What’s New In Python 3.7

Date:November 15, 2017

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 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

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.)

New Modules

  • None yet.

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.)


parse_multipart() 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.)


contextlib.asynccontextmanager() has been added. (Contributed by Jelle Zijlstra in bpo-29679.)


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 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.)


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 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.)


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.)


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 os.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.)


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.)


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 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.


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.)


register_function() of xmlrpc.server.SimpleXMLRPCDispatcher and its subclasses can be used as a decorator. (Contributed by Xiang Zhang in bpo-7769.)


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


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.)


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.)


Function zipapp.create_archive() now accepts an optional filter argument to allow the user to select which files should be included in the archive, and an optional compressed argument to generate a compressed archive.

A command line option --compress has also been added to support compression.


  • 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.)
  • selectors.EpollSelector.modify(), selectors.PollSelector.modify() and selectors.DevpollSelector.modify() may be around 10% faster under heavy loads. (Contributed by Giampaolo Rodola’ in bpo-30014)

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.)


Changes in the C API

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 asterix. 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.)


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.

Porting to Python 3.7

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

Changes in the Python API

  • 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.)
  • 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.

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.)


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.)