What's New In Python 3.0
************************

Author:
   Guido van Rossum

This article explains the new features in Python 3.0, compared to 2.6.
Python 3.0, also known as "Python 3000" or "Py3K", is the first ever
*intentionally backwards incompatible* Python release. Python 3.0 was
released on December 3, 2008. There are more changes than in a typical
release, and more that are important for all Python users.
Nevertheless, after digesting the changes, you'll find that Python
really hasn't changed all that much -- by and large, we're mostly
fixing well-known annoyances and warts, and removing a lot of old
cruft.

This article doesn't attempt to provide a complete specification of
all new features, but instead tries to give a convenient overview. For
full details, you should refer to the documentation for Python 3.0,
and/or the many PEPs referenced in the text. If you want to understand
the complete implementation and design rationale for a particular
feature, PEPs usually have more details than the regular
documentation; but note that PEPs usually are not kept up-to-date once
a feature has been fully implemented.

Due to time constraints this document is not as complete as it should
have been.  As always for a new release, the "Misc/NEWS" file in the
source distribution contains a wealth of detailed information about
every small thing that was changed.


Common Stumbling Blocks
=======================

This section lists those few changes that are most likely to trip you
up if you're used to Python 2.5.


Print Is A Function
-------------------

The "print" statement has been replaced with a "print()" function,
with keyword arguments to replace most of the special syntax of the
old "print" statement (**PEP 3105**).  Examples:

   Old: print "The answer is", 2*2
   New: print("The answer is", 2*2)

   Old: print x,           # Trailing comma suppresses newline
   New: print(x, end=" ")  # Appends a space instead of a newline

   Old: print              # Prints a newline
   New: print()            # You must call the function!

   Old: print >>sys.stderr, "fatal error"
   New: print("fatal error", file=sys.stderr)

   Old: print (x, y)       # prints repr((x, y))
   New: print((x, y))      # Not the same as print(x, y)!

You can also customize the separator between items, e.g.:

   print("There are <", 2**32, "> possibilities!", sep="")

which produces:

   There are <4294967296> possibilities!

Note:

* The "print()" function doesn't support the "softspace" feature of
  the old "print" statement.  For example, in Python 2.x, "print
  "A\n", "B"" would write ""A\nB\n""; but in Python 3.0, "print("A\n",
  "B")" writes ""A\n B\n"".

* Initially, you'll be finding yourself typing the old "print x" a lot
  in interactive mode.  Time to retrain your fingers to type
  "print(x)" instead!

* When using the "2to3" source-to-source conversion tool, all "print"
  statements are automatically converted to "print()" function calls,
  so this is mostly a non-issue for larger projects.


Views And Iterators Instead Of Lists
------------------------------------

Some well-known APIs no longer return lists:

* "dict" methods "dict.keys()", "dict.items()" and "dict.values()"
  return "views" instead of lists.  For example, this no longer works:
  "k = d.keys(); k.sort()".  Use "k = sorted(d)" instead (this works
  in Python 2.5 too and is just as efficient).

* Also, the "dict.iterkeys()", "dict.iteritems()" and
  "dict.itervalues()" methods are no longer supported.

* "map()" and "filter()" return iterators.  If you really need a list
  and the input sequences are all of equal length, a quick fix is to
  wrap "map()" in "list()", e.g. "list(map(...))", but a better fix is
  often to use a list comprehension (especially when the original code
  uses "lambda"), or rewriting the code so it doesn't need a list at
  all.  Particularly tricky is "map()" invoked for the side effects of
  the function; the correct transformation is to use a regular "for"
  loop (since creating a list would just be wasteful).

  If the input sequences are not of equal length, "map()" will stop at
  the termination of the shortest of the sequences. For full
  compatibility with "map()" from Python 2.x, also wrap the sequences
  in "itertools.zip_longest()", e.g. "map(func, *sequences)" becomes
  "list(map(func, itertools.zip_longest(*sequences)))".

* "range()" now behaves like "xrange()" used to behave, except it
  works with values of arbitrary size.  The latter no longer exists.

* "zip()" now returns an iterator.


