9.2.1. Scopes and Namespaces Example

9.2.1. 作用域和命名空间示例

   def scope_test(): 
       def do_local(): 
           spam = "local spam" 
 
       def do_nonlocal(): 
           nonlocal spam 
           spam = "nonlocal spam" 
 
       def do_global(): 
           global spam 
           spam = "global spam" 
 
       spam = "test spam" 
       do_local() 
       print("After local assignment:", spam) 
       do_nonlocal() 
       print("After nonlocal assignment:", spam) 
       do_global() 
       print("After global assignment:", spam) 
 
   scope_test() 
   print("In global scope:", spam) 
 

   After local assignment: test spam 
   After nonlocal assignment: nonlocal spam 
   After global assignment: nonlocal spam 
   In global scope: global spam 
 

9.3. A First Look at Classes

9.3. 初探类

9.3.1. Class Definition Syntax

9.3.1. 类定义语法

   class ClassName: 
        
       . 
       . 
       . 
        
 

9.3.2. Class Objects

9.3.2. 类对象

   class MyClass: 
       """A simple example class""" 
       i = 12345 
 
       def f(self): 
           return 'hello world' 
 

   x = MyClass() 
 

   def __init__(self): 
       self.data = [] 
 

   x = MyClass() 
 

   >>> class Complex: 
   ...     def __init__(self, realpart, imagpart): 
   ...         self.r = realpart 
   ...         self.i = imagpart 
   ... 
   >>> x = Complex(3.0, -4.5) 
   >>> x.r, x.i 
   (3.0, -4.5) 
 
 

9.3.3. Instance Objects

9.3.3. 实例对象

   x.counter = 1 
   while x.counter < 10: 
       x.counter = x.counter * 2 
   print(x.counter) 
   del x.counter 
 

9.3.4. Method Objects

9.3.4. 方法对象

   x.f() 
 

   xf = x.f 
   while True: 
       print(xf()) 
 

9.3.5. Class and Instance Variables

9.3.5. 类和实例变量

   class Dog: 
 
       kind = 'canine'         # class variable shared by all instances