
What's New In Python 3.5
************************

Release:
   3.5.0a3

Date:
   March 30, 2015

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

For full details, see the Misc/NEWS file.

Note: 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:

* "zipapp": *Improving Python ZIP Application Support* (**PEP
  441**).

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
===========


zipapp
------

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
================


argparse
--------

* "ArgumentParser" now allows to disable *abbreviated usage* of long
  options by setting *allow_abbrev* to "False". (Contributed by
  Jonathan Paugh, Steven Bethard, paul j3 and Daniel Eriksson.)


cgi
---

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


code
----

* The "code.InteractiveInterpreter.showtraceback()" method now
  prints the full chained traceback, just like the interactive
  interpreter. (Contributed by Claudiu Popa in issue 17442.)


compileall
----------

* "compileall.compile_dir()" and "compileall"'s command-line
  interface can now do parallel bytecode compilation. (Contributed by
  Claudiu Popa in issue 16104.)


contextlib
----------

* The new "contextlib.redirect_stderr()" context manager(similar to
  "contextlib.redirect_stdout()") makes it easier for utility scripts
  to handle inflexible APIs that write their output to "sys.stderr"
  and don't provide any options to redirect it. (Contributed by Berker
  Peksag in issue 22389.)


difflib
-------

* 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.)


distutils
---------

* 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.)


doctest
-------

* "doctest.DocTestSuite()" returns an empty "unittest.TestSuite" if
  *module* contains no docstrings instead of raising "ValueError".
  (Contributed by Glenn Jones in issue 15916.)


glob
----

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


imaplib
-------

* "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.)


imghdr
------

* "what()" now recognizes the OpenEXR format.  (Contributed by
  Martin Vignali and Claudiu Popa in issue 20295.)


importlib
---------

* "importlib.util.LazyLoader" allows for the lazy loading of modules
  in applications where startup time is paramount. (Contributed by
  Brett Cannon in issue 17621.)

* "importlib.abc.InspectLoader.source_to_code()" is now a static
  method to make it easier to work with source code in a string. With
  a module object that you want to initialize you can then use
  "exec(code, module.__dict__)" to execute the code in the module.

* "importlib.util.module_from_spec()" is now the preferred way to
  create a new module. Compared to "types.ModuleType", this new
  function will set the various import-controlled attributes based on
  the passed-in spec object.


inspect
-------

* "inspect.Signature" and "inspect.Parameter" are now picklable and
  hashable.  (Contributed by Yury Selivanov in issue 20726 and issue
  20334.)

* New class method "inspect.Signature.from_callable()", which makes
  subclassing of "Signature" easier.  (Contributed by Yury Selivanov
  and Eric Snow in issue 17373.)


ipaddress
---------

* "ipaddress.IPv4Network" and "ipaddress.IPv6Network" now accept an
  "(address, netmask)" tuple argument, so as to easily construct
  network objects from existing addresses.  (Contributed by Peter
  Moody and Antoine Pitrou in issue 16531.)


json
----

* The output of "json.tool" command line interface is now in the
  same order as the input. Use the *--sort-keys* option to sort the
  output of dictionaries alphabetically by key.  (Contributed by
  Berker Peksag in issue 21650.)

* JSON decoder now raises "json.JSONDecodeError" instead of
  "ValueError".   (Contributed by Serhiy Storchaka in issue 19361.)


os
--

* New "os.scandir()" function that exposes file information from the
  operating system when listing a directory. "os.scandir()" returns an
  iterator of "os.DirEntry" objects corresponding to the entries in
  the directory given by *path*. (Contributed by Ben Hoyt with the
  help of Victor Stinner in issue 22524.)

* "os.stat_result" now has a "st_file_attributes" attribute on
  Windows.  (Contributed by Ben Hoyt in issue 21719.)


re
--

* 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.)


math
----

* "math.inf" and "math.nan" constants added.  (Contributed by Mark
  Dickinson in issue 23185.)


shutil
------

* "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.)


signal
------

* 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.)


smtpd
-----

* 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.


smtplib
-------

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


sndhdr
------

* "what()" and "whathdr()" now return "namedtuple()". (Contributed
  by Claudiu Popa in issue 18615.)


socket
------

* New "socket.socket.sendfile()" method allows to send a file over a
  socket by using high-performance "os.sendfile()" function on UNIX
  resulting in uploads being from 2x to 3x faster than when using
  plain "socket.socket.send()". (Contributed by Giampaolo Rodola' in
  issue 17552.)


sysconfig
---------

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


tarfile
-------

* The "tarfile.open()" function now supports "'x'" (exclusive
  creation) mode.  (Contributed by Berker Peksag in issue 21717.)


time
----

* The "time.monotonic()" function is now always available.
  (Contributed by Victor Stinner in issue 22043.)


urllib
------

* 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.)


wsgiref
-------

* *headers* parameter of "wsgiref.headers.Headers" is now optional.
  (Contributed by Pablo Torres Navarrete and SilentGhost in issue
  5800.)


xmlrpc
------

* "xmlrpc.client.ServerProxy" is now a *context manager*.
  (Contributed by Claudiu Popa in issue 20627.)


faulthandler
------------

* "enable()", "register()", "dump_traceback()" and
  "dump_traceback_later()" functions now accept file descriptors.
  (Contributed by Wei Wu in issue 23566.)


zipfile
-------

* 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.)


Optimizations
=============

The following performance enhancements have been added:

* "os.walk()" has been sped up by 3-5x on POSIX systems and 7-20x on
  Windows. This was done using the new "os.scandir()" function, which
  exposes file information from the underlying "readdir" and
  "FindFirstFile"/"FindNextFile" system calls. (Contributed by Ben
  Hoyt with help from Victor Stinner in issue 23605.)

* Construction of "bytes(int)" (filled by zero bytes) is faster and
  use less memory for large objects. "calloc()" is used instead of
  "malloc()" to allocate memory for these objects.

* Some operations on "IPv4Network" and "IPv6Network" have been
  massively sped up, such as "subnets()", "supernet()",
  "summarize_address_range()", "collapse_addresses()". The speed up
  can range from 3x to 15x. (issue 21486, issue 21487, issue 20826)

* Many operations on "io.BytesIO" are now 50% to 100% faster.
  (Contributed by Serhiy Storchaka in issue 15381 and David Wilson in
  issue 22003.)

* "marshal.dumps()" is now faster (65%-85% with versions 3--4,
  20-25% with versions 0--2 on typical data, and up to 5x in best
  cases). (Contributed by Serhiy Storchaka in issue 20416 and issue
  23344.)


Build and C API Changes
=======================

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

* New "calloc" functions:

  * "PyMem_RawCalloc()"

  * "PyMem_Calloc()"

  * "PyObject_Calloc()"

  * "_PyObject_GC_Calloc()"


Deprecated
==========


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.


Removed
=======


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:

  * "os.open()", "open()"

  * "os.read()", "os.write()"

  * "time.sleep()"

* 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)
