多态

要了解什麼是多态,我們首先要對資料類型再作一點說明,當我們定義一個class的時候,

我們實際上就定義了一種資料類型,我們定義的資料類型和Python自帶的資料類型,比如str,list,dict沒什麼兩樣：


class Student(object):
    def __init__(self, name, score):
        self.name = name
        self.score = score

    def print_score(self):
        print '%s: %s' % (self.name, self.score -2)


from   mycompany.web.Student  import *
c = Student('a',88)
print c.name
print isinstance(c, Student)
print c.print_score()

C:\Python27\python.exe C:/Users/TLCB/PycharmProjects/untitled/a1.py
a
True
a: 86
None

a = list() # a是list類型
b = Animal() # b是Animal類型
c = Dog() # c是Dog類型
判斷一個變量是否是某個類型可以用isinstance()判斷：

>>> isinstance(a, list)
True
>>> isinstance(b, Animal)
True
>>> isinstance(c, Dog)
Tru

from   mycompany.web.Dog  import *

c = Dog()
c.run()
print isinstance(c, Dog)
print  isinstance(c, Animal)

C:\Python27\python.exe C:/Users/TLCB/PycharmProjects/untitled/a1.py
yy
Animal is running...
True
True

from   mycompany.web.Dog  import *

b = Animal()
print b.run()

C:\Python27\python.exe C:/Users/TLCB/PycharmProjects/untitled/a1.py
yy
Animal isxx running...
None

from   mycompany.web.Dog import *
def run_twice(animal):
    animal.run()

print '----------------'
run_twice(Animal())
run_twice(Dog())


C:\Python27\python.exe C:/Users/TLCB/PycharmProjects/untitled/a1.py
----------------
Animal isxx running...
Dog isxx running...

Process finished with exit code 0



看上去沒啥意思，但是仔細想想，現在，如果我們再定義一個Tortoise類型，也從Animal派生：

class Tortoise(Animal):
    def run(self):
        print 'Tortoise is running slowly...'



from   mycompany.web.Tortoise import *
def run_twice(animal):
    animal.run()

print '----------------'
run_twice(Animal())

run_twice(Tortoise())

C:\Python27\python.exe C:/Users/TLCB/PycharmProjects/untitled/a1.py
----------------
Animal isxx running...
Tortoise is running slowly...

Process finished with exit code 0


你會發現,新增一個Animal的子類,不必對run_twice()做任何修改,

實際上,任何依賴Animal 作為的函數的函數或者方法都可以不加修改地正常運作,原因在于多态





from   mycompany.web.Tortoise import *
def run_twice(animal):
    animal.run()

print '----------------'
run_twice(Animal())

run_twice(Tortoise())



C:\Python27\python.exe C:/Users/TLCB/PycharmProjects/untitled/a1.py
xxxx
----------------
Animal isxx running...
Animal isxx running...