What’s New In Python 3.7

This article explains the new features in Python 3.7, compared to 3.6.

For full details, see the changelog.


Prerelease users should be aware that this document is currently in draft form. It will be updated substantially as Python 3.7 moves towards release, so it’s worth checking back even after reading earlier versions.

Summary – Release highlights

New Features

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 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.

See also

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

PEP 553: Built-in breakpoint()

PEP 553 describes a new built-in called breakpoint() which makes it easy and consistent to enter the Python debugger. Built-in breakpoint() calls sys.breakpointhook(). By default, this 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. Or, 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: A New C-API for Thread-Local Storage in CPython

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

It is sometimes convenient to customize or otherwise have control over access to module attributes. A typical example is managing deprecation warnings. Typical workarounds are assigning __class__ of a module object to a custom subclass of types.ModuleType or replacing the sys.modules item with a custom wrapper instance. This procedure is now simplified by recognizing __getattr__ defined directly in a module that would act like a normal __getattr__() method, except that it will be defined on module instances.

See also

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

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 can be enabled on a per-module basis in Python 3.7 using a __future__ import, like this:

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 564: Add new time functions with nanosecond resolution

Add six new “nanosecond” variants of existing functions to the time module:

While similar to the existing functions without the _ns suffix, they provide nanosecond resolution: they return a number of nanoseconds as a Python int.

The time.time_ns() resolution is 3 times better than the time.time() resolution on Linux and Windows.

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 540: Add a new UTF-8 mode

Add a new UTF-8 mode to ignore the locale, use the UTF-8 encoding, and change sys.stdin and sys.stdout error handlers to surrogateescape. This mode is enabled by default in the POSIX locale, but otherwise disabled by default.

The new -X utf8 command line option and PYTHONUTF8 environment variable are added to control the UTF-8 mode.

See also

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

PEP 557: Data Classes

Adds a new module dataclasses. It provides a class decorator dataclass() which inspects the class’s variable annotations (see PEP 526) and using them, adds methods such as __init__, __repr__, and __eq__ to the class. It is similar to typing.NamedTuple, but also works on classes with mutable instances, among other features.

For example:

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

PEP 567: Context Variables

Adds a new module contextvars, that provides APIs to manage, store, and access non-local state.

Context variables are natively supported in asyncio and are ready to be used without any extra configuration.

The decimal module was updated to use contextvars to store the current decimal context. This allows decimal operations to work with the correct context in async/await code.

See also

PEP 567 – Context Variables
PEP written and implemented by Yury Selivanov

New Development Mode: -X dev

Add a new “development mode”: -X dev command line option and PYTHONDEVMODE environment variable to enable CPython’s “development mode”, introducing additional runtime checks which are too expensive to be enabled by default. See -X dev documentation for the effects of the development mode.

Hash-based pycs

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.

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.)
  • 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.)

New Modules


This module provides several new APIs and one new ABC for access to, opening, and reading resources inside packages. Resources are roughly akin to files inside of 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. (see the PyPI package importlib_resources as a compatible back port for older Python versions).

Improved Modules


The parse_intermixed_args() supports letting the user intermix options and positional arguments on the command line, as is possible in many unix commands. It supports most but not all argparse features. (Contributed by paul.j3 in bpo-14191.)


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


asynccontextmanager() and AbstractAsyncContextManager have been added. (Contributed by Jelle Zijlstra in bpo-29679 and bpo-30241.)

contextlib.AsyncExitStack has been added. (Contributed by Alexander Mohr and Ilya Kulakov in bpo-29302.)


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


Added support for the Blowfish method. (Contributed by Serhiy Storchaka in bpo-31664.)

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


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


The dis() function now is able to disassemble nested code objects (the code of comprehensions, generator expressions and nested functions, and the code used for building nested classes). (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.)

distutils.core.setup now warns if the classifiers, keywords and platforms fields are not specified as a list or a string. (Contributed by Berker Peksag in bpo-19610.)

The 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.)


Add configurable blocksize to HTTPConnection and HTTPSConnection for improved upload throughput. (Contributed by Nir Soffer in bpo-31945.)


SimpleHTTPRequestHandler 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.)

Add the parameter directory to the SimpleHTTPRequestHandler and the --directory to the command line of the module http.server. 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 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.)


The importlib.abc.ResourceReader ABC was introduced to support the loading of resource from packages.


Added another argument monetary in format_string() of locale. If monetary is true, the conversion uses monetary thousands separator and grouping strings. (Contributed by Garvit in bpo-10379.)

The getpreferredencoding() function now always returns 'UTF-8' on Android or in the UTF-8 mode (-X utf8 option), the locale and the do_setlocale argument are ignored.


New remainder() function, implementing the IEEE 754-style remainder operation. (Contributed by Mark Dickinson in bpo-29962.)


Added support for bytes paths in fwalk(). (Contributed by Serhiy Storchaka in bpo-28682.)

Added support for file descriptors in scandir() on Unix. (Contributed by Serhiy Storchaka in bpo-25996.)

New function register_at_fork() allows registering Python callbacks to be executed on a process fork. (Contributed by Antoine Pitrou in bpo-16500.)


set_trace() now takes an optional header keyword-only argument. If given, this 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 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.)


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


The ssl module now uses OpenSSL’s builtin API instead of match_hostname() to check host name or IP address. Values are validated during TLS handshake. Any cert validation error including a failing host name match 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 host_flags. (Contributed by Christian Heimes in bpo-31399.)


