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1,500 | obj pickle load(open('shopfile pkl''rb')obj serverobj chef (obj order('lsp'lsp orders from bob makes pizza oven bakes lsp pays for item to this just runs simulation as isbut we might extend the shop to keep track of inventoryrevenueand so on--saving it to its file after changes would retain its updated state see the st... |
1,501 | (see and for more on the __dict__ attributeyou can use the approach of this module' wrapper class to manage access to any object with attributes--listsdictionariesand even classes and instances herethe wrapper class simply prints trace message on each attribute access and delegates the attribute request to the embedded... |
1,502 | we'll return to this issue in the next as new-style class changeand see it live in and in the context of managed attributes and decorators for nowkeep in mind that for delegation coding patternsyou may need to redefine operator overloading methods in wrapper classes (either by handby toolsor by superclassesif they are ... |
1,503 | and contexts of useyou may find this feature to be more useful in your own code than some programmers realize name mangling overview here' how name mangling workswithin class statement onlyany names that start with two underscores but don' end with two underscores are automatically expanded to include the name of the e... |
1,504 | class def metha(self)self def methb(self)print(self xme too both of these classes work by themselves the problem arises if the two classes are ever mixed together in the same class treeclass ( ) (only in inowthe value that each class gets back when it says self will depend on which class assigned it last because all as... |
1,505 | pseudoprivate attributes are also useful in larger frameworks or toolsboth to avoid introducing new method names that might accidentally hide definitions elsewhere in the class tree and to reduce the chance of internal methods being replaced by names defined lower in the tree if method is intended for use only within c... |
1,506 | methods in generaland bound methods in particularsimplify the implementation of many design goals in python we met bound methods briefly while studying __call__ in the full storywhich we'll flesh out hereturns out to be more general and flexible than you might expect in we learned how functions can be processed as norm... |
1,507 | object doit('hello world'reallythougha bound method object is generated along the wayjust before the method call' parentheses in factwe can fetch bound method without actually calling it an object name expression evaluates to an object as all expressions do in the followingit returns bound method object that packages t... |
1,508 | in python xthe language has dropped the notion of unbound methods what we describe as an unbound method here is treated as simple function in for most purposesthis makes no difference to your codeeither wayan instance will be passed to method' first argument when it' called through an instance programs that do explicit... |
1,509 | the next is not needed in for methods without self argument that are called only through the class nameand never through an instance--such methods are run as simple functionswithout receiving an instance argument in xsuch calls are errors unless an instance is passed manually or the method is marked as being static (mo... |
1,510 | like simple functionsbound method objects have introspection information of their ownincluding attributes that give access to the instance object and method function they pair calling the bound method simply dispatches the pairbound double bound __self__bound __func__ (bound __self__ base bound(calls bound __func__(bou... |
1,511 | better coded as simple function than class with constructorbut the class here serves to illustrate its callable natureclass negatedef __init__(selfval)self val -val def __repr__(self)return str(self valclasses are callables too but called for objectnot work instance print format actions [squaresobjectpobject methodnega... |
1,512 | class myguidef handler(self)use self attr for state def makewidgets(self) button(text='spam'command=self handlerherethe event handler is self handler-- bound method object that remembers both self and mygui handler because self will refer to the original instance when handler is later invoked on eventsthe method will h... |
1,513 | print(messageclass persondef __init__(selfnamejob=none)self name name self job job object factory(spammake spam object object factory(person"arthur""king"make person object object factory(personname='brian'dittowith keywords and default in this codewe define an object generator function called factory it expects to be ... |