Ordering Comparisons
--------------------

Python 3.0 has simplified the rules for ordering comparisons:

* The ordering comparison operators ("<", "<=", ">=", ">") raise a
  TypeError exception when the operands don't have a meaningful
  natural ordering.  Thus, expressions like "1 < ''", "0 > None" or
  "len <= len" are no longer valid, and e.g. "None < None" raises
  "TypeError" instead of returning "False".  A corollary is that
  sorting a heterogeneous list no longer makes sense -- all the
  elements must be comparable to each other.  Note that this does not
  apply to the "==" and "!=" operators: objects of different
  incomparable types always compare unequal to each other.

* "builtin.sorted()" and "list.sort()" no longer accept the *cmp*
  argument providing a comparison function.  Use the *key* argument
  instead. N.B. the *key* and *reverse* arguments are now "keyword-
  only".

* The "cmp()" function should be treated as gone, and the "__cmp__()"
  special method is no longer supported.  Use "__lt__()" for sorting,
  "__eq__()" with "__hash__()", and other rich comparisons as needed.
  (If you really need the "cmp()" functionality, you could use the
  expression "(a > b) - (a < b)" as the equivalent for "cmp(a, b)".)


Integers
--------

* **PEP 237**: Essentially, "long" renamed to "int". That is, there is
  only one built-in integral type, named "int"; but it behaves mostly
  like the old "long" type.

* **PEP 238**: An expression like "1/2" returns a float.  Use "1//2"
  to get the truncating behavior.  (The latter syntax has existed for
  years, at least since Python 2.2.)

* The "sys.maxint" constant was removed, since there is no longer a
  limit to the value of integers.  However, "sys.maxsize" can be used
  as an integer larger than any practical list or string index.  It
  conforms to the implementation's "natural" integer size and is
  typically the same as "sys.maxint" in previous releases on the same
  platform (assuming the same build options).

* The "repr()" of a long integer doesn't include the trailing "L"
  anymore, so code that unconditionally strips that character will
  chop off the last digit instead.  (Use "str()" instead.)

* Octal literals are no longer of the form "0720"; use "0o720"
  instead.


Text Vs. Data Instead Of Unicode Vs. 8-bit
------------------------------------------

Everything you thought you knew about binary data and Unicode has
changed.

* Python 3.0 uses the concepts of *text* and (binary) *data* instead
  of Unicode strings and 8-bit strings.  All text is Unicode; however
  *encoded* Unicode is represented as binary data.  The type used to
  hold text is "str", the type used to hold data is "bytes".  The
  biggest difference with the 2.x situation is that any attempt to mix
  text and data in Python 3.0 raises "TypeError", whereas if you were
  to mix Unicode and 8-bit strings in Python 2.x, it would work if the
  8-bit string happened to contain only 7-bit (ASCII) bytes, but you
  would get "UnicodeDecodeError" if it contained non-ASCII values.
  This value-specific behavior has caused numerous sad faces over the
  years.

* As a consequence of this change in philosophy, pretty much all code
  that uses Unicode, encodings or binary data most likely has to
  change.  The change is for the better, as in the 2.x world there
  were numerous bugs having to do with mixing encoded and unencoded
  text.  To be prepared in Python 2.x, start using "unicode" for all
  unencoded text, and "str" for binary or encoded data only.  Then the
  "2to3" tool will do most of the work for you.

* You can no longer use "u"..."" literals for Unicode text. However,
  you must use "b"..."" literals for binary data.

* As the "str" and "bytes" types cannot be mixed, you must always
  explicitly convert between them.  Use "str.encode()" to go from
  "str" to "bytes", and "bytes.decode()" to go from "bytes" to "str".
  You can also use "bytes(s, encoding=...)" and "str(b,
  encoding=...)", respectively.

* Like "str", the "bytes" type is immutable.  There is a separate
  *mutable* type to hold buffered binary data, "bytearray".  Nearly
  all APIs that accept "bytes" also accept "bytearray".  The mutable
  API is based on "collections.MutableSequence".

