What’s New In Python 3.5

Date:March 29, 2015

This article explains the new features in Python 3.5, compared to 3.4.

For full details, see the Misc/NEWS file.


Prerelease users should be aware that this document is currently in draft form. It will be updated substantially as Python 3.5 moves towards release, so it’s worth checking back even after reading earlier versions.

See also

PEP 478 - Python 3.5 Release Schedule

Summary – Release highlights

New syntax features:

  • None yet.

New library modules:

New built-in features:

  • None yet.

Implementation improvements:

  • When the LC_TYPE locale is the POSIX locale (C locale), sys.stdin and sys.stdout are now using the surrogateescape error handler, instead of the strict error handler (issue 19977).

Significantly Improved Library Modules:

  • None yet.

Security improvements:

  • None yet.

Please read on for a comprehensive list of user-facing changes.

PEP 471 - os.scandir() function – a better and faster directory iterator

PEP 471 adds a new directory iteration function, os.scandir(), to the standard library. Additionally, os.walk() is now implemented using os.scandir(), which speeds it up by 3-5 times on POSIX systems and by 7-20 times on Windows systems.

PEP and implementation written by Ben Hoyt with the help of Victor Stinner.

See also

PEP 471 – os.scandir() function – a better and faster directory iterator

PEP 475: Retry system calls failing with EINTR

PEP 475 adds support for automatic retry of system calls failing with EINTR: this means that user code doesn’t have to deal with EINTR or InterruptedError manually, and should make it more robust against asynchronous signal reception.

See also

PEP 475 – Retry system calls failing with EINTR

PEP 486: Make the Python Launcher aware of virtual environments

PEP 486 makes the Windows launcher (see PEP 397) aware of an active virtual environment. When the default interpreter would be used and the VIRTUAL_ENV environment variable is set, the interpreter in the virtual environment will be used.

See also

PEP 486 – Make the Python Launcher aware of virtual environments

Other Language Changes

Some smaller changes made to the core Python language are:

  • Added the 'namereplace' error handlers. The 'backslashreplace' error handlers now works with decoding and translating. (Contributed by Serhiy Storchaka in issue 19676 and issue 22286.)
  • The -b option now affects comparisons of bytes with int. (Contributed by Serhiy Storchaka in issue 23681)

New Modules


The new zipapp module (specified in PEP 441) provides an API and command line tool for creating executable Python Zip Applications, which were introduced in Python 2.6 in issue 1739468 but which were not well publicised, either at the time or since.

With the new module, bundling your application is as simple as putting all the files, including a __main__.py file, into a directory myapp and running:

$ python -m zipapp myapp
$ python myapp.pyz

Improved Modules



  • FieldStorage now supports the context management protocol. (Contributed by Berker Peksag in issue 20289.)





  • The charset of the HTML document generated by difflib.HtmlDiff.make_file() can now be customized by using charset keyword-only parameter. The default charset of HTML document changed from 'ISO-8859-1' to 'utf-8'. (Contributed by Berker Peksag in issue 2052.)


  • The build and build_ext commands now accept a -j option to enable parallel building of extension modules. (Contributed by Antoine Pitrou in issue 5309.)



  • iglob() and glob() now support recursive search in subdirectories using the “**” pattern. (Contributed by Serhiy Storchaka in issue 13968.)


  • IMAP4 now supports the context management protocol. When used in a with statement, the IMAP4 LOGOUT command will be called automatically at the end of the block. (Contributed by Tarek Ziadé and Serhiy Storchaka in issue 4972.)








  • Number of capturing groups in regular expression is no longer limited by 100. (Contributed by Serhiy Storchaka in issue 22437.)
  • Now unmatched groups are replaced with empty strings in re.sub() and re.subn(). (Contributed by Serhiy Storchaka in issue 1519638.)



  • move() now accepts a copy_function argument, allowing, for example, copy() to be used instead of the default copy2() if there is a need to ignore metadata. (Contributed by Claudiu Popa in issue 19840.)


  • Different constants of signal module are now enumeration values using the enum module. This allows meaningful names to be printed during debugging, instead of integer “magic numbers”. (Contributed by Giampaolo Rodola’ in issue 21076.)


  • Both SMTPServer and smtpd.SMTPChannel now accept a decode_data keyword to determine if the DATA portion of the SMTP transaction is decoded using the utf-8 codec or is instead provided to process_message() as a byte string. The default is True for backward compatibility reasons, but will change to False in Python 3.6. (Contributed by Maciej Szulik in issue 19662.)
  • It is now possible to provide, directly or via name resolution, IPv6 addresses in the SMTPServer constructor, and have it successfully connect. (Contributed by Milan Oberkirch in issue 14758.)
  • SMTPServer now supports RFC 6531 via the enable_SMTPUTF8 constructor argument and a user-provided process_smtputf8_message() method.


  • A new auth() method provides a convenient way to implement custom authentication mechanisms. (Contributed by Milan Oberkirch in issue 15014.)




  • The user scripts directory on Windows is now versioned. (Contributed by Paul Moore in issue 23437.)