1,514 | interface objects in our scriptsbut might instead create them at runtime according to the contents of configuration file such file might simply give the string name of stream class to be imported from moduleplus an optional constructor call argument factory-style functions or code might come in handy here because they ... |
1,515 | methods the second of these search rules is explained fully in the new-style class discussion in the next though difficult to understand without the next code (and somewhat rare to create yourself)diamond patterns appear when multiple classes in tree share common superclassthe new-style search order is designed to visi... |
1,516 | for exampleas we've seenpython' default way to print class instance object isn' incredibly usefulclass spamdef __init__(self)self data "foodx spam(print(xno __repr__ or __str__ defaultclass name address (idsame in xbut says "instanceas you saw in both ' case study and ' operator overloading coverageyou can provide __st... |
1,517 | let' get started with the simple case--listing attributes attached to an instance the following classcoded in the file listinstance pydefines mix-in called listinstance that overloads the __str__ method for all classes that include it in their header lines because this is coded as classlistinstance is generic tool whos... |
1,518 | names and values of all instance attributes the dictionary' keys are sorted to finesse any ordering differences across python releases in these respectslistinstance is similar to ' attribute displayin factit' largely just variation on theme our class here uses two additional techniquesthoughit displays the instance' me... |
1,519 | have one or more superclasses this is where multiple inheritance comes in handyby adding listinstance to the list of superclasses in class header ( mixing it in)you get its __str__ "for freewhile still inheriting from the existing superclass(esthe file testmixin py demonstrates with first-cut testing scriptfile testmix... |
1,520 | strings herein keeping with ' factories pattern ""import importlib def tester(listerclasssept=false)class superdef __init__(self)self data 'spamdef ham(self)pass superclass __init__ create instance attrs class sub(superlisterclass)def __init__(self)super __init__(selfself data 'eggsself data def spam(self)pass mix in h... |
1,521 | and tests of two other lister classes coming up the listinstance class we've coded so far works in any class it' mixed into because self refers to an instance of the subclass that pulls this class inwhatever that may be againin sensemix-in classes are the class equivalent of modules--packages of methods useful in varie... |
1,522 | __repr__or else this loops when printing bound methods""def __attrnames(self)result 'for attr in dir(self)instance dir(if attr[: ='__and attr[- :='__'skip internals result +'\ % \nattr elseresult +'\ % =% \ (attrgetattr(selfattr)return result def __str__(self)return 'self __class__ __name__id(self)self __attrnames()my ... |
1,523 | <instance of subaddress _listinherited__attrnames=__class__ __delattr__ __dict__ __dir__ __doc__ __eq__ more names omitted total __repr__ __setattr__ __sizeof__ __str__ __subclasshook__ __weakref__ data =spam data =eggs data = hamsub more >spamsub more >as one possible improvement to address the proliferation of inheri... |
1,524 | <instance of subaddress unders__class____delattr____dict____dir____doc____eq____format____ge____getattribute____gt____hash____init____le____lt____module____ne____new____qualname____reduce____reduce_ex____repr____setattr____sizeof____str____subclasshook____weakref__ others_listinherited__attrnames=<bound method sub __at... |
1,525 | constructed stringuses __x attr names to avoid impacting clientsrecurses to superclasses explicitlyuses str format(for clarity""def __attrnames(selfobjindent)spaces (indent result 'for attr in sorted(obj __dict__)if attr startswith('__'and attr endswith('__')result +spaces '{ }\nformat(attrelseresult +spaces '{ }={ }\n... |
1,526 | simplerand which we'll omit here for space and timethis class is coded to keep its business as explicit as possiblethoughto maximize clarity for exampleyou could replace the __listclass method' loop statement in the first of the following with the implicitly run generator expression in the secondbut the second seems un... |
1,527 | nowto testrun this class' module file as beforeit passes the listtree class to testmixin py to be mixed in with subclass in the test function the file' tree-sketcher output in python is as followsc:\codec:\python \python listtree py <instance of subaddress _listtree__visited={data =spam data =eggs data = <class subaddr... |
1,528 | <instance of subaddress _listtree__visited={data =spam data =eggs data = <class subaddress __doc__ __init__ __module__ __qualname__ spamsub spam at <class superaddress __dict__ __doc__ __init__ __module__ __qualname__ __weakref__ hamsuper ham at <class objectaddress __class__ __delattr__ __dir__ __doc__ __eq__ more omi... |
