Print 是一個函式¶

print 陳述式已經被print() 函式所取代，且舊print 陳述式的大部分特殊語法也被關鍵字引數所取代 (PEP 3105)。範例如下：

Old:print"The answer is",2*2New:print("The answer is",2*2)Old:printx,# Trailing comma suppresses newlineNew:print(x,end=" ")# Appends a space instead of a newlineOld:print# Prints a newlineNew: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:

• Theprint() function doesn't support the "softspace" feature ofthe oldprint 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\nB\n".

• Initially, you'll be finding yourself typing the oldprintxa lot in interactive mode. Time to retrain your fingers to typeprint(x) instead!

• When using the2to3 source-to-source conversion tool, allprint statements are automatically converted toprint() function calls, so this is mostly a non-issue forlarger projects.

Views And Iterators Instead Of Lists¶

一些眾所周知的 API 不再回傳串列：

• dict methodsdict.keys(),dict.items() anddict.values() return "views" instead of lists. For example,this no longer works:k=d.keys();k.sort(). Usek=sorted(d) instead (this works in Python 2.5 too and is justas efficient).

• Also, thedict.iterkeys(),dict.iteritems() anddict.itervalues() methods are no longer supported.

• map() andfilter() return iterators. If you really needa list and the input sequences are all of equal length, a quickfix is to wrapmap() inlist(), e.g.list(map(...)),but a better fix isoften to use a list comprehension (especially when the original codeuseslambda), or rewriting the code so it doesn't need alist at all. Particularly tricky ismap() invoked for theside effects of the function; the correct transformation is to use aregularfor loop (since creating a list would just bewasteful).

  If the input sequences are not of equal length,map() willstop at the termination of the shortest of the sequences. For fullcompatibility withmap() from Python 2.x, also wrap the sequences initertools.zip_longest(), e.g.map(func,*sequences) becomeslist(map(func,itertools.zip_longest(*sequences))).

• range() now behaves likexrange() used to behave, exceptit works with values of arbitrary size. The latter no longerexists.

• zip() 現在會回傳一個疊代器。

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 ameaningful natural ordering. Thus, expressions like1<'',0>None orlen<=len are no longer valid, and e.g.None<None raisesTypeError instead of returningFalse. A corollary is that sorting a heterogeneous listno longer makes sense -- all the elements must be comparable to eachother. Note that this does not apply to the== and!=operators: objects of different incomparable types always compareunequal to each other.

• sorted() andlist.sort() no longer accept thecmp argument providing a comparison function. Use thekeyargument instead. N.B. thekey andreverse arguments are now"keyword-only".

• Thecmp() 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 thecmp() functionality, you could use theexpression(a>b)-(a<b) as the equivalent forcmp(a,b).)

整數¶

• PEP 237: Essentially,long renamed toint.That is, there is only one built-in integral type, namedint; but it behaves mostly like the oldlong type.

• PEP 238: An expression like1/2 returns a float. Use1//2 to get the truncating behavior. (The latter syntax hasexisted for years, at least since Python 2.2.)

• Thesys.maxint constant was removed, since there is nolonger a limit to the value of integers. However,sys.maxsizecan be used as an integer larger than any practical list or stringindex. It conforms to the implementation's "natural" integer sizeand is typically the same assys.maxint in previous releaseson the same platform (assuming the same build options).

• Therepr() of a long integer doesn't include the trailingLanymore, so code that unconditionally strips that character willchop off the last digit instead. (Usestr() instead.)

• Octal literals are no longer of the form0720; use0o720instead.

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

Everything you thought you knew about binary data and Unicode haschanged.

• Python 3.0 uses the concepts oftext and (binary)data insteadof Unicode strings and 8-bit strings. All text is Unicode; howeverencoded Unicode is represented as binary data. The type used tohold text isstr, the type used to hold data isbytes. The biggest difference with the 2.x situation isthat any attempt to mix text and data in Python 3.0 raisesTypeError, whereas if you were to mix Unicode and 8-bitstrings in Python 2.x, it would work if the 8-bit string happened tocontain only 7-bit (ASCII) bytes, but you would getUnicodeDecodeError if it contained non-ASCII values. Thisvalue-specific behavior has caused numerous sad faces over theyears.

• As a consequence of this change in philosophy, pretty much all codethat uses Unicode, encodings or binary data most likely has tochange. The change is for the better, as in the 2.x world therewere numerous bugs having to do with mixing encoded and unencodedtext. To be prepared in Python 2.x, start usingunicodefor all unencoded text, andstr for binary or encoded dataonly. Then the2to3 tool will do most of the work for you.

• You can no longer useu"..." literals for Unicode text.However, you must useb"..." literals for binary data.