  • A new urllib.request.HTTPBasicPriorAuthHandler allows HTTP Basic Authentication credentials to be sent unconditionally with the first HTTP request, rather than waiting for a HTTP 401 Unauthorized response from the server. (Contributed by Matej Cepl in issue 19494.)





  • Added support for writing ZIP files to unseekable streams. (Contributed by Serhiy Storchaka in issue 23252.)
  • The zipfile.ZipFile.open() function now supports 'x' (exclusive creation) mode. (Contributed by Serhiy Storchaka in issue 21717.)


The following performance enhancements have been added:

Build and C API Changes

Changes to Python’s build process and to the C API include:


Unsupported Operating Systems

  • None yet.

Deprecated Python modules, functions and methods

  • The formatter module has now graduated to full deprecation and is still slated for removal in Python 3.6.
  • smtpd has in the past always decoded the DATA portion of email messages using the utf-8 codec. This can now be controlled by the new decode_data keyword to SMTPServer. The default value is True, but this default is deprecated. Specify the decode_data keyword with an appropriate value to avoid the deprecation warning.
  • Directly assigning values to the key, value and coded_value of Morsel objects is deprecated. Use the set() method instead. In addition, the undocumented LegalChars parameter of set() is deprecated, and is now ignored.
  • Passing a format string as keyword argument format_string to the format() method of the string.Formatter class has been deprecated.

Deprecated functions and types of the C API

  • None yet.

Deprecated features

  • None yet.


API and Feature Removals

The following obsolete and previously deprecated APIs and features have been removed:

  • The __version__ attribute has been dropped from the email package. The email code hasn’t been shipped separately from the stdlib for a long time, and the __version__ string was not updated in the last few releases.
  • The internal Netrc class in the ftplib module was deprecated in 3.4, and has now been removed. (Contributed by Matt Chaput in issue 6623.)

Porting to Python 3.5

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

Changes in the Python API

  • PEP 475: the following functions are now retried when interrupted instead of raising InterruptedError if the signal handler does not raise an exception:
  • Before Python 3.5, a datetime.time object was considered to be false if it represented midnight in UTC. This behavior was considered obscure and error-prone and has been removed in Python 3.5. See issue 13936 for full details.
  • ssl.SSLSocket.send() now raises either ssl.SSLWantReadError or ssl.SSLWantWriteError on a non-blocking socket if the operation would block. Previously, it would return 0. See issue 20951.
  • The __name__ attribute of generator is now set from the function name, instead of being set from the code name. Use gen.gi_code.co_name to retrieve the code name. Generators also have a new __qualname__ attribute, the qualified name, which is now used for the representation of a generator (repr(gen)). See issue 21205.
  • The deprecated “strict” mode and argument of HTMLParser, HTMLParser.error(), and the HTMLParserError exception have been removed. (Contributed by Ezio Melotti in issue 15114.) The convert_charrefs argument of HTMLParser is now True by default. (Contributed by Berker Peksag in issue 21047.)
  • Although it is not formally part of the API, it is worth noting for porting purposes (ie: fixing tests) that error messages that were previously of the form “‘sometype’ does not support the buffer protocol” are now of the form “a bytes-like object is required, not ‘sometype’”. (Contributed by Ezio Melotti in issue 16518.)
  • If the current directory is set to a directory that no longer exists then FileNotFoundError will no longer be raised and instead find_spec() will return None without caching None in sys.path_importer_cache which is different than the typical case (issue 22834).
  • HTTP status code and messages from http.client and http.server were refactored into a common HTTPStatus enum. The values in http.client and http.server remain available for backwards compatibility. (Contributed by Demian Brecht in issue 21793.)
  • When an import loader defines exec_module() it is now expected to also define create_module() (raises a DeprecationWarning now, will be an error in Python 3.6). If the loader inherits from importlib.abc.Loader then there is nothing to do, else simply define create_module() to return None (issue 23014).
  • re.split() always ignored empty pattern matches, so the 'x*' pattern worked the same as 'x+', and the '\b' pattern never worked. Now re.split() raises a warning if the pattern could match an empty string. For compatibility use patterns that never match an empty string (e.g. 'x+' instead of 'x*'). Patterns that could only match an empty string (such as '\b') now raise an error.
  • The Morsel dict-like interface has been made self consistent: morsel comparison now takes the key and value into account, copy() now results in a Morsel instance rather than a dict, and update() will no raise an exception if any of the keys in the update dictionary are invalid. In addition, the undocumented LegalChars parameter of set() is deprecated and is now ignored. (issue 2211)

Changes in the C API

  • The undocumented format member of the (non-public) PyMemoryViewObject structure has been removed.

    All extensions relying on the relevant parts in memoryobject.h must be rebuilt.

  • The PyMemAllocator structure was renamed to PyMemAllocatorEx and a new calloc field was added.

  • Removed non-documented macro PyObject_REPR which leaked references. Use format character %R in PyUnicode_FromFormat()-like functions to format the repr() of the object.

  • Because the lack of the __module__ attribute breaks pickling and introspection, a deprecation warning now is raised for builtin type without the __module__ attribute. Would be an AttributeError in future. (issue 20204)