What’s New In Python 3.5

Date:April 21, 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:

  • PEP 465, a new matrix multiplication operator: a @ b.

New library modules:

New built-in features:

  • bytes % args, bytearray % args: PEP 461 - Adding % formatting to bytes and bytearray

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).
  • PEP 488, the elimination of .pyo files.

Significantly Improved Library Modules:

  • None yet.

Security improvements:

  • None yet.

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

PEP 461 - Adding % formatting to bytes and bytearray

This PEP proposes adding % formatting operations similar to Python 2’s str type to bytes and bytearray.


>>> b'Hello %s!' % b'World'
b'Hello World!'
>>> b'x=%i y=%f' % (1, 2.5)
b'x=1 y=2.500000'

Unicode is not allowed for %s, but it is accepted by %a (equivalent of repr(obj).encode('ascii', 'backslashreplace')):

>>> b'Hello %s!' % 'World'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: %b requires bytes, or an object that implements __bytes__, not 'str'
>>> b'price: %a' % '10€'
b"price: '10\\u20ac'"

See also

PEP 461 – Adding % formatting to bytes and bytearray

PEP 465 - A dedicated infix operator for matrix multiplication

This PEP proposes a new binary operator to be used for matrix multiplication, called @. (Mnemonic: @ is * for mATrices.)

See also

PEP 465 – A dedicated infix operator for matrix multiplication

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

PEP 488: Elimination of PYO files

PEP 488 does away with the concept of .pyo files. This means that .pyc files represent both unoptimized and optimized bytecode. To prevent the need to constantly regenerate bytecode files, .pyc files now have an optional opt- tag in their name when the bytecode is optimized. This has the side-effect of no more bytecode file name clashes when running under either -O or -OO, thus allowing unoptimized, -O, and -OO bytecode files to all exist simultaneously. importlib.util.cache_from_source() has an updated API to help with this change.

See also

PEP 488 – Elimination of PYO files

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.)
  • It’s now possible to compare lists of byte strings with difflib.diff_bytes() (fixes a regression from Python 2).


  • 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.)








  • New commonpath() function that extracts common path prefix. Unlike the commonprefix() function, it always returns a valid patch. (Contributed by Rafik Draoui and Serhiy Storchaka in issue 10395.)


  • Serializing more “lookupable” objects (such as unbound methods or nested classes) now are supported with pickle protocols < 4. (Contributed by Serhiy Storchaka in issue 23611.)


  • 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.)


  • On Windows, signal.set_wakeup_fd() now also supports socket handles. (Contributed by Victor Stinner in issue 22018.)
  • 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.)
  • Additional debuglevel (2) shows timestamps for debug messages in smtplib.SMTP. (Contributed by Gavin Chappell and Maciej Szulik in issue 16914.)





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


  • The tarfile.open() function now supports 'x' (exclusive creation) mode. (Contributed by Berker Peksag in issue 21717.)
  • The extractall() and extract() methods now take a keyword parameter numeric_only. If set to True, the extracted files and directories will be owned by the numeric uid and gid from the tarfile. If set to False (the default, and the behavior in versions prior to 3.5), they will be owned bythe named user and group in the tarfile. (Contributed by Michael Vogt and Eric Smith in issue 23193.)



  • A new HTTPPasswordMgrWithPriorAuth allows HTTP Basic Authentication credentials to be managed so as to eliminate unnecessary 401 response handling, or to unconditionally send credentials on the first request in order to communicate with servers that return a 404 response instead of a 401 if the Authorization header is not sent. (Contributed by Matej Cepl in issue 19494 and Akshit Khurana in issue 7159.)




  • SAX parsers now support a character stream of InputSource object. (Contributed by Serhiy Storchaka in issue 2175.)



  • 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

  • Windows XP - Per PEP 11, Microsoft support of Windows XP has ended.

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.)
  • The concept of .pyo files has been removed.
  • The JoinableQueue class in the provisional asyncio module was deprecated in 3.4.4 and is now removed (issue 23464).

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

  • The socket module now exports the CAN_RAW_FD_FRAMES constant on linux 3.6 and greater.
  • The pygettext.py Tool now uses the standard +NNNN format for timezones in the POT-Creation-Date header.
  • The smtplib module now uses sys.stderr instead of previous module level stderr variable for debug output. If your (test) program depends on patching the module level variable to capture the debug output, you will need to update it to capture sys.stderr instead.

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)