* All backslashes in raw string literals are interpreted literally.
  This means that "'\U'" and "'\u'" escapes in raw strings are not
  treated specially.  For example, "r'\u20ac'" is a string of 6
  characters in Python 3.0, whereas in 2.6, "ur'\u20ac'" was the
  single "euro" character.  (Of course, this change only affects raw
  string literals; the euro character is "'\u20ac'" in Python 3.0.)

* The built-in "basestring" abstract type was removed.  Use "str"
  instead.  The "str" and "bytes" types don't have functionality
  enough in common to warrant a shared base class.  The "2to3" tool
  (see below) replaces every occurrence of "basestring" with "str".

* Files opened as text files (still the default mode for "open()")
  always use an encoding to map between strings (in memory) and bytes
  (on disk).  Binary files (opened with a "b" in the mode argument)
  always use bytes in memory.  This means that if a file is opened
  using an incorrect mode or encoding, I/O will likely fail loudly,
  instead of silently producing incorrect data.  It also means that
  even Unix users will have to specify the correct mode (text or
  binary) when opening a file.  There is a platform-dependent default
  encoding, which on Unixy platforms can be set with the "LANG"
  environment variable (and sometimes also with some other platform-
  specific locale-related environment variables).  In many cases, but
  not all, the system default is UTF-8; you should never count on this
  default.  Any application reading or writing more than pure ASCII
  text should probably have a way to override the encoding. There is
  no longer any need for using the encoding-aware streams in the
  "codecs" module.

* The initial values of "sys.stdin", "sys.stdout" and "sys.stderr" are
  now unicode-only text files (i.e., they are instances of
  "io.TextIOBase").  To read and write bytes data with these streams,
  you need to use their "io.TextIOBase.buffer" attribute.

* Filenames are passed to and returned from APIs as (Unicode) strings.
  This can present platform-specific problems because on some
  platforms filenames are arbitrary byte strings.  (On the other hand,
  on Windows filenames are natively stored as Unicode.)  As a work-
  around, most APIs (e.g. "open()" and many functions in the "os"
  module) that take filenames accept "bytes" objects as well as
  strings, and a few APIs have a way to ask for a "bytes" return
  value.  Thus, "os.listdir()" returns a list of "bytes" instances if
  the argument is a "bytes" instance, and "os.getcwdb()" returns the
  current working directory as a "bytes" instance.  Note that when
  "os.listdir()" returns a list of strings, filenames that cannot be
  decoded properly are omitted rather than raising "UnicodeError".

* Some system APIs like "os.environ" and "sys.argv" can also present
  problems when the bytes made available by the system is not
  interpretable using the default encoding.  Setting the "LANG"
  variable and rerunning the program is probably the best approach.

* **PEP 3138**: The "repr()" of a string no longer escapes non-ASCII
  characters.  It still escapes control characters and code points
  with non-printable status in the Unicode standard, however.

* **PEP 3120**: The default source encoding is now UTF-8.

* **PEP 3131**: Non-ASCII letters are now allowed in identifiers.
  (However, the standard library remains ASCII-only with the exception
  of contributor names in comments.)

* The "StringIO" and "cStringIO" modules are gone.  Instead, import
  the "io" module and use "io.StringIO" or "io.BytesIO" for text and
  data respectively.

* See also the Unicode HOWTO, which was updated for Python 3.0.


Overview Of Syntax Changes
==========================

This section gives a brief overview of every *syntactic* change in
Python 3.0.


New Syntax
----------

* **PEP 3107**: Function argument and return value annotations.  This
  provides a standardized way of annotating a function's parameters
  and return value.  There are no semantics attached to such
  annotations except that they can be introspected at runtime using
  the "__annotations__" attribute.  The intent is to encourage
  experimentation through metaclasses, decorators or frameworks.

* **PEP 3102**: Keyword-only arguments.  Named parameters occurring
  after "*args" in the parameter list *must* be specified using
  keyword syntax in the call.  You can also use a bare "*" in the
  parameter list to indicate that you don't accept a variable-length
  argument list, but you do have keyword-only arguments.

* Keyword arguments are allowed after the list of base classes in a
  class definition.  This is used by the new convention for specifying
  a metaclass (see next section), but can be used for other purposes
  as well, as long as the metaclass supports it.