• As thestr andbytes types cannot be mixed, youmust always explicitly convert between them. Usestr.encode()to go fromstr tobytes, andbytes.decode()to go frombytes tostr. You can also usebytes(s,encoding=...) andstr(b,encoding=...),respectively.

• Likestr, thebytes type is immutable. There is aseparatemutable type to hold buffered binary data,bytearray. Nearly all APIs that acceptbytes alsoacceptbytearray. The mutable API is based oncollections.MutableSequence.

• All backslashes in raw string literals are interpreted literally.This means that'\U' and'\u' escapes in raw strings are nottreated specially. For example,r'\u20ac' is a string of 6characters in Python 3.0, whereas in 2.6,ur'\u20ac' was thesingle "euro" character. (Of course, this change only affects rawstring literals; the euro character is'\u20ac' in Python 3.0.)

• The built-inbasestring abstract type was removed. Usestr instead. Thestr andbytes typesdon't have functionality enough in common to warrant a shared baseclass. The2to3 tool (see below) replaces every occurrence ofbasestring withstr.

• Files opened as text files (still the default mode foropen())always use an encoding to map between strings (in memory) and bytes(on disk). Binary files (opened with ab in the mode argument)always use bytes in memory. This means that if a file is openedusing an incorrect mode or encoding, I/O will likely fail loudly,instead of silently producing incorrect data. It also means thateven Unix users will have to specify the correct mode (text orbinary) when opening a file. There is a platform-dependent defaultencoding, which on Unixy platforms can be set with theLANGenvironment variable (and sometimes also with some otherplatform-specific locale-related environment variables). In manycases, but not all, the system default is UTF-8; you should nevercount on this default. Any application reading or writing more thanpure ASCII text should probably have a way to override the encoding.There is no longer any need for using the encoding-aware streamsin thecodecs module.

• The initial values ofsys.stdin,sys.stdout andsys.stderr are now unicode-only text files (i.e., they areinstances ofio.TextIOBase). To read and write bytes datawith these streams, you need to use theirio.TextIOBase.bufferattribute.

• Filenames are passed to and returned from APIs as (Unicode) strings.This can present platform-specific problems because on someplatforms filenames are arbitrary byte strings. (On the other hand,on Windows filenames are natively stored as Unicode.) As awork-around, most APIs (e.g.open() and many functions in theos module) that take filenames acceptbytes objectsas well as strings, and a few APIs have a way to ask for abytes return value. Thus,os.listdir() returns alist ofbytes instances if the argument is abytesinstance, andos.getcwdb() returns the current workingdirectory as abytes instance. Note that whenos.listdir() returns a list of strings, filenames thatcannot be decoded properly are omitted rather than raisingUnicodeError.

• Some system APIs likeos.environ andsys.argv canalso present problems when the bytes made available by the system isnot interpretable using the default encoding. Setting theLANGvariable and rerunning the program is probably the best approach.

• PEP 3138: Therepr() of a string no longer escapesnon-ASCII characters. It still escapes control characters and codepoints 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 exceptionof contributor names in comments.)

• TheStringIO andcStringIO modules are gone. Instead,import theio module and useio.StringIO orio.BytesIO for text and data respectively.

• 也請見為了 Python 3.0 所更新的Unicode HOWTO。

新增語法¶

• PEP 3107: Function argument and return value annotations. Thisprovides a standardized way of annotating a function's parametersand return value. There are no semantics attached to suchannotations except that they can be introspected at runtime usingthe__annotations__ attribute. The intent is toencourage experimentation through metaclasses, decorators or frameworks.

• PEP 3102: Keyword-only arguments. Named parameters occurringafter*args in the parameter listmust be specified usingkeyword syntax in the call. You can also use a bare* in theparameter list to indicate that you don't accept a variable-lengthargument list, but you do have keyword-only arguments.

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

• PEP 3104:nonlocal statement. Usingnonlocalxyou can now assign directly to a variable in an outer (butnon-global) scope.nonlocal is a new reserved word.

• PEP 3132: Extended Iterable Unpacking. You can now write thingslikea,b,*rest=some_sequence. And even*rest,a=stuff. Therest object is always a (possibly empty) list; theright-hand side may be any iterable. Example:

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

  這會將a 設為0、將b 設為4，並將rest 設為[1,2,3]。

• Dictionary comprehensions:{k:vfork,vinstuff} means thesame thing asdict(stuff) but is more flexible. (This isPEP 274 vindicated. :-)

• Set literals, e.g.{1,2}. Note that{} is an emptydictionary; useset() for an empty set. Set comprehensions arealso supported; e.g.,{xforxinstuff} means the same thing asset(stuff) but is more flexible.

