KiwiPyCon2011, Wellington, Sunday, Track 1, Patterns in Python by Glenn Ramsay

3. ME (1993, U of Auckland) mechanical engineering – control systems

4. Freelance developer (mainly C++, now Python)

5. Based at Hikutaia

6. Row Pro (digitalrowing.com) since 2001

7. Interest in software is primarily for modelling and simulation

8. PhD candidate at U of A (FEA horse's hoof, FORTRAN, Perl) thesis submitted Hikutaia

10. General software design / pattern concepts (brief)

11. Specific examples of “Gang of Four” patterns in Python

12. Motivation “ 16 of 23 [GoF] patterns have a qualitatively simpler implementation in Lisp or Dylan than in C++, for at least some uses of each pattern” Peter Norvig (http://norvig.com/design-patterns/) “ Patterns are not needed in Python because design patterns are a sign of a deficiency of a language” … ... for the purpose that the design pattern addresses. How is observer implemented in Python? (equivalent to Boost.Signals, Qt Slot/Signals, .NET events) Coming from C++ or Java, if you already know the GoF patterns then it would be informative to see how they are implemented in Python. This talk documents part of my journey from C++ to Ptyhon.

13. Software vs Engineering design Physical construction – E.g. building a house Software construction The code is the design! “ Software may be cheap to build, but it is incredibly expensive to design” J W Reeves, What Is Software Design?, www.developerdotstar.com Design stage output

14. Software design How does one design software, compared to physical engineering design? Data + algorithms? - only a part of the solution Structure and Interpretation of Computer Programs. H Abelson, G Sussman, J Sussman. http://mitpress.mit.edu/sicp/full-text/book/book.html Software Design Concepts (wikipedia) Abstraction – categorize and group concepts Refinement – convert high level to program statements Modularity – isolate independent features Software architecture – overall structure of the software Control Hierarchy – program structure Structural partitioning – horizontal vs vertical ? Data structure – logical relationship among elements Software procedure – an operation within a module Information hiding - information contained within a module is inaccessible to others Not especially helpful - too abstract!

17. Abstractions should not depend on details. Details should depend on abstractions.

20. Christopher Alexander - Architect

21. Patterns are discovered – not invented

22. Patterns are not independent from the programming language. Example: subroutines in assembler. Gamma, Helm, Johnson, Vlissades (1995): Design patterns: elements of reusable object oriented software. Addison Wesley.

23. Pattern classes Purpose Creational Structural Behavioural Scope Class Factory Method Adapter (class) Interpreter Template Method Object Abstract factory Builder Prototype Singleton Adapter (object) Bridge Composite Decorator Facade Flyweight Proxy Chain of responsibility Command Iterator Mediator Memento Observer State Strategy Visitor Gamma, Helm, Johnson, Vlissades (1995): Design patterns: elements of reusable object oriented software. Addison Wesley. Invisible or simplified in Python due to: first class types first class functions other Fixed at compile time Can change at runtime Object creation Compostion of classes or objects Class and object interactions

27. can be passed as a parameter to a subroutine

28. can be returned as the result of a subroutine

29. can be constructed at run-time

33. This means that a base class is not always needed

34. Therefore a lot of infrastructure code can be avoided

37. If you need to do something special. E.g. # Put in const.py...: class _const : class ConstError (TypeError): pass def __setattr__ ( self ,name,value): if self .__dict__.has_key(name): raise self .ConstError, "Can't rebind const (%s)" %name self .__dict__[name]=value import sys sys.modules[__name__]=_const() # that's all -- now any client-code can import const # and bind an attribute ONCE: const.magic = 23 # but NOT re-bind it: const.magic = 88 # raises const.ConstError # you may also want to add the obvious __delattr__ Alex Martelli http://code.activestate.com/recipes/65207-constants-in-python/

40. Known uses: undo/redo.

41. OO replacement for callbacks.

42. Specify, queue and execute requests at different times. http://www.cs.mcgill.ca/~hv/classes/CS400/01.hchen/doc/command/command.html

43. Command – GoF style Rahul Verma, Chetan Giridhar. Design Patterns in Python. www.testingperspective.com class Command : """The Command Abstract class""" def __init__ ( self ): pass #Make changes def execute ( self ): #OVERRIDE raise NotImplementedError class FlipUpCommand (Command): """The Command class for turning on the light""" def __init__ ( self , light): self .__light = light def execute ( self ): self .__light.turnOn()