The improved host name check requires an OpenSSL 1.0.2 or 1.1 compatible libssl. OpenSSL 0.9.8 and 1.0.1 are no longer supported. LibreSSL is temporarily not supported until it gains the necessary OpenSSL 1.0.2 APIs.

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. 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 update. Default cipher suite selection can be configured on compile time. (Contributed by Christian Heimes in bpo-31429.)

Added support for validating server certificates containing internationalized domain names (IDNs). As part of this change, the ssl.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. (Contributed by Christian Heimes in bpo-32947, bpo-20995, bpo-29136, and bpo-30622)

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 as minimum_version and maximum_version. Supported protocols are indicated by new flags like 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.)


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.)


Added sys.flags.dev_mode flag for the new development mode.


Added tkinter.ttk.Spinbox. (Contributed by Alan Moore in bpo-32585.)


The PEP 564 added six new functions with nanosecond resolution:

Add new clock identifiers:

  • 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 timer from the CPU.
  • time.CLOCK_UPTIME (FreeBSD, OpenBSD): Time whose absolute value is the time the system has been running and not suspended, providing accurate uptime measurement, both absolute and interval.

Added functions time.thread_time() and time.thread_time_ns() to get per-thread CPU time measurements. (Contributed by Antoine Pitrou in bpo-32025.)


The internal unicodedata database has been upgraded to use Unicode 10. (Contributed by Benjamin Peterson.)


Added new command-line option -k to filter tests to run with a substring or Unix shell-like pattern. For example, python -m unittest -k foo runs the tests 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.)

New function seal() will disable the creation of mock children by preventing to get or set any new attribute on the sealed mock. The sealing process is performed recursively. (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 is never quoted by default. (Contributed by Christian Theune and Ratnadeep Debnath in bpo-16285.)


Function encode() 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 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 precedence order:

    • the BytesWarning filter for -b (or -bb)
    • any filters specified with -W
    • any filters specified with PYTHONWARNINGS
    • 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)


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.)


register_function() of SimpleXMLRPCDispatcher and its subclasses can 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.)


  • Added two new opcodes: LOAD_METHOD and CALL_METHOD to avoid instantiation of bound method objects for method calls, which results in method calls being faster up to 20%. (Contributed by Yury Selivanov and INADA Naoki in bpo-26110.)
  • Searching some unlucky Unicode characters (like Ukrainian capital “Є”) in a string was up to 25 times slower than searching other characters. Now it is slower only by 3 times in the worst case. (Contributed by Serhiy Storchaka in bpo-24821.)
  • Fast implementation from standard C library is now used for functions erf() and erfc() in the math module. (Contributed by Serhiy Storchaka in bpo-26121.)
  • The os.fwalk() function has been sped up by 2 times. This was done using the os.scandir() function. (Contributed by Serhiy Storchaka in bpo-25996.)
  • The shutil.rmtree() function has been sped up to 20–40%. This was done using 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.)
  • 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 is moved from peephole optimizer to new AST optimizer. (Contributed by Eugene Toder and INADA Naoki in bpo-29469)
  • 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)

Build and C API Changes

Other CPython Implementation Changes

  • Trace hooks may now opt out of receiving line events from the interpreter by setting the new f_trace_lines attribute to False on the frame being traced. (Contributed by Nick Coghlan in bpo-31344.)
  • Trace hooks may now opt in to receiving opcode events from the interpreter by setting the new f_trace_opcodes attribute to True 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.


  • 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.

  • 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+ they will emit SyntaxError. (Contributed by Serhiy Storchaka in bpo-10544.)

Windows Only

  • 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.)


Platform Support Removals

  • FreeBSD 9 and older are no longer supported.

API and Feature Removals

  • 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 directly socket.socketpair() which is available on all platforms since Python 3.5 (before, it wasn’t available on Windows). asyncio.windows_utils.socketpair was just an alias to socket.socketpair on Python 3.5 and newer.
  • asyncio: The module doesn’t export selectors and _overlapped modules as asyncio.selectors and asyncio._overlapped. Replace from asyncio import selectors with import selectors for example.

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

  • 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 [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.)

Changes in the Python API

  • socketserver.ThreadingMixIn.server_close() now waits until all non-daemon threads complete. Use daemonic threads by setting socketserver.ThreadingMixIn.daemon_threads to True to not wait until threads complete. (Contributed by Victor Stinner in bpo-31233.)

  • socketserver.ForkingMixIn.server_close() now waits until all child processes complete. (Contributed by Victor Stinner in bpo-31151.)

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

  • pkgutil.walk_packages() now raises 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.)

  • Module, FunctionDef, AsyncFunctionDef, and ClassDef AST nodes now have a new docstring field. The first statement in their body is not considered as a docstring anymore. co_firstlineno and co_lnotab of code object for class and module are affected by this change. (Contributed by INADA Naoki and Eugene Toder in bpo-29463.)

  • 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() returns now 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 you won’t be able to socket.fromshare() a socket share()-ed in older Python versions.

  • repr for datetime.timedelta has changed to include 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 direcory is failed.

  • Support of 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, so to facilitate this 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.

Changes in the C API

  • The function PySlice_GetIndicesEx() is considered not safe 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

Other CPython implementation changes

  • In preparation for potential future changes to the public CPython runtime initialization API (see PEP 432 for details), 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. (Contributed by Nick Coghlan and Eric Snow as part of bpo-22257.)
  • 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.
  • CPython’ 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.


PEP 545: Python Documentation Translations

PEP 545 describes the process to translate Python documentation, and two translations have been added:

(Contributed by Julien Palard, Inada Naoki, and Victor Stinner in bpo-26546.)