Python 3000 Py Con 24 Feb02007 Guido van

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 – Py 3 k 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 Foundation 2

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 Foundation 3

Python 3. 0 Timeline • PEPs to be completed: April 2007� • 3. 0 a 1: June 2007 • 3. 0 final: June 2008 � For comparison, the 2. 6 timeline: • 2. 6 a 1: December 2007 • 2. 6 final: April 2008 There will also be a 2. 7 timeline 24 Feb 02007 (c) 2007 Python Software Foundation 4

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 Foundation 5

No More Classic Classes • In 2. 2 … 2. 9: – class C(object): # classic class (0. 1 … 2. 1) # 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 Foundation 6

Print is a Function • print x, y • print x, • print >>f, x -> print(x, y) -> print(x, end=" ") -> print(x, file=f) • Automatic translation is 98% correct • Fails for cases involving softspace cleverness: – print "xn", "y" doesn 't insert a space before y – print("xn", "y") does – ditto for print "xt", "y" 24 Feb 02007 (c) 2007 Python Software Foundation 7

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 Foundation 8

Default Comparison Changed • Default ==, != compare object identity – (this is unchanged) • Default <, <=, >, >= raise Type. Error • Example: [1, 2, ""]. sort() raises Type. Error • Rationale: 2. x default ordering is bogus – depends on type names – depends on addresses 24 Feb 02007 (c) 2007 Python Software Foundation 9

**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 Foundation 10

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 x. DD or 12" • bytes has. decode() method returning a string • str has a. encode() method returning bytes 24 Feb 02007 (c) 2007 Python Software Foundation 11

**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 Foundation 12

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 Foundation 13

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 Foundation 14

**Raise and Except Changes • All exceptions must derive from Base. Exception • 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 Foundation 15

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 Foundation 16

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 Type. Error 24 Feb 02007 (c) 2007 Python Software Foundation 17

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 Foundation 18

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 <full-package-name> import foo 24 Feb 02007 (c) 2007 Python Software Foundation 19

**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 24 Feb 02007 42" (c) 2007 Python Software Foundation 20

**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 Foundation 21

**Abstract Base Classes? • Still highly speculative (no PEP yet) – wiki. python. org/moin/Abstract. Base. Classes • 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 Foundation 22

**Switch/Case Statement? ? ? • Highly speculative; see PEP 3103 – switch EXPR: case EXPR: SUITE … [else: SUITE] # or case in EXPRLIST: • Problem: when to compile EXPR? – Would prefer precompilation for faster execution – But this would introduce unusual semantics 24 Feb 02007 (c) 2007 Python Software Foundation 23

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 Foundation 24

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 Foundation 25

**Library Reform • • Not my priority Others are interested, but effort seems stalled

*C API Changes • Too early to tell what will happen • 3 rd 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 Foundation 27

Converting 2. x Code to 3. 0 • Generic conversion tool exists – sandbox/2 to 3 – 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 Foundation 28

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 Foundation 29

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 Foundation 30

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 24 Feb 02007 (c) 2007 Python Software Foundation 31