bpo-26121.)

Other CPython Implementation Changes

  • Trace hooks may now opt out of receiving the line and opt into receiving the opcode events from the interpreter by setting the corresponding new f_trace_lines and f_trace_opcodes attributes 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__.origin is also set to None (previously the string "namespace"). See bpo-32305. Also, the namespace module object’s __spec__.loader is set to the same value as __loader__ (previously, the former was set to None). See bpo-32303.
  • The locals() dictionary now displays in the lexical order that variables were defined. Previously, the order was undefined. (Contributed by Raymond Hettinger in bpo-32690.)
  • The distutils upload command 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.)

Deprecated Python Behavior

Yield expressions (both yield and 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.)

Deprecated Python modules, functions and methods


aifc.openfp() has been deprecated and will be removed in Python 3.9. Use aifc.open() instead. (Contributed by Brian Curtin in bpo-31985.)


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. See Using locks, conditions and semaphores in the async with statement for more information. (Contributed by Andrew Svetlov in bpo-32253.)

The asyncio.Task.current_task() and asyncio.Task.all_tasks() methods have been deprecated. (Contributed by Andrew Svetlov in bpo-32250.)


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 'r' and '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.)


Using non-integer value for selecting a plural form in gettext is now deprecated. It never correctly worked. (Contributed by Serhiy Storchaka in bpo-28692.)


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)

The importlib.abc.ResourceLoader ABC has been deprecated in favour of importlib.abc.ResourceReader.


locale.format() has been deprecated, use locale.format_string() instead. (Contributed by Garvit in bpo-10379.)


The macpath is now deprecated and will be removed in Python 3.8. (Contributed by Chi Hsuan Yen in bpo-9850.)


dummy_threading and _dummy_thread have been deprecated. It is no longer possible to build Python with threading disabled. Use threading instead. (Contributed by Antoine Pitrou in bpo-31370.)


The silent argument value trunctation 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.)


ssl.wrap_socket() is deprecated. Use ssl.SSLContext.wrap_socket() instead. (Contributed by Christian Heimes in bpo-28124.)


sunau.openfp() has been deprecated and will be removed in Python 3.9. Use sunau.open() instead. (Contributed by Brian Curtin in bpo-31985.)


Deprecated sys.set_coroutine_wrapper() and sys.get_coroutine_wrapper().

The undocumented sys.callstats() function has been deprecated and will be removed in a future Python version. (Contributed by Victor Stinner in bpo-28799.)


wave.openfp() has been deprecated and will be removed in Python 3.9. Use wave.open() instead. (Contributed by Brian Curtin in bpo-31985.)

Deprecated functions and types of the C API

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 0x03050400 and 0x03060000 (not inclusive), or is 0x03060100 or higher. (Contributed by Serhiy Storchaka in bpo-27867.)

PyOS_AfterFork() has been deprecated. Use PyOS_BeforeFork(), PyOS_AfterFork_Parent() or PyOS_AfterFork_Child() instead. (Contributed by Antoine Pitrou in bpo-16500.)

Platform Support Removals

  • 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.UTF-8 (full locale), C.utf8 (full locale) or UTF-8 (LC_CTYPE-only locale) as an alternative to the legacy ASCII-based C locale.

  • 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.

API and Feature Removals

The following features and APIs have been removed from Python 3.7:

  • The 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.
  • The splitunc() function in the ntpath module 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 _source attribute 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.)
  • Functions bool(), float(), list() and tuple() no longer take keyword arguments. The first argument of int() can now be passed only as positional argument.
  • Removed previously deprecated in Python 2.4 classes Plist, Dict and _InternalDict in 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.
  • The 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.socketpair was just an alias to socket.socketpair on Python 3.5 and newer.
  • asyncio no longer exports the selectors and _overlapped modules as asyncio.selectors and asyncio._overlapped. Replace from asyncio import selectors with import selectors.
  • 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.)
  • The unused distutils install_misc command has been removed. (Contributed by Eric N. Vander Weele in bpo-29218.)

Module Removals

The 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.)

Windows-only Changes

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.)

Porting to Python 3.7

This section lists previously described changes and other bugfixes that may require changes to your code.

Changes in Python Behavior

  • async and await names are now reserved keywords. Code using these names as identifiers will now raise a SyntaxError. (Contributed by Jelle Zijlstra in bpo-30406.)

