Python 3000 (PyCon, 24-Feb-02007) Guido van Rossum

24 Feb 02007(c) 2007 Python Software Foundation2 What Is Python 3000? The next major Python release –To be released as Python 3.0 The first one in a long time to be incompatible –But not completely different or unusual Concept first formed around 2000 –Py3k nickname was a play on Windows 2000 Goal: to correct my early design mistakes –Those that would require incompatibility to fix –Reduce cognitive load for first-time learners Work and thinking started for real last year

24 Feb 02007(c) 2007 Python Software Foundation3 Activity Since Last Year Lots of design discussions –(too many, if you ask me :-) Some PEPs were written –(but not enough…) Lots of code was written –(just the right amount!) –(but we're not done yet!!)

24 Feb 02007(c) 2007 Python Software Foundation4 Python 3.0 Timeline PEPs to be completed: April a1: June final: June 2008 For comparison, the 2.6 timeline: 2.6a1: December final: April 2008 There will also be a 2.7 timeline

24 Feb 02007(c) 2007 Python Software Foundation5 Rest of the Talk Highlight some of the most visible changes –print function, dict views, comparisons, unicode, … How to convert 2.x to 3.0 code Notational convention: *= incompletely implemented **= not yet implemented

24 Feb 02007(c) 2007 Python Software Foundation6 No More Classic Classes In 2.2 … 2.9: –class C:# classic class (0.1 … 2.1) –class C(object):# new-style class (old now :-) In 3.0: –both are new-style classes (just say "classes") Differences are subtle, few of you will notice

24 Feb 02007(c) 2007 Python Software Foundation7 Print is a Function print x, y-> print(x, y) print x,-> print(x, end=" ") print >>f, x-> print(x, file=f) Automatic translation is 98% correct Fails for cases involving softspace cleverness: –print "x\n", "y" doesn 't insert a space before y –print("x\n", "y") does –ditto for print "x\t", "y"

24 Feb 02007(c) 2007 Python Software Foundation8 Dictionary Views Inspired by Java Collections Framework Remove.iterkeys(),.iteritems(),.itervalues() Change.keys(),.items(),.values() These return a dict view –Not an iterator –A lightweight object that can be iterated repeatedly –.keys(),.items() have set semantics –.values() has "collection" semantics supports iteration and not much else

24 Feb 02007(c) 2007 Python Software Foundation9 Default Comparison Changed Default ==, != compare object identity –(this is unchanged) Default, >= raise TypeError Example: [1, 2, ""].sort() raises TypeError Rationale: 2.x default ordering is bogus –depends on type names –depends on addresses

24 Feb 02007(c) 2007 Python Software Foundation10 **Unicode Strings Java-like model: –strings (the str type) are always Unicode –separate bytes type –must explicitly specify encoding to go between these Open issues: –implementation fixed-width characters for O(1) indexing maybe 3 internal widths: 1, 2, 4 byte characters C API issues (many C APIs use C char* pointers) –optimize slicing and concatenation??? lots of issues, supporters, detractors

24 Feb 02007(c) 2007 Python Software Foundation11 The Bytes Type A mutable sequence of small ints (0…255) –b[0] is an int; b[:1] is a new bytes object Implemented efficiently as unsigned char[] Has some list-like methods, e.g..extend() Has some string-like methods, e.g..find() –But none that depend on locale bytes literals: b"ascii or \xDD or \012" bytes has.decode() method returning a string str has a.encode() method returning bytes

24 Feb 02007(c) 2007 Python Software Foundation12 **New I/O Library Stackable components (inspired by Java, Perl) –Lowest level: unbuffered byte I/O platform-specific; don't use C stdio –Add buffering –Add unicode encoding/decoding encoding explicitly specified or somehow guessed –Add CRLF/LF mapping Compatible API –open(filename) returns a buffered text file read() and readline() return strings –open(filename, "b") returns a buffered binary file read() returns bytes; can't use readline()

24 Feb 02007(c) 2007 Python Software Foundation13 Int/Long Unification There is only one built-in integer type Its name is int Its implementation is like long in Python 2.x C API is a bit murky Performance could use a boost

24 Feb 02007(c) 2007 Python Software Foundation14 Int Division Returns a Float Always! Same effect in 2.x with –from __future__ import division Use // for int division Use -Q option to Python 2.x to find old usage

24 Feb 02007(c) 2007 Python Software Foundation15 **Raise and Except Changes All exceptions must derive from BaseException Exceptions have __traceback__ attribute Must use raise E(arg) instead of raise E, arg Can still use raise E and raise without args Use raise E(arg).with_traceback(tb) –instead of raise E, arg, tb Use "except E as v:" instead of "except E, v:" Variable v is deleted at end of except block!!!