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

• New binary literals, e.g.0b1010 (already in 2.6), andthere is a new corresponding built-in function,bin().

• Bytes literals are introduced with a leadingb orB, andthere is a new corresponding built-in function,bytes().

語法變更¶

• PEP 3109 andPEP 3134: newraise statement syntax:raise[expr[fromexpr]]. See below.

• as andwith are now reserved words. (Since2.6, actually.)

• True,False, andNone are reserved words. (2.6 partially enforcedthe restrictions onNone already.)

• Change fromexceptexc,var toexceptexcasvar. SeePEP 3110.

• PEP 3115: New Metaclass Syntax. Instead of:

  classC:__metaclass__=M...

  現在必須使用：

  classC(metaclass=M):...

  The module-global__metaclass__ variable is no longersupported. (It was a crutch to make it easier to default tonew-style classes without deriving every class fromobject.)

• List comprehensions no longer support the syntactic form[...forvarinitem1,item2,...]. Use[...forvarin(item1,item2,...)] instead.Also note that list comprehensions have different semantics: theyare closer to syntactic sugar for a generator expression inside alist() constructor, and in particular the loop controlvariables are no longer leaked into the surrounding scope.

• Theellipsis (...) can be used as an atomic expressionanywhere. (Previously it was only allowed in slices.) Also, itmust now be spelled as.... (Previously it could also bespelled as..., by a mere accident of the grammar.)

已被移除的語法¶

• PEP 3113: Tuple parameter unpacking removed. You can no longerwritedeffoo(a,(b,c)):....Usedeffoo(a,b_c):b,c=b_c instead.

• 移除反引號（請改用repr()）。

• 移除<>（請改用!=）。

• Removed keyword:exec() is no longer a keyword; it remains asa function. (Fortunately the function syntax was also accepted in2.x.) Also note thatexec() no longer takes a stream argument;instead ofexec(f) you can useexec(f.read()).

• Integer literals no longer support a trailingl orL.

• String literals no longer support a leadingu orU.

• Thefrommoduleimport* syntax is onlyallowed at the module level, no longer inside functions.

• The only acceptable syntax for relative imports isfrom.[module]importname. Allimport forms not starting with. areinterpreted as absolute imports. (PEP 328)

• Classic classes are gone.

Operators And Special Methods¶

• != now returns the opposite of==, unless== returnsNotImplemented.

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

• __getslice__(),__setslice__() and__delslice__()were killed. The syntaxa[i:j] now translates toa.__getitem__(slice(i,j)) (or__setitem__() or__delitem__(), when used as an assignment or deletion target,respectively).

• PEP 3114: the standardnext() method has been renamed to__next__().

• The__oct__() and__hex__() special methods are removed--oct() andhex() use__index__() now to convertthe argument to an integer.

• 移除對__members__ 和__methods__ 的支援。

• The function attributes namedfunc_X have been renamed touse the__X__ form, freeing up these names in the functionattribute 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__() 現在為__bool__()。

Builtins¶

• PEP 3135: Newsuper(). You can now invokesuper()without arguments and (assuming this is in a regular instance methoddefined inside aclass statement) the right class andinstance will automatically be chosen. With arguments, the behaviorofsuper() is unchanged.

• PEP 3111:raw_input() was renamed toinput(). Thatis, the newinput() function reads a line fromsys.stdin and returns it with the trailing newline stripped.It raisesEOFError if the input is terminated prematurely.To get the old behavior ofinput(), useeval(input()).

• A new built-in functionnext() was added to call the__next__() method on an object.

• Theround() function rounding strategy and return type havechanged. Exact halfway cases are now rounded to the nearest evenresult instead of away from zero. (For example,round(2.5) nowreturns2 rather than3.)round(x[,n]) nowdelegates tox.__round__([n]) instead of always returning afloat. It generally returns an integer when called with a singleargument and a value of the same type asx when called with twoarguments.

• Movedintern() tosys.intern().

• 移除apply()。請使用f(*args) 來替換apply(f,args)。

• Removedcallable(). Instead ofcallable(f) you can useisinstance(f,collections.Callable). Theoperator.isCallable()function is also gone.

• Removedcoerce(). This function no longer serves a purposenow that classic classes are gone.

• 移除execfile()。請使用exec(open(fn).read()) 來替換execfile(fn)。

• Removed thefile type. Useopen(). There are now severaldifferent kinds of streams that open can return in theio module.

• Removedreduce(). Usefunctools.reduce() if you reallyneed it; however, 99 percent of the time an explicitforloop is more readable.

• 移除reload()。請使用imp.reload()。

• Removed.dict.has_key() -- use thein operatorinstead.