1,529 | dictionary works to avoid repeats in the output because class objects are hashable and thus may be dictionary keysa set would provide similar functionality technicallycycles are not generally possible in class inheritance trees-- class must already have been defined to be named as superclassand python raises an excepti... |
1,530 | recurses to superclasses explicitlyuses str format(for clarity__module__=__main__ __str__this test' output is much larger in and may justify isolating underscore names in general as we did earlier in factthis test may not even work in some currently recent releases as isc:\codec:\python \python listtree py etc file "li... |
1,531 | "to fixwrap the format call in try statement to catch the exceptionuse formatting expressions instead of the str format methoduse one of the aforementioned still-working str format usage modes and hope it does not change tooor wait for repair of this in later release here' the recommended workaround using the tried-and... |
1,532 | _name= _tclcommands=[_w children={mastermuch more omitted str(bprint( [: ]or print just the first part experiment arbitrarily on your own the main point here is that oop is all about code reuseand mix-in classes are powerful example like almost everything else in programmingmultiple inheritance can be useful device whe... |
1,533 | like most softwarethere' much more we could do here the following gives some pointers on extensions you may wish to explore some are interesting projectsand two serve as segue to the next but for space will have to remain in the suggested exercise category here general ideasguisbuilt-ins grouping double-underscore name... |
1,534 | on instanceseven though their names don' appear in instance namespace dictionaries properties and descriptors are associated with instances toobut they don' reserve space in the instancetheir computed nature is much more explicitand they may seem closer to class-level methods than instance data as we'll see in the next... |
1,535 | may be bit of pipe dream anyhow techniques like those outlined here may address slots and propertiesbut some attributes are entirely dynamicwith no physical basis at allthose computed on fetch by generic method such as __get attr__ are not data in the classic sense tools that attempt to display data in wildly dynamic l... |
1,536 | python--if not for your codethen for the code of others you may need to understand firstthoughhere' another quick quiz to review test your knowledgequiz what is multiple inheritance what is delegation what is composition what are bound methods what are pseudoprivate attributes used fortest your knowledgeanswers multipl... |
1,537 | advanced class topics this concludes our look at oop in python by presenting few more advanced class-related topicswe will survey subclassing built-in types"new styleclass changes and extensionsstatic and class methodsslots and propertiesfunction and class decoratorsthe mro and the super calland more as we've seenpytho... |
1,538 | extending built-in types besides implementing new kinds of objectsclasses are sometimes used to extend the functionality of python' built-in types to support more exotic data structures for instanceto add queue insert and delete methods to listsyou can code classes that wrap (embeda list object and export insert and de... |
1,539 | def __iter__(self)return 'set:repr(self dataprint(self)return iter(self datafor in selfto use this classwe make instancescall methodsand run defined operators as usualfrom setwrapper import set set([ ]print( union(set([ ]))print( set([ ])prints set:[ prints set:[ overloading operations such as indexing and iteration al... |
1,540 | enough to do the trickpython typesubclass py [' '' '' '[' '' '' '(indexing [' '' '' 'at (indexing [' '' '' 'at [' '' '' ''spam'['spam'' '' '' 'this output also includes tracing text the class prints on indexing of coursewhether changing indexing this way is good idea in general is another issue--users of your mylist cl... |
1,541 | def __and__(selfother)return self intersect(otherdef __or__(selfother)return self union(otherdef __repr__(self)return 'set:list __repr__(selfif __name__ ='__main__' set([ , , , ] set([ , , , , ]print(xylen( )print( intersect( ) union( )print( yx yx reverse()print(xhere is the output of the self-test code at the end of ... |
1,542 | to be considered "new styleand enable and obtain all new-style behavior classes without this are "classic because all classes are automatically new-style in xthe features of new-style classes are simply normal class features in that line 've opted to keep their descriptions in this section separatehoweverin deference t... |