* **PEP 3104**: "nonlocal" statement.  Using "nonlocal x" you can now
  assign directly to a variable in an outer (but non-global) scope.
  "nonlocal" is a new reserved word.

* **PEP 3132**: Extended Iterable Unpacking.  You can now write things
  like "a, b, *rest = some_sequence".  And even "*rest, a = stuff".
  The "rest" object is always a (possibly empty) list; the right-hand
  side may be any iterable.  Example:

     (a, *rest, b) = range(5)

  This sets *a* to "0", *b* to "4", and *rest* to "[1, 2, 3]".

* Dictionary comprehensions: "{k: v for k, v in stuff}" means the same
  thing as "dict(stuff)" but is more flexible.  (This is **PEP 274**
  vindicated. :-)

* Set literals, e.g. "{1, 2}".  Note that "{}" is an empty dictionary;
  use "set()" for an empty set.  Set comprehensions are also
  supported; e.g., "{x for x in stuff}" means the same thing as
  "set(stuff)" but is more flexible.

* New octal literals, e.g. "0o720" (already in 2.6).  The old octal
  literals ("0720") are gone.

* New binary literals, e.g. "0b1010" (already in 2.6), and there is a
  new corresponding built-in function, "bin()".

* Bytes literals are introduced with a leading "b" or "B", and there
  is a new corresponding built-in function, "bytes()".


Changed Syntax
--------------

* **PEP 3109** and **PEP 3134**: new "raise" statement syntax: "raise
  [*expr* [from *expr*]]".  See below.

* "as" and "with" are now reserved words.  (Since 2.6, actually.)

* "True", "False", and "None" are reserved words.  (2.6 partially
  enforced the restrictions on "None" already.)

* Change from "except" *exc*, *var* to "except" *exc* "as" *var*.  See
  **PEP 3110**.

* **PEP 3115**: New Metaclass Syntax.  Instead of:

     class C:
         __metaclass__ = M
         ...

  you must now use:

     class C(metaclass=M):
         ...

  The module-global "__metaclass__" variable is no longer supported.
  (It was a crutch to make it easier to default to new-style classes
  without deriving every class from "object".)

* List comprehensions no longer support the syntactic form "[... for
  *var* in *item1*, *item2*, ...]".  Use "[... for *var* in (*item1*,
  *item2*, ...)]" instead. Also note that list comprehensions have
  different semantics: they are closer to syntactic sugar for a
  generator expression inside a "list()" constructor, and in
  particular the loop control variables are no longer leaked into the
  surrounding scope.

* The *ellipsis* ("...") can be used as an atomic expression anywhere.
  (Previously it was only allowed in slices.)  Also, it *must* now be
  spelled as "...".  (Previously it could also be spelled as ". . .",
  by a mere accident of the grammar.)


Removed Syntax
--------------

* **PEP 3113**: Tuple parameter unpacking removed.  You can no longer
  write "def foo(a, (b, c)): ...". Use "def foo(a, b_c): b, c = b_c"
  instead.

* Removed backticks (use "repr()" instead).

* Removed "<>" (use "!=" instead).

* Removed keyword: "exec()" is no longer a keyword; it remains as a
  function.  (Fortunately the function syntax was also accepted in
  2.x.)  Also note that "exec()" no longer takes a stream argument;
  instead of "exec(f)" you can use "exec(f.read())".

* Integer literals no longer support a trailing "l" or "L".

* String literals no longer support a leading "u" or "U".

* The "from" *module* "import" "*" syntax is only allowed at the
  module level, no longer inside functions.

* The only acceptable syntax for relative imports is "from .[*module*]
  import *name*".  All "import" forms not starting with "." are
  interpreted as absolute imports.  (**PEP 328**)

* Classic classes are gone.


Changes Already Present In Python 2.6
=====================================

Since many users presumably make the jump straight from Python 2.5 to
Python 3.0, this section reminds the reader of new features that were
originally designed for Python 3.0 but that were back-ported to Python
2.6.  The corresponding sections in What's New in Python 2.6 should be
consulted for longer descriptions.