44. Command in Python def greet (who): print "Hello %s" % who greet_command = lambda : greet( "World" ) # pass the callable around, and invoke it later greet_command() class MoveFileCommand (object): def __init__ ( self , src, dest): self .src = src self .dest = dest self () def __call__ ( self ): os.rename( self .src, self .dest) def undo ( self ): os.rename( self .dest, self .src) undo_stack = [] undo_stack.append(MoveFileCommand( 'foo.txt' , 'bar.txt' )) undo_stack.append(MoveFileCommand( 'bar.txt' , 'baz.txt' )) # foo.txt is now renamed to baz.txt undo_stack.pop().undo() # Now it's bar.txt undo_stack.pop().undo() # and back to foo.txt http://stackoverflow.com/questions/1494442/general-command-pattern-and-command-dispatch-pattern-in-python (Ants Aasma) Simple case: Just use a callable More complex case: Use a command object but no need for a base class

46. makes you seem stupid

47. it's global

48. creates very strong coupling with client classes http://en.csharp-online.net

49. Singleton GoF style http://code.activestate.com/recipes/52558-the-singleton-pattern-implemented-with-python/ class Singleton : class __impl : """ Implementation of the singleton interface """ def spam ( self ): """ Test method, return singleton id """ return id( self ) # storage for the instance reference __instance = None def __init__ ( self ): """ Create singleton instance """ # Check whether we already have an instance if Singleton.__instance is None : # Create and remember instance Singleton.__instance = Singleton.__impl() # Store instance reference as the only member in the handle self .__dict__[ '_Singleton__instance' ] = Singleton.__instance def __getattr__ ( self , attr): """ Delegate access to implementation """ return getattr( self .__instance, attr) def __setattr__ ( self , attr, value): return setattr( self .__instance, attr, value)

53. Strategy - statically typed class Bisection (FindMinima): def algorithm ( self ,line): Return ( 5.5 , 6.6 ) class ConjugateGradient (FindMinima): def algorithm ( self ,line): Return ( 3.3 , 4.4 ) class MinimaSolver : # context class strategy= '' def __init__ ( self ,strategy): self .strategy=strategy def minima ( self ,line): return self .strategy.algorithm(line) def changeAlgorithm ( self , newAlgorithm): self .strategy = newAlgorithm def test (): solver=MinimaSolver(ConjugateGradient()) print solver.minima(( 5.5 , 5.5 )) solver.changeAlgorithm(Bisection()) print solver.minima(( 5.5 , 5.5 )) From J Gregorio http://assets.en.oreilly.com/1/event/12/_The%20Lack%20of_%20Design%20Patterns%20in%20Python%20Presentation.pdf

54. Strategy in Python def bisection (line): Return 5.5 , 6.6 def conjugate_gradient (line): Return 3.3 , 4.4 def test (): solver = conjugate_gradient print solver(( 5.5 , 5.5 )) solver = bisection print solver(( 5.5 , 5.5 )) From J Gregorio http://assets.en.oreilly.com/1/event/12/_The%20Lack%20of_%20Design%20Patterns%20in%20Python%20Presentation.pdf Invisible because Python has first class functions

56. Known uses: model – view – controller, Qt Signals/Slots, Boost.Signals, most GUI toolkits http://en.wikipedia.org/wiki/File:Observer.svg

58. Detach during notify()

59. Pass parameters to the observer e.g. http://code.activestate.com/recipes/131499-observer-pattern/

62. Not the same as Python decorators Objects enclose other objects that share similar interfaces. The decorating object appears to mask or modify or annotate the enclosed object. http://en.wikipedia.org/wiki/File:Decorator_UML_class_diagram.svg

63. Decorator GoF style class Writer (object): def write ( self , s): print s class WriterDecorator (object): def __init__ ( self , wrappee): self .wrappee = wrappee def write ( self , s): self .wrappee.write(s) class UpperWriter (WriterDecorator): def write ( self , s): self .wrappee.write(s.upper()) class ShoutWriter (WriterDecorator): def write ( self , s): self .wrappee.write( '!' .join( [t for t in s.split( ' ' ) if t]) + '!' ) Magnus Therning http://therning.org/magnus/archives/301 w = Writer() w.write( 'hello' ) uw = UpperWriter(w) uw.write( 'hello' ) wd = WriterDecorator(w) wd.write( 'hello' ) sw1 = ShoutWriter(w) sw1.write( 'hello again' ) sw2 = ShoutWriter(uw) sw2.write( 'hello again' ) >>> hello HELLO hello hello!again! HELLO!AGAIN!