1,543 | it has for some two decades howeverbecause they modify core class behaviorsnew-style classes had to be introduced in python as distinct tool so as to avoid impacting any existing code that depends on the prior model for examplesome subtle differencessuch as diamond pattern inheritance search and the interaction of buil... |
1,544 | classes and types mergedtype testing classes are now typesand types are now classes in factthe two are essentially synonymsthough the metaclasses that now subsume types are still somewhat distinct from normal classes the type(ibuilt-in returns the class an instance is made frominstead of generic instance typeand is nor... |
1,545 | and xeven though they are an option in the latter this and book sometimes label features as changes to contrast with traditional codebut some are technically introduced by new-style classes--which are mandated in xbut can show up in code too for spacethis distinction is called out often but not dogmatically here compli... |
1,546 | reflects conundrum introduced by the metaclass model because classes are now instances of metaclassesand because metaclasses can define built-in operator methods to process the classes they generatea method call run for class must skip the class itself and look one level higher to pick up method that processes the clas... |
1,547 | print(x__str__ spam class (object)rest of class unchanged classic doesn' inherit default new-style in and (built-ins not routed to getattr [ typeerror'cobject does not support indexing print(xthough apparently rationalized in the name of class metaclass methods and optimizing built-in operationsthis divergence is not a... |
1,548 | in more realistic delegation scenariothis means that built-in operations like expressions no longer work the same as their traditional direct-call equivalent asymmetricallydirect calls to built-in method names still workbut equivalent expressions do not because through-type calls fail for names not at the class level a... |
1,549 | getattrupper upper(getattrupper 'spamx[ getitem 'px __getitem__( getitem 'ptype(x__getitem__( getitem 'px 'eggsaddeggs 'spameggsx __add__('eggs'addeggs 'spameggstype(x__add__( 'eggs'addeggs 'spameggsbuilt-in operation (implicittraditional equivalence (explicitnew-style equivalence ditto for and others for more details ... |
1,550 | components of wrapped object may be inherited from arbitrary sourcesother approachesa web search today will uncover numerous additional tools that similarly populate proxy classes with overloading methodsit' widespread concernagainin we'll also see how to code straightforward and general superclasses once that provide ... |
1,551 | practical contexts where this type/class merging becomes most obvious is when we do explicit type testing with python ' classic classesthe type of class instance is generic "instance,but the types of built-in objects are more specificc:\codec:\python \python class cpass classic classes in (instances are made from class... |
1,552 | (classes and built-in types work the same as you can seein classes are typesbut types are also classes technicallyeach class is generated by metaclass-- class that is normally either type itselfor subclass of it customized to augment or manage generated classes besides impacting code that does type testingthis turns ou... |
1,553 | in in this regard--comparing instance types compares the instancesclasses automaticallyc:\codec:\python \python class (object)pass class (object)pass cd () (type( =type(dfalse new-stylesame as all in type( )type( ( __class__d __class__ (of courseas 've pointed out numerous times in this booktype checking is usually the... |
1,554 | are classes in the new-style model--built-in types are now classesand their instances derive from objecttootype('spam')type(str(isinstance('spam'objecttrue isinstance(strobjecttrue same for built-in types (classesin facttype itself derives from objectand object derives from typeeven though the two are different objects... |
1,555 | this means all classes get defaults in __bases__ (, (__repr__ this model also makes for fewer special cases than the prior type/class distinction of classic classesand it allows us to write code that can safely assume and use an object superclass ( by assuming it as an "anchorin some super built-in roles described ahea... |
1,556 | once when it is accessible from multiple subclasses it' arguably better than dflrbut applies to small subset of python user codeas we'll seethoughthe new-style class model itself makes diamonds much more commonand the mro more important at the same timethe new mro will locate attributes differentlycreating potential in... |