* PEP 343: The 'with' statement.  The "with" statement is now a
  standard feature and no longer needs to be imported from the
  "__future__". Also check out Writing Context Managers and The
  contextlib module.

* PEP 366: Explicit Relative Imports From a Main Module.  This
  enhances the usefulness of the "-m" option when the referenced
  module lives in a package.

* PEP 370: Per-user site-packages Directory.

* PEP 371: The multiprocessing Package.

* PEP 3101: Advanced String Formatting.  Note: the 2.6 description
  mentions the "format()" method for both 8-bit and Unicode strings.
  In 3.0, only the "str" type (text strings with Unicode support)
  supports this method; the "bytes" type does not.  The plan is to
  eventually make this the only API for string formatting, and to
  start deprecating the "%" operator in Python 3.1.

* PEP 3105: print As a Function.  This is now a standard feature and
  no longer needs to be imported from "__future__".  More details were
  given above.

* PEP 3110: Exception-Handling Changes.  The "except" *exc* "as" *var*
  syntax is now standard and "except" *exc*, *var* is no longer
  supported.  (Of course, the "as" *var* part is still optional.)

* PEP 3112: Byte Literals.  The "b"..."" string literal notation (and
  its variants like "b'...'", "b"""..."""", and "br"..."") now
  produces a literal of type "bytes".

* PEP 3116: New I/O Library.  The "io" module is now the standard way
  of doing file I/O.  The built-in "open()" function is now an alias
  for "io.open()" and has additional keyword arguments *encoding*,
  *errors*, *newline* and *closefd*.  Also note that an invalid *mode*
  argument now raises "ValueError", not "IOError".  The binary file
  object underlying a text file object can be accessed as "f.buffer"
  (but beware that the text object maintains a buffer of itself in
  order to speed up the encoding and decoding operations).

* PEP 3118: Revised Buffer Protocol.  The old builtin "buffer()" is
  now really gone; the new builtin "memoryview()" provides (mostly)
  similar functionality.

* PEP 3119: Abstract Base Classes.  The "abc" module and the ABCs
  defined in the "collections" module plays a somewhat more prominent
  role in the language now, and built-in collection types like "dict"
  and "list" conform to the "collections.MutableMapping" and
  "collections.MutableSequence" ABCs, respectively.

* PEP 3127: Integer Literal Support and Syntax.  As mentioned above,
  the new octal literal notation is the only one supported, and binary
  literals have been added.

* PEP 3129: Class Decorators.

* PEP 3141: A Type Hierarchy for Numbers.  The "numbers" module is
  another new use of ABCs, defining Python's "numeric tower".  Also
  note the new "fractions" module which implements "numbers.Rational".


Library Changes
===============

Due to time constraints, this document does not exhaustively cover the
very extensive changes to the standard library.  **PEP 3108** is the
reference for the major changes to the library.  Here's a capsule
review:

* Many old modules were removed.  Some, like "gopherlib" (no longer
  used) and "md5" (replaced by "hashlib"), were already deprecated by
  **PEP 4**.  Others were removed as a result of the removal of
  support for various platforms such as Irix, BeOS and Mac OS 9 (see
  **PEP 11**).  Some modules were also selected for removal in Python
  3.0 due to lack of use or because a better replacement exists.  See
  **PEP 3108** for an exhaustive list.

* The "bsddb3" package was removed because its presence in the core
  standard library has proved over time to be a particular burden for
  the core developers due to testing instability and Berkeley DB's
  release schedule.  However, the package is alive and well,
  externally maintained at
  https://www.jcea.es/programacion/pybsddb.htm.