  • 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.)

  • object.__aiter__() methods can no longer be declared as asynchronous. (Contributed by Yury Selivanov in bpo-31709.)

  • Due to an oversight, earlier Python versions erroneously accepted the following syntax:

    f(1 for x in [1],)
    class C(1 for x in [1]):

    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.getcwd() or os.path.dirname(__main__.__file__), depending on why the code was checking for the empty string in the first place).

Changes in the Python API

  • socketserver.ThreadingMixIn.server_close() now waits until all non-daemon threads complete. Set the new socketserver.ThreadingMixIn.block_on_close class attribute to False to 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_close class attribute to False to get the pre-3.7 behaviour. (Contributed by Victor Stinner in bpo-31151 and bpo-33540.)

  • The locale.localeconv() function now temporarily sets the LC_CTYPE locale to the value of LC_NUMERIC in some cases. (Contributed by Victor Stinner in bpo-31900.)

  • pkgutil.walk_packages() now raises a ValueError if path is a string. Previously an empty list was returned. (Contributed by Sanyam Khurana in bpo-24744.)

  • A format string argument for string.Formatter.format() is now positional-only. Passing it as a keyword argument was deprecated in Python 3.5. (Contributed by Serhiy Storchaka in bpo-29193.)

  • 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.)

  • The struct.Struct.format type is now str instead of bytes. (Contributed by Victor Stinner in bpo-21071.)

  • 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.)

  • Due to internal changes in socket, calling socket.fromshare() on a socket created by socket.share in older Python versions is not supported.

  • repr for BaseException has changed to not include the trailing comma. Most exceptions are affected by this change. (Contributed by Serhiy Storchaka in bpo-30399.)

  • repr for datetime.timedelta has changed to include the keyword arguments in the output. (Contributed by Utkarsh Upadhyay in bpo-30302.)

  • Because shutil.rmtree() is now implemented using the os.scandir() function, the user specified handler onerror is now called with the first argument os.scandir instead of os.listdir when 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 '--', '&&', '~~', and '||'. 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*?$' will not only match empty strings at positions 2 and 3, but also the string '\n' at positions 2–3. To match only blank lines, the pattern should be rewritten as r'(?m)^[^\S\n]*$'.

    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’).

    (Contributed by Serhiy Storchaka in bpo-25054 and bpo-32308.)

  • Change re.escape() to only escape regex special characters instead of escaping all characters other than ASCII letters, numbers, and '_'. (Contributed by Serhiy Storchaka in bpo-29995.)

  • tracemalloc.Traceback frames are now sorted from oldest to most recent to be more consistent with traceback. (Contributed by Jesse Bakker in bpo-32121.)

  • 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_STREAM work 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=False to 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.)

  • asyncio.Server.sockets now returns a copy of the internal list of server sockets, instead of returning it directly. (Contributed by Yury Selivanov in bpo-32662.)

  • Struct.format is now a str instance instead of a bytes instance. (Contributed by Victor Stinner in bpo-21071.)

  • ast.literal_eval() is now stricter. Addition and subtraction of arbitrary numbers are no longer allowed. (Contributed by Serhiy Storchaka in bpo-31778.)

  • Calendar.itermonthdates will now consistently raise an exception when a date falls outside of the 0001-01-01 through 9999-12-31 range. 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.)

  • collections.ChainMap now preserves the order of the underlying mappings. (Contributed by Raymond Hettinger in bpo-32792.)

  • The 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.)

Changes in the C API

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.)

CPython bytecode changes

There are two new opcodes: LOAD_METHOD and CALL_METHOD. (Contributed by Yury Selivanov and INADA Naoki in bpo-26110.)

The STORE_ANNOTATION opcode has been removed. (Contributed by Mark Shannon in bpo-32550.)

Windows-only Changes

The file used to override sys.path is now called <python-executable>._pth instead of 'sys.path'. See Finding modules for more information. (Contributed by Steve Dower in bpo-28137.)

Other CPython implementation changes

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:

  • PySys_AddWarnOptionUnicode() is not currently usable by embedding applications due to the requirement to create a Unicode object prior to calling Py_Initialize. Use PySys_AddWarnOption() instead.
  • 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.)