1,557 | thoughnew-style classes visit first otherwisec could be essentially pointless in diamond context for any names in too--it could not customize and would be used only for names unique to explicit conflict resolution of coursethe problem with assumptions is that they assume thingsif this search order deviation seems too s... |
1,558 | class (bc)pass ( meth( meth use default search order will vary per class type defaults to classic order in class (bc)meth meth ( meth( meth <=pick ' methodnew-style (and xclass (bc)meth meth ( meth( meth <=pick ' methodclassic herewe select methods by explicitly assigning to names lower in the tree we might also simply... |
1,559 | in sumby defaultthe diamond pattern is searched differently for classic and new-style classesand this is non-backward-compatible change keep in mindthoughthat this change primarily affects diamond pattern cases of multiple inheritancenew-style class inheritance works the same for most other inheritance tree structures ... |
1,560 | details of the mro are bit too arcane and academic for this text as rulethis book avoids formal algorithms and prefers to teach informally by example on the other handsome readers may still have an interest in the formal theory behind new-style mro if this set includes youit' described in full detail onlinesearch pytho... |
1,561 | object root)--to the topand then to the right ( dflrdepth first and left to rightthe model used for all classic classes in )class apass class ( )pass nondiamondsorder same as classic class cpass depth firstthen left to right class (bc)pass __mro__ (the mro of the following treefor exampleis the same as the earlier diam... |
1,562 | unless classes derive from object strictly speakingnew-style classes also have class mro(method used in the prior example for varietyit' called at class instantiation time and its return value is list used to initialize the __mro__ attribute when the class is created (the method is available for customization in metacl... |
1,563 | def filterdictvals(dv)""dict with entries for value removed filterdictvals(dict( = = = ) ={' ' ""return {kv for (kv in items(if !vdef invertdict( )""dict with values changed to keys (grouped by valuesvalues must all be hashable to work as dict/set keys invertdict(dict( = = = )={ [' '' '] [' ']""def keysof( )return sort... |
1,564 | return attr obj if not bysource else invertdict(attr objif __name__ ='__main__'print('classic classes in xnew-style in 'class aattr class ( )attr class ( )attr class (bc)pass (print('py=>%si attr python' search =ourstrace(inheritance( )'inh\ '[inheritance ordertrace(mapattrs( )'attrs\ 'attrs =source trace(mapattrs(ibys... |
1,565 | '__module__''attr ''attr 'objs {['attr ']['attr ']['__doc__''__module__']new-style classes in and py=> inh (attrs {'__dict__''__doc__''__module__''__weakref__''attr ''attr 'objs {['__dict__''__weakref__']['attr ']['attr ']['__doc__''__module__']as larger application of these toolsthe following is our inheritance simula... |
1,566 | trace(mapattrs( ){'_listinstance__attrnames''__init__''__str__'etc 'data ''data ''data ''ham''spam'trace(mapattrs(ibysource=true){['data ''data ''data ']['_listinstance__attrnames''__str__']['__dict__''__weakref__''ham']['__doc__''__init__''__module__''__qualname__''spam']trace(mapattrs(iwithobject=true){'_listinstance... |
1,567 | class from which they are acquiredeven though they are not physically stored in the instance' __dict__ itselfmapattrs-slots pytest __slots__ attribute inheritance from mapattrs import mapattrstrace class (object)__slots__ [' '' '] class ( )__slots__ [' '' 'class ( ) class (bc) def __init__(self)self name 'bob' (trace(m... |
1,568 | and but appears to have an issue in pythons and where it raises wrong-number-arguments exception internally for the objects displayed here since 've already devoted too much space to covering transitory python defectsand since this has been repaired in the versions of python used in this editionwe'll leave working arou... |
1,569 | looks like instance data age ape illegalnot in __slots__ attributeerror'limiterobject has no attribute 'apethis feature is envisioned as both way to catch typo errors like this (assignments to illegal attribute names not in __slots__ are detectedas well as an optimization mechanism allocating namespace dictionary for e... |
1,570 | does not include to be clearthis is major incompatibility with the traditional class model--one that can complicate any code that accesses attributes genericallyand may even cause some programs to fail altogether for instanceprograms that list or access instance attributes by name string may need to use more storage-ne... |