* Some modules were renamed because their old name disobeyed **PEP
  8**, or for various other reasons.  Here's the list:

  +-------------------------+-------------------------+
  | Old Name                | New Name                |
  |=========================|=========================|
  | _winreg                 | winreg                  |
  +-------------------------+-------------------------+
  | ConfigParser            | configparser            |
  +-------------------------+-------------------------+
  | copy_reg                | copyreg                 |
  +-------------------------+-------------------------+
  | Queue                   | queue                   |
  +-------------------------+-------------------------+
  | SocketServer            | socketserver            |
  +-------------------------+-------------------------+
  | markupbase              | _markupbase             |
  +-------------------------+-------------------------+
  | repr                    | reprlib                 |
  +-------------------------+-------------------------+
  | test.test_support       | test.support            |
  +-------------------------+-------------------------+

* A common pattern in Python 2.x is to have one version of a module
  implemented in pure Python, with an optional accelerated version
  implemented as a C extension; for example, "pickle" and "cPickle".
  This places the burden of importing the accelerated version and
  falling back on the pure Python version on each user of these
  modules.  In Python 3.0, the accelerated versions are considered
  implementation details of the pure Python versions. Users should
  always import the standard version, which attempts to import the
  accelerated version and falls back to the pure Python version.  The
  "pickle" / "cPickle" pair received this treatment.  The "profile"
  module is on the list for 3.1.  The "StringIO" module has been
  turned into a class in the "io" module.

* Some related modules have been grouped into packages, and usually
  the submodule names have been simplified.  The resulting new
  packages are:

  * "dbm" ("anydbm", "dbhash", "dbm", "dumbdbm", "gdbm", "whichdb").

  * "html" ("HTMLParser", "htmlentitydefs").

  * "http" ("httplib", "BaseHTTPServer", "CGIHTTPServer",
    "SimpleHTTPServer", "Cookie", "cookielib").

  * "tkinter" (all "Tkinter"-related modules except "turtle").  The
    target audience of "turtle" doesn't really care about "tkinter".
    Also note that as of Python 2.6, the functionality of "turtle" has
    been greatly enhanced.

  * "urllib" ("urllib", "urllib2", "urlparse", "robotparse").

  * "xmlrpc" ("xmlrpclib", "DocXMLRPCServer", "SimpleXMLRPCServer").

Some other changes to standard library modules, not covered by **PEP
3108**:

* Killed "sets".  Use the built-in "set()" class.

* Cleanup of the "sys" module: removed "sys.exitfunc()",
  "sys.exc_clear()", "sys.exc_type", "sys.exc_value",
  "sys.exc_traceback".  (Note that "sys.last_type" etc. remain.)

* Cleanup of the "array.array" type: the "read()" and "write()"
  methods are gone; use "fromfile()" and "tofile()" instead.  Also,
  the "'c'" typecode for array is gone -- use either "'b'" for bytes
  or "'u'" for Unicode characters.

* Cleanup of the "operator" module: removed "sequenceIncludes()" and
  "isCallable()".

* Cleanup of the "thread" module: "acquire_lock()" and
  "release_lock()" are gone; use "acquire()" and "release()" instead.

* Cleanup of the "random" module: removed the "jumpahead()" API.

* The "new" module is gone.

* The functions "os.tmpnam()", "os.tempnam()" and "os.tmpfile()" have
  been removed in favor of the "tempfile" module.

* The "tokenize" module has been changed to work with bytes.  The main
  entry point is now "tokenize.tokenize()", instead of
  generate_tokens.

* "string.letters" and its friends ("string.lowercase" and
  "string.uppercase") are gone.  Use "string.ascii_letters" etc.
  instead.  (The reason for the removal is that "string.letters" and
  friends had locale-specific behavior, which is a bad idea for such
  attractively named global "constants".)

* Renamed module "__builtin__" to "builtins" (removing the
  underscores, adding an 's').  The "__builtins__" variable found in
  most global namespaces is unchanged.  To modify a builtin, you
  should use "builtins", not "__builtins__"!


**PEP 3101**: A New Approach To String Formatting
=================================================

* A new system for  built-in string formatting operations replaces the
  "%" string  formatting operator.   (However, the "%"  operator is
  still supported;  it will  be deprecated in  Python 3.1  and removed
  from the language at some later time.)  Read **PEP 3101** for the
  full scoop.