24 Feb 02007(c) 2007 Python Software Foundation16 Signature Annotations NOT type declarations! Example: –def foo(x: "whatever", y: list(range(3))) -> 42*2: … Argument syntax is (roughly): –NAME [':' expr] ['=' expr] Both expressions are evaluated at 'def' time –foo.func_annotations is: {'a': "whatever", 'b': [0, 1, 2], "return": 84} –NO other use is made of these annotations

24 Feb 02007(c) 2007 Python Software Foundation17 Keyword-Only Parameters Example def: –def foo(a, b=1, *, c=42, d): … Example call: –foo(1, 2, d=3) Cannot use: –foo(1, 2, 3) # raises TypeError

24 Feb 02007(c) 2007 Python Software Foundation18 Set Literals {1, 2, 3} is the same as set([1, 2, 3]) No empty set literal; use set() No frozenset literal; use frozenset({…}) **Set comprehensions: –{f(x) for x in S if P(x)} same as set(f(x) for x in S if P(x))

24 Feb 02007(c) 2007 Python Software Foundation19 Absolute Import Same effect in 2.5 with –from __future__ import absolute_import Within a package "import foo" does NOT search the package path, only sys.path Use "from. import foo" for relative import Or use from import foo

24 Feb 02007(c) 2007 Python Software Foundation20 **String Formatting Examples (see PEP 3101 for more): –"See {0}, {1} and {foo}".format("A", "B", foo="C") "See A, B and C" –"my name is {0} :-{{}}".format("Fred") "my name is Fred :-{}" –"File name {0.foo}".format(open("foo.txt")) File name foo.txt –"Name is {0[name]}".format({"name": "Fred"}) "Name is Fred" –Shoe size {0:8}".format(42) "Shoe size 42"

24 Feb 02007(c) 2007 Python Software Foundation21 **Nonlocal Statement def outer(): x = 42 def inner(): nonlocal x# <---- new print(x) x += 1 return inner Doesn't work today; x becomes a local in inner Different keywords proposed: –nonlocal, global, outer, … (see PEP 3104)

24 Feb 02007(c) 2007 Python Software Foundation22 **Abstract Base Classes? Still highly speculative (no PEP yet) –wiki.python.org/moin/AbstractBaseClasses Introduce a standard abstract class hierarchy for type categories like file, container, sequence, iterable etc. Standard types to use these as base classes User-defined types may use these When used, can help distinguishing e.g. sequence from mapping, or file-like behavior, or "stringiness", or "numericity", etc.

24 Feb 02007(c) 2007 Python Software Foundation23 **Switch/Case Statement??? Highly speculative; see PEP 3103 –switch EXPR: case EXPR: SUITE case EXPR:# or case in EXPRLIST: SUITE … [else: SUITE] Problem: when to compile EXPR? –Would prefer precompilation for faster execution –But this would introduce unusual semantics

24 Feb 02007(c) 2007 Python Software Foundation24 Miscellaneous Changes exec becomes a function again range() becomes xrange() input() becomes raw_input() zip() returns an iterator Moved intern() into sys module Renamed __nonzero__ to __bool__ 'as' and 'with' are keywords And more, planned and implemented

24 Feb 02007(c) 2007 Python Software Foundation25 Miscellaneous Removals classic classes: new-style classes default backticks: use repr() Removed <>: use != apply(): use func(*args) coerce(), __coerce__: not needed dict.has_key(): use key in dict 'softspace' attribute on file objects

24 Feb 02007(c) 2007 Python Software Foundation26 **Library Reform Not my priority Others are interested, but effort seems stalled Need help! May happen after 3.0a1 is released

24 Feb 02007(c) 2007 Python Software Foundation27 *C API Changes Too early to tell what will happen 3rd party extension authors want to know For now, these simple rules: –Adding APIs is okay (of course) –Deleting APIs is okay –Changing APIs incompatibly is NOT OKAY

24 Feb 02007(c) 2007 Python Software Foundation28 Converting 2.x Code to 3.0 Generic conversion tool exists –sandbox/2to3 –accurate source-to-source transformation –parse tree decorated with whitespace & comments New conversions are easily added –create a class from boilerplate –add a class variable PATTERN to match nodes –add a method transform() to transform one node Separately, Python 2.6 will help –can warn about out-of-date usages –can provide forward-compatible alternatives

24 Feb 02007(c) 2007 Python Software Foundation29 Examples of What It Can Do apply(fun, args, kwds) -> fun(*args, **kwds) d.iterkeys() -> d.keys() exec a in b, c -> exec(a, b, c) print >>sys.stderr, x, -> print(x, end=" ", file=sys.stderr) except E, v: -> except E as v: d.has_key(k) -> k in d intern(s) -> sys.intern(s) a <> b -> a != b; `x` -> repr(x); int -> long automatically adds parentheses where needed

24 Feb 02007(c) 2007 Python Software Foundation30 Examples of What It Can't Do detect whether d is a dict (in d.iterkeys()) detect whether you use d.keys() as a list later turn int()/int() into int()//int() fix code that depends on int() < str() remove redundant code fix custom classes emulating dictionaries fix string exceptions, non-Exception exceptions in general: limited to syntactic conversions –can't follow control flow, doesn't do type inference

24 Feb 02007(c) 2007 Python Software Foundation31 What You Can Do Today Don't worry about stuff that can be automated Don't try to write source-level compatible code –Use Python 2.6 when it comes out –Write unit tests with maximal coverage –Use keys = sorted(d.iterkeys()) –Use list(d.iterkeys()) when you really need a list –Derive all exceptions from Exception –Derive all classes from object –Don't rely on subtle print/softspace semantics use print line.rstrip("\n") instead of print line, –Use // for int division

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