1,571 | __slots__in order to create an attribute namespace dictionary tooclass d__slots__ [' '' ''__dict__' def __init__(self)self ( attributeerrora name __dict__ to include one too class attrs work normally stored in __dict__a is slot all instance attrs undefined until assigned in this caseboth storage mechanisms are used thi... |
1,572 | isa name' absence in the lowest __slots__ list does not preclude its existence in higher __slots__ because slot names become class-level attributesinstances acquire the union of all slot names anywhere in the treeby the normal inheritance ruleclass e__slots__ [' '' 'class ( )__slots__ [' ''__dict__' ( ax ( superclass h... |
1,573 | on request slots are the most data-centric of thesebut are representative of larger category such attributes require inclusive approachesspecial handlingor general avoidance-the latter of which becomes unsatisfactory as soon as any programmer uses slots in subject code reallyclass-level instance attributes like slots p... |
1,574 | instance __dict__and may even imply its absencenew-style classes must instead generally run attribute assignments by routing them to the object superclass in practicethis may make this method fundamentally different in some classic and new-style classes slot usage rules slot declarations can appear in multiple classes ... |
1,575 | class c__slots__ [' 'class ( )pass ( __dict__ {' ' __dict__ keys(dict_keys(' ''__slots__']bullet slots in super but not sub makes instance dict for nonslots but slot name still managed in class class c__slots__ [' 'class ( )__slots__ [' 'bullet only lowest slot accessible class c__slots__ [' '] bullet no class-level de... |
1,576 | class (listtree)__slots__ [' '' ' ( print(xoksuperclass produces __dict__ displays at xa and at the following classes display correctly as well--any nonslot class like listtree generates an instance __dict__and can thus safely assume its presenceclass a__slots__ [' 'class (alisttree)pass class a__slots__ [' 'class (ali... |
1,577 | finallywhile slots primarily optimize memory usetheir speed impact is less clear-cut here' simple test script using the timeit techniques we studied in for both the slots and nonslots (instance dictionarystorage modelsit makes , instancesassigns and fetches attributes on eachand repeats , times--for both models taking ... |
1,578 | our next new-style extension is properties-- mechanism that provides another way for new-style classes to define methods called automatically for access or assignment to instance attributes this feature is similar to properties ( "gettersand "setters"in languages like java and #but in python is generally best used spar... |
1,579 | operators( age name attributeerrorname runs __getattr__ runs __getattr__ here is the same examplecoded with properties insteadnote that properties are available for all classes but require the new-style object derivation in to work properly for intercepting attribute assignments (and won' complain if you forget this--b... |
1,580 | properties coded in other classes)class operatorsdef __getattr__(selfname)if name ='age'return elseraise attributeerror(namedef __setattr__(selfnamevalue)print('set% % (namevalue)if name ='age'self __dict__['_age'value elseself __dict__[namevalue operators( age age setage _age age job 'trainersetjob trainer job 'traine... |
1,581 | to make sense of this decorator syntaxthoughwe must move ahead __getattribute__ and descriptorsattribute tools also in the class extensions departmentthe __getattribute__ operator overloading methodavailable for new-style classes onlyallows class to intercept all attribute referencesnot just undefined references this m... |
1,582 | at roughly the same timestatic and class methodsdecoratorsand more many of the changes and feature additions of new-style classes integrate with the notion of subclassable types mentioned earlier in this because subclassable types and new-style classes were introduced in conjunction with merging of the type/class dicho... |
1,583 | and its processing are associated with the class rather than its instances that isthe information is usually stored on the class itself and processed apart from any instance for such taskssimple functions coded outside class can often suffice--because they can access class attributes through the class namethey have acc... |
1,584 | whether they are called through an instance or class by contrastin python we are required to pass an instance to method only if the method expects one--methods that do not include an instance argument can be called through the class without passing an instance that is allows simple functions in classas long as they do ... |
1,585 | workbut calls from instances failc:\codec:\python \python from spam import spam spam( spam( spam(can call functions in class in calls through instances still pass self spam printnuminstances(differs in number of instances created printnuminstances(typeerrorprintnuminstances(takes positional arguments but was given that... |
1,586 | spam spam numinstances and cannot be changed via inheritance because the class name is accessible to the simple function as global variablethis works fine alsonote that the name of the function becomes globalbut only to this single moduleit will not clash with names in other files of the program prior to static methods... |
1,587 | to be passed in when invoked to designate such methodsclasses call the built-in functions staticmethod and classmethodas hinted in the earlier discussion of new-style classes both mark function object as special--that isas requiring no instance if static and requiring class argument if class method for examplein the fi... |
1,588 | [ static methodsby contrastare called without an instance argument unlike simple functions outside classtheir names are local to the scopes of the classes in which they are definedand they may be looked up by inheritance instance-less functions can be called through class normally in python xbut never by default in usi... |
1,589 | number of instances instance argument not passed compared to simply moving printnuminstances outside the classas prescribed earlierthis version requires an extra staticmethod call (or an line we'll see aheadhoweverit also localizes the function name in the class scope (so it won' clash with other names in the module)mo... |
1,590 | numinstances use class method instead of static def __init__(self)spam numinstances + def printnuminstances(cls)print("number of instances%scls numinstancesprintnuminstances classmethod(printnuminstancesthis class is used in the same way as the prior versionsbut its printnuminstances method receives the spam classnot t... |
1,591 | and python passes the lowest classsubto the class method all is well in this case-since sub' redefinition of the method calls the spam superclass' version explicitlythe superclass method in spam receives its own class in its first argument but watch what happens for an object that inherits the class method verbatimz ot... |
1,592 | numinstancesy numinstancesz numinstances ( spam numinstancessub numinstancesother numinstances ( per-class datastatic and class methods have additional advanced roleswhich we will finesse heresee other resources for more use cases in recent python versionsthoughthe static and class method designations have become even ... |
1,593 | object interface python provides few built-in function decorators for operations such as marking static and class methods and defining properties (as sketched earlierthe property built-in works as decorator automatically)but programmers can also code arbitrary decorators of their own although they are not strictly tied... |
1,594 | numinstances def __init__(self)spam numinstances spam numinstances @staticmethod def printnuminstances()print("number of instances created%sspam numinstancesfrom spam_static_deco import spam spam( spam( spam(spam printnuminstances(number of instances created printnuminstances(number of instances created calls from clas... |
1,595 | the next section explains first look at user-defined function decorators although python provides handful of built-in functions that can be used as decoratorswe can also write custom decorators of our own because of their wide utilitywe're going to devote an entire to coding decorators in the final part of this book as... |
1,596 | __call__ would be passed tracer instance onlyas we'll see in part viiithere are variety of ways to code function decoratorsincluding nested def statementssome of the alternatives are better suited to methods than the version shown here for exampleby using nested functions with enclosing scopes for stateinstead of calla... |
1,597 | hook to automatically augment the classes with instance counters and any other data requireddef count(aclass)aclass numinstances return aclass return class itselfinstead of wrapper @count class spamsame as spam count(spam@count class sub(spam)numinstances not needed here @count class other(spam)in factas codedthis deco... |
1,598 | my creation routed to meta like meta(' '()}in python xthe effect is the samebut the coding differs--use class attribute instead of keyword argument in the class headerclass c__metaclass__ meta my creation routed to meta in either linepython calls class' metaclass to create the new class objectpassing in the data define... |
1,599 | to see python at work in more substantial examples than much of the rest of the book was able to provide for nowlet' move on to our final class-related topic the super built-in functionfor better or worseso fari've mentioned python' super built-in function only briefly in passing because it is relatively uncommon and m... |
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