Changes To Exceptions
=====================

The APIs for raising and catching exception have been cleaned up and
new powerful features added:

* **PEP 352**: All exceptions must be derived (directly or indirectly)
  from "BaseException".  This is the root of the exception hierarchy.
  This is not new as a recommendation, but the *requirement* to
  inherit from "BaseException" is new.  (Python 2.6 still allowed
  classic classes to be raised, and placed no restriction on what you
  can catch.)  As a consequence, string exceptions are finally truly
  and utterly dead.

* Almost all exceptions should actually derive from "Exception";
  "BaseException" should only be used as a base class for exceptions
  that should only be handled at the top level, such as "SystemExit"
  or "KeyboardInterrupt".  The recommended idiom for handling all
  exceptions except for this latter category is to use "except"
  "Exception".

* "StandardError" was removed.

* Exceptions no longer behave as sequences.  Use the "args" attribute
  instead.

* **PEP 3109**: Raising exceptions.  You must now use "raise
  *Exception*(*args*)" instead of "raise *Exception*, *args*".
  Additionally, you can no longer explicitly specify a traceback;
  instead, if you *have* to do this, you can assign directly to the
  "__traceback__" attribute (see below).

* **PEP 3110**: Catching exceptions.  You must now use "except
  *SomeException* as *variable*" instead of "except *SomeException*,
  *variable*".  Moreover, the *variable* is explicitly deleted when
  the "except" block is left.

* **PEP 3134**: Exception chaining.  There are two cases: implicit
  chaining and explicit chaining.  Implicit chaining happens when an
  exception is raised in an "except" or "finally" handler block.  This
  usually happens due to a bug in the handler block; we call this a
  *secondary* exception.  In this case, the original exception (that
  was being handled) is saved as the "__context__" attribute of the
  secondary exception. Explicit chaining is invoked with this syntax:

     raise SecondaryException() from primary_exception

  (where *primary_exception* is any expression that produces an
  exception object, probably an exception that was previously caught).
  In this case, the primary exception is stored on the "__cause__"
  attribute of the secondary exception.  The traceback printed when an
  unhandled exception occurs walks the chain of "__cause__" and
  "__context__" attributes and prints a separate traceback for each
  component of the chain, with the primary exception at the top.
  (Java users may recognize this behavior.)

* **PEP 3134**: Exception objects now store their traceback as the
  "__traceback__" attribute.  This means that an exception object now
  contains all the information pertaining to an exception, and there
  are fewer reasons to use "sys.exc_info()" (though the latter is not
  removed).

* A few exception messages are improved when Windows fails to load an
  extension module.  For example, "error code 193" is now "%1 is not a
  valid Win32 application".  Strings now deal with non-English
  locales.


Miscellaneous Other Changes
===========================


Operators And Special Methods
-----------------------------

* "!=" now returns the opposite of "==", unless "==" returns
  "NotImplemented".

* The concept of "unbound methods" has been removed from the language.
  When referencing a method as a class attribute, you now get a plain
  function object.

* "__getslice__()", "__setslice__()" and "__delslice__()" were killed.
  The syntax "a[i:j]" now translates to "a.__getitem__(slice(i, j))"
  (or "__setitem__()" or "__delitem__()", when used as an assignment
  or deletion target, respectively).

* **PEP 3114**: the standard "next()" method has been renamed to
  "__next__()".

* The "__oct__()" and "__hex__()" special methods are removed --
  "oct()" and "hex()" use "__index__()" now to convert the argument to
  an integer.

* Removed support for "__members__" and "__methods__".

* The function attributes named "func_X" have been renamed to use the
  "__X__" form, freeing up these names in the function attribute
  namespace for user-defined attributes.  To wit, "func_closure",
  "func_code", "func_defaults", "func_dict", "func_doc",
  "func_globals", "func_name" were renamed to "__closure__",
  "__code__", "__defaults__", "__dict__", "__doc__", "__globals__",
  "__name__", respectively.

* "__nonzero__()" is now "__bool__()".


Builtins
--------

* **PEP 3135**: New "super()".  You can now invoke "super()" without
  arguments and (assuming this is in a regular instance method defined
  inside a "class" statement) the right class and instance will
  automatically be chosen.  With arguments, the behavior of "super()"
  is unchanged.

