Object Oriented Programming Enthought, Inc. 97

Transcription

1 Object Oriented Programming 97

2 Classes A reusable collection of behavior and data Definition Subclasses Properties 98

3 Modeling a Power Plant ATTRIBUTES # Data that describes the # modeled "object" name: Comanche Peak maximum_output: 2.3 Gigawatts current_output:? Gigawatts status: normal night_watchman: Homer Simpson (!)... BEHAVIORS (METHODS) # tasks or operations that the # model does. start_up() shut_down() emergency_shutdown()... 99

4 Working with Objects # Our befuddled friend. >>> homer = Person(first='Homer', last='simpson') # Create the power plant he is in charge of... >>> plant = PowerPlant(name='Comanche Peak', maximum_output=2.3, night_watchman=homer) # Start the plant using the 'start_up' method. >>> plant.start_up() # Homer does his thing. >>> homer.eat('donut') >>> homer.take_nap() # Check the status (an attribute) of # the plant. No Bueno. >>> if plant.status!= 'normal':... print 'status:', plant.status... homer.speak('doh!')... plant.emergency_shutdown() status: meltdown Homer says 'Doh!' 100

5 Class Definition class PowerPlant(object): # Constructor method def init (self, # self is ALWAYS the first argument. name='', maximum_output=0.0, night_watchman=none): # assign passed-in arguments to new object self.name = name self.maximum_output = maximum_output self.night_watchman = night_watchman # Intialize other attributes. self.current_output = 0.0 self.status = 'normal' # class methods def start_up(self): self.start_reactor_cooling_pump() self.reduce_boric_acid_concentration() self.remove_control_rods() def shut_down(self):... <other methods> 101

6 Subclasses and Inheritance PowerPlant NuclearPowerPlant HydroPowerPlant 102

7 Base Class class PowerPlant(object): # Constructor method def init (self, # "self" is ALWAYS the first argument. name='', maximum_output=0.0, night_watchman=none): # assign passed-in arguments to new object self.name = name self.maximum_output = maximum_output self.night_watchman = night_watchman # Initialize other attributes. self.current_output = 0.0 self.status = 'normal' # Default implementation for methods def start_up(self): self.current_output = self.maximum_output def shut_down(self): self.current_output =

8 Sub-Classing # Derive 'specialized' classes from the PowerPlant base class. # They will 'inherit' the methods of the base class. class NuclearPowerPlant(PowerPlant): # over-ride methods that need custom behavior. def start_up(self): self.start_reactor_cooling_pump() self.reduce_boric_acid_concentration() self.remove_control_rods() def shut_down(self):

9 Using Super # Use 'super' to call the "super-class" (parent class) methods. class HydroPowerPlant(PowerPlant): def init (self, name='', river_name='', maximum_output=0.0, night_watchman=none): # Use 'super' to call an over-ridden method from the # base class. super(hydropowerplant, self). init (name=name, maximum_output=maximum_output, night_watchman=night_watchman) self.river_name = river_name # over-ride methods that need custom behavior. def start_up(self): self.open_sluice_gate() self.unlock_turbine_shaft() self.remove_control_rods() 105

10 Interchangeable Classes # The same code will work without modification for a # NuclearPowerPlant or a HydroPowerPlant. >>> homer = Person(first='Homer', last='simpson') # Create the power plant he is in charge of... >>> plant = HydroPowerPlant(name='Comanche Peak', maximum_output=2.3, night_watchman=homer) # Start the plant using the 'start_up' method. >>> plant.start_up() # Homer does his thing. >>> homer.eat('donut') >>> homer.take_nap() # Check the status (an attribute) of the plant. No Bueno. >>> if plant.status!= 'normal':... print 'status:', plant.status... homer.speak('doh!')... plant.emergency_shutdown() status: Fish stuck in impeller. Homer says 'Doh!' 106

11 Particle Class 107

12 Particle Class Example SIMPLE PARTICLE CLASS >>> class Particle(object):... # Initializer method... def init (self, m, v):... self.mass = m # Assign attribute values of new object... self.velocity = v... # Method for calculating object momentum... def momentum(self):... return self.mass * self.velocity... # A "magic" method defines object's string representation.... # Evaluating the repr recovers the Particle object.... def repr (self):... return "Particle({0}, {1})".format(... repr(self.mass), repr(self.velocity)) EXAMPLE >>> a = Particle(3.2, 4.1) >>> print a.momentum() >>> a Particle(3.2, 4.1) 108

13 Overloading Addition SIMPLE PARTICLE CLASS Classes can override many behaviors using special method names including numeric behavior.... def add (self, other):... if not isinstance(other, Particle):... return NotImplemented... mnew = self.mass + other.mass... vnew = (self.momentum() + other.momentum())/mnew... return Particle(mnew, vnew) EXAMPLE (cont'd) >>> b = Particle(8.6, 10.2) >>> b, a (Particle(8.6, 10.2), Particle(3.2, 4.1)) >>> c = a + b >>> c Particle(11.8, ) >>> print c.momentum()

14 Properties For example, the energy and momentum methods of the Particle class can be implemented as read-only properties: # My_particle_class.py class Particle(object): def _get_momentum(self): return self.mass * self.velocity def _get_energy(self): return 0.5 * self.mass * self.velocity**2 def _not_allowed(self, value): raise RuntimeError("Can not set energy or momentum directly.") energy = property(_get_energy, _not_allowed) momentum = property(_get_momentum, _not_allowed) 110

15 Properties Then, for example: >>> p = Particle(mass=3.0, velocity=4.0) >>> p.momentum 12.0 >>> p.energy = 100 Traceback (most recent call last): File "<stdin>", line 1, in <module> File "particle_with_properties.py", line 23, in _not_allowed raise RuntimeError("can not set energy or momentum directly.") RuntimeError: can not set energy or momentum directly. 111

16 General Scoping Rules Variables are looked up from namespaces in the following order: Local Function Scope Enclosing Function Scope Global Scope Builtin Scope 112

17 Class Scoping Rules CLASS SCOPING # global variable var = 0 class MyClass(object): # class variable var = 1 def access_class_c(self): print 'class var:', self.var def access_global_c(self): print 'global var:', var EXAMPLES >>> obj = MyClass() >>> obj.access_class_c() class var: 1 >>> obj.access_global_c() global var: 0 >>> obj.write_class_c() class var: 2 >>> obj.write_instance_c() instance var: 3 def write_class_c(self): # Modify the class variable. MyClass.var = 2 print 'class var:', self.var def write_instance_c(self): # Create an instance variable. self.var = 3 print 'instance var:', self.var See demo/variable_scoping/ class_scoping.py. 115