64. Decorator using function decorators def uppercase (f): def wrapper (*args, **kwargs): orig = f(*args, **kwargs) return orig.upper() return wrapper def shout (f): def wrapper (*args, **kwargs): orig = f(*args, **kwargs) return '!' .join( [t for t in orig.split( ' ' ) if t]) + '!' return wrapper @shout def s_writer (s): return s print s_writer( "hello again" ) @shout @uppercase def su_writer (s): return s print su_writer( "hello again" ) >>> HELLO AGAIN HELLO!AGAIN!

65. Decorator using delegation import string class UpSeq : def __init__ ( self , seqobj): self .seqobj = seqobj def __str__ ( self ): return string.upper( self .seqobj.seq) def __getattr__ ( self ,attr): return getattr( self .seqobj, attr) class DNA (): def __init__ ( self , name, seq): self .seq = seq self .name = name def __getitem__ ( self , item): return self .seq.__getitem__(item) def first ( self ): return self .seq[ 0 ] s=UpSeq(DNA(name= '1' , seq= ' atcgctgtc ' )) >>> print s ATCGCTGTC >>> print s[ 0 : 3 ] atc >>> print s.first() a Adapted from http://www.pasteur.fr/formation/infobio/python/ UpSeq delegates to DNA

67. Very common

68. Known uses: TCP, GUIs http://en.wikipedia.org/wiki/File:State_Design_Pattern_UML_Class_Diagram.svg

69. State GoF style class State(object): """Base state. This is to share functionality""" def scan( self ): """Scan the dial to the next station""" self .pos += 1 if self .pos == len( self .stations): self .pos = 0 print "Scanning… Station is" , self .stations[ self .pos], self .name class AmState(State): def __init__( self , radio): self .radio = radio self .stations = [ "1250" , "1380" , "1510" ] self .pos = 0 self .name = "AM" def toggle_amfm( self ): print "Switching to FM" self .radio.state = self .radio.fmstate class FmState(State): def __init__( self , radio): self .radio = radio self .stations = [ "81.3" , "89.1" , "103.9" ] self .pos = 0 self .name = "FM" def toggle_amfm( self ): print "Switching to AM" self .radio.state = self .radio.amstate class Radio(object): """A radio. It has a scan button, and an AM/FM toggle switch.""" def __init__( self ): """We have an AM state and an FM state""" self .amstate = AmState( self ) self .fmstate = FmState( self ) self .state = self .amstate def toggle_amfm( self ): self .state.toggle_amfm() def scan( self ): self .state.scan() # Test radio = Radio() actions = [radio.scan] * 2 + [radio.toggle_amfm] + [radio.scan] * 2 actions = actions * 2 for action in actions: action() Jeff ? http://ginstrom.com/scribbles/2007/10/08/design-patterns-python-style/ Scanning... Station is 1380 AM Scanning... Station is 1510 AM Switching to FM Scanning... Station is 89.1 FM Scanning... Station is 103.9 FM Scanning... Station is 81.3 FM Scanning... Station is 89.1 FM Switching to AM Scanning... Station is 1250 AM Scanning... Station is 1380 AM Note lack of state methods “Abstract” state Context Concrete states

71. State in Python (method) class Sequencer(): def __init__( self ): self ._action_impl = self .action1 self .count = 1 def action( self ): self ._action_impl() def next( self ): self .count += 1 if self .count > 3 : self .count = 1 self ._action_impl = getattr( self , "action" +str( self .count)) def action1( self ): print "1" def action2( self ): print "2" def action3( self ): print "3" s = Sequencer() actions = [s.action] + [s.next] actions = actions * 3 for f in actions: f() >>> 1 2 3 >>> Switch methods output Use Bridge so that the binding of Sequencer.action doesn't change

72. State in Python (class) >>> First 1 Second 2 Third 3 First 4 First 1 Second 2 Second 3 class Base (): def __init__ ( self ): self .state = 0 def action ( self ): self .state += 1 print self .__class__.__name__, self .state def change_state ( self , next_class): self .__class__ = next_class class Third (Base): def transition ( self ): self .change_state( First ) class Second (Base): def transition ( self ): self .change_state( Third ) class First (Base): def transition ( self ): self .change_state( Second ) state = First() state.action() state.transition() state.action() state.transition() state.action() state.transition() state.action() state = First() actions = [state.action] + [state.transition] actions = actions * 3 for action in actions: action() output This doesn't work because state is always First

74. Strategy is behavioural – interchange algorithms

75. Bridge is structural – implementation varies independently from abstraction

76. C++ pimpl http://atlas.kennesaw.edu/~dbraun/csis4650/A&D/GoF_Patterns

78. Factory Method GOF style class Person : def __init__ ( self ): self .name = None self .gender = None def getName ( self ): return self .name def getGender ( self ): return self .gender class Male (Person): def __init__ ( self , name): print "Hello Mr." + name class Female (Person): def __init__ ( self , name): print "Hello Miss." + name class Factory : def getPerson ( self , name, gender): if gender == 'M' : return Male(name) if gender == 'F' : return Female(name) From: dpip.testingperspective.com factory = Factory() person = factory.getPerson( " Chetan " , "M" ) person = factory.getPerson( " Money " , "F" ) >>> Hello Mr.Chetan Hello Miss.Money

79. Factory method in Python class Male (object): def __init__ ( self , name): print "Hello Mr." + name class Female (object): def __init__ ( self , name): print "Hello Ms." + name factory = dict(F=Female, M=Male) if __name__ == '__main__' : person = factory[ "F" ]( "Money" ) person = factory[ "M" ]( " Powers " ) >>> Hello Ms.Money Hello Mr.Powers Adapted from: http://www.rmi.net/~lutz/talk.html # variable length arg lists def factory (aClass, *args, **kwargs): return aClass(*args, **kwargs) class Spam : def __init__ ( self ): print self .__class__.__name__ def doit ( self , message): print message class Person : def __init__ ( self , name, job): self .name = name self .job = job print self .__class__.__name__, name, job object1 = factory(Spam) object2 = factory(Person, " Guido " , "guru" ) >>> Spam Person Guido guru

81. Abstract Factory GoF style class PetShop : def __init__ ( self , animal_factory= None ): """pet_factory is our abstract factory. We can set it at will.""" self .pet_factory = animal_factory def show_pet ( self ): """Creates and shows a pet using the abstract factory""" pet = self .pet_factory.get_pet() print "This is a lovely" , pet print "It says" , pet.speak() print "It eats" , self .pet_factory.get_food() class Dog : def speak ( self ): return "woof" def __str__ ( self ): return "Dog" class Cat : def speak ( self ): return " meow " def __str__ ( self ): return "Cat" http://ginstrom.com/scribbles/2007/10/08/design-patterns-python-style/ class DogFactory : def get_pet ( self ): return Dog() def get_food ( self ): return "dog food" class CatFactory : def get_pet ( self ): return Cat() def get_food ( self ): return "cat food" # Create the proper family def get_factory (): return random.choice([DogFactory, CatFactory])() # Show pets with various factories shop = PetShop() for i in range( 3 ): shop.pet_factory = get_factory() shop.show_pet() print "=" * 10 >>> This is a lovely Dog It says woof It eats dog food ========== This is a lovely Cat It says meow It eats cat food ========== This is a lovely Dog It says woof It eats dog food ==========

84. Flyweight in Python http://codesnipers.com/?q=python-flyweights import weakref class Card (object): _CardPool = weakref.WeakValueDictionary() def __new__ (cls, value, suit): obj = Card._CardPool.get(value + suit, None ) if not obj: obj = object.__new__(cls) Card._CardPool[value + suit] = obj obj.value, obj.suit = value, suit return obj c1 = Card( '9' , 'h' ) c2 = Card( '9' , 'h' ) c3 = Card( '2' , 's' ) print c1 == c2, c1 == c3, c2 == c3 print id(c1), id(c2), id(c3) >>> True False False 38958096 38958096 38958128

86. Template method

88. Bridge

89. Composite

90. Memento

92. Duck typing – base classes may be optional

93. Ability to override special methods

96. Thank you http://dl.dropbox.com/u/10287301/Ramsey-KiwiPycon-2011.pdf