* **PEP 3111**: "raw_input()" was renamed to "input()".  That is, the
  new "input()" function reads a line from "sys.stdin" and returns it
  with the trailing newline stripped. It raises "EOFError" if the
  input is terminated prematurely. To get the old behavior of
  "input()", use "eval(input())".

* A new built-in function "next()" was added to call the "__next__()"
  method on an object.

* The "round()" function rounding strategy and return type have
  changed.  Exact halfway cases are now rounded to the nearest even
  result instead of away from zero.  (For example, "round(2.5)" now
  returns "2" rather than "3".)  "round(x[, n])" now delegates to
  "x.__round__([n])" instead of always returning a float.  It
  generally returns an integer when called with a single argument and
  a value of the same type as "x" when called with two arguments.

* Moved "intern()" to "sys.intern()".

* Removed: "apply()".  Instead of "apply(f, args)" use "f(*args)".

* Removed "callable()".  Instead of "callable(f)" you can use
  "isinstance(f, collections.Callable)".  The "operator.isCallable()"
  function is also gone.

* Removed "coerce()".  This function no longer serves a purpose now
  that classic classes are gone.

* Removed "execfile()".  Instead of "execfile(fn)" use
  "exec(open(fn).read())".

* Removed the "file" type.  Use "open()".  There are now several
  different kinds of streams that open can return in the "io" module.

* Removed "reduce()".  Use "functools.reduce()" if you really need it;
  however, 99 percent of the time an explicit "for" loop is more
  readable.

* Removed "reload()".  Use "imp.reload()".

* Removed. "dict.has_key()" -- use the "in" operator instead.


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

Due to time constraints, here is a *very* incomplete list of changes
to the C API.

* Support for several platforms was dropped, including but not limited
  to Mac OS 9, BeOS, RISCOS, Irix, and Tru64.

* **PEP 3118**: New Buffer API.

* **PEP 3121**: Extension Module Initialization & Finalization.

* **PEP 3123**: Making "PyObject_HEAD" conform to standard C.

* No more C API support for restricted execution.

* "PyNumber_Coerce()", "PyNumber_CoerceEx()", "PyMember_Get()", and
  "PyMember_Set()" C APIs are removed.

* New C API "PyImport_ImportModuleNoBlock()", works like
  "PyImport_ImportModule()" but won't block on the import lock
  (returning an error instead).

* Renamed the boolean conversion C-level slot and method: "nb_nonzero"
  is now "nb_bool".

* Removed "METH_OLDARGS" and "WITH_CYCLE_GC" from the C API.


Performance
===========

The net result of the 3.0 generalizations is that Python 3.0 runs the
pystone benchmark around 10% slower than Python 2.5.  Most likely the
biggest cause is the removal of special-casing for small integers.
There's room for improvement, but it will happen after 3.0 is
released!


Porting To Python 3.0
=====================

For porting existing Python 2.5 or 2.6 source code to Python 3.0, the
best strategy is the following:

* (Prerequisite:) Start with excellent test coverage.

* Port to Python 2.6.  This should be no more work than the average
  port from Python 2.x to Python 2.(x+1).  Make sure all your tests
  pass.

* (Still using 2.6:) Turn on the "-3" command line switch. This
  enables warnings about features that will be removed (or change) in
  3.0.  Run your test suite again, and fix code that you get warnings
  about until there are no warnings left, and all your tests still
  pass.

* Run the "2to3" source-to-source translator over your source code
  tree.  (See 2to3 --- Automated Python 2 to 3 code translation for
  more on this tool.)  Run the result of the translation under Python
  3.0.  Manually fix up any remaining issues, fixing problems until
  all tests pass again.

It is not recommended to try to write source code that runs unchanged
under both Python 2.6 and 3.0; you'd have to use a very contorted
coding style, e.g. avoiding "print" statements, metaclasses, and much
more.  If you are maintaining a library that needs to support both
Python 2.6 and Python 3.0, the best approach is to modify step 3 above
by editing the 2.6 version of the source code and running the "2to3"
translator again, rather than editing the 3.0 version of the source
code.

For porting C extensions to Python 3.0, please see Μεταφορά Extension
Modules στην Python 3.
