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Python 3.0 where we break all your code Abhinav Sarkar
[email protected]
[email protected]
BARCAMP BANGALORE 7
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introduction
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xkcd#365
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python 3.0 aka Python 3000 aka Py3k aka p3yk
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where we break all your code
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backwards incompatible
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more precisely:
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almost every program will need changes
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almost every program will need changes
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xkcd#349
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justifications
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Python is 16 years old
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b/w compatibility is important
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python: pre 1.0 lots of changes
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1.0 -> 2.0 2.0 -> 2.6
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no b/w incompatible changes
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none important enough to give presentations on
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annoying features
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py3k is the chance to fix
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mistakes duplications ugly things
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mistakes coerce() backticks
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duplication TIOOWTDI not true
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map list comprehensions
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string functions string methods
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apply(foo, args, kwargs) foo(*args, **kwargs)
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email package rfc822 modules
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how did this happen?
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ugly
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`backticks` repr()
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reduce() sum() replaces 95% of uses
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print >> somefile, “stuff”
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smaller language == better language
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“python fits in your brain”
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easier to learn easier to use
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3.0
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language changes
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<> -> !=
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`backticks` -> repr()
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removed: print statement
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!!
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print() now a function
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print a, b, c -> print(a, b, c)
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print >> fp, “stuff” -> print(“stuff”, file=fp)
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new print feature: customize separators
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print “,”.join((a,b,c,d)) -> print(a,b,c,d, sep=“,”)
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new string formatting
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str.format() “name: {0} {1}”.format(first, last) fmt = “name: {first} {last}” fmt.format(first=“abhinav”, last=“sarkar”)
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format() builtin >>> format(3.0, “06.1f”) '0003.0'
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new keywords: as True False None nonlocal
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basic types
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strings
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unicode string default
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but sometimes you want bytes network protocols binary files
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bytes() bytes([0x0A, 0x0B, 0x42]) bytes(string, encoding)
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new ascii() builtin and “%a” string format
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non-ascii identifiers
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numbers
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no more long() ints are long by default no more 123L
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integer division >>> 1/2 0.5
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old style >>> 1//2 0
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dicts
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dictob.has_key(key) -> key in dictob
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removed: dict.iter*
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instead: dictionary views
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for k in dict: for k,v in dict.items():
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sets
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set literals {1,2,3}
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{} == dict() empty set: set()
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set comprehensions
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iterables
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next() -> __next__()
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new feature unpacking: first, *rest, last = list
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head, *rest = somelist *ignore, tail = somelist
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map(), filter(), zip() -> iterators
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exceptions
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no more string exceptions exceptions must derive from BaseException
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2-arg raise raise MyExcp, val -> raise MyExcp(val)
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3-arg raise raise MyExcp, val, tback -> raise MyExcp(val).with_traceback(tback)
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except excp, e -> except excp as e
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classes
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new classes only as old-style classes
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class A(object) -> class A()
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class decorators @decorator class MyClass: pass
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more more powerful metaclasses Abstract Base Classes (ABC) abstract methods @abstractproperty
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functions
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keyword only args can't be positional
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buggy example: def fun(arg1, arg2, flag=False): pass fun(1,2,3) # flag = 3 ??
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the change: keyword args after *args
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def fun(arg1, arg2, *, flag=False): pass >>> fun(1,2,3) Traceback (most recent call last): File “”, line 1, in TypeError: fun() takes exactly 2 positional arguments (3 given)
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annotations
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attaching metadata to arguments
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syntax: def func(arg: expression)->returnValue: pass
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expressions can be anything
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documentation def doEvil(plan: “a plan”) -> “evil results”: .... types def foo(a: float, b: int, c:list) -> dict: .... more complex def processFiles(*files: “one or more of filenames”, delete: “delete when done” = False) -> “a boolean”: ....
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Python ignore annotations
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stored in __annotations__
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3rd party tools: optimizers, docs, IDEs, typechecking, ...
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files
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text vs binary text files must have encoding produce unicode binary files produce bytestrings
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new I/O layer raw I/O buffered I/O text I/O
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modules
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absolute imports
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import foo always imports from the top level
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import from same package: from . import foo
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import from parent package: from .. import foo
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stdlib
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goals: PEP8 compliance some structure cleaning out cruft
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PEP 0008 “Modules should have short, all-lowercase names.”
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BaseHTTPServer cPickle cStringIO HTMLParser ....
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either renamed, or Flushed.
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structure
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current structure
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duplication and near duplications bsddb gdbm dbm dumbdbm
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urllib vs urllib2
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new packages: http html xmlrpc json dbm urllib
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lots of renames: cStringIO -> io.StringIO SocketServer -> socketserver httplib -> http.client urllib + urllib2 -> urllib
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cruft
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lots of purges: old email modules old hash modules old platform modules
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thread gone use threading
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UserDict and friends -> subclass from dict
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porting approach
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take 2.5 code get working on 2.6 turn -3 flag while True: run through 2to3 run unit tests under 3.0 fix 2.x code
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2.x -> 2.6 -> 3.0
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python 2.6 interim release -3 flag: turns on warnings from __future__ from future_builtins import enables some backports
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2to3 ships with 2.6, 3.0 does mechanical rewrites handles a lot never going to be perfect
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unit tests
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eventually ... drop 2.x version switch to 3.0 version
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things not in py3k
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antigravity xkcd#353
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case insensitivity death to lambda implicit self macros
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not a complete rewrite
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what happens to python 2.x?
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No content
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2.x is not going away will continue to be supported
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please try 3.0 betas
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(and report bugs and send fixes)
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References PEPs http://www.python.org/dev/peps/ http://www.python.org/dev/peps/pep-3100/ Anthony Baxter's OSCON'08 Talk http://www.interlink.com.au/anthony/tech/talks/OSCON2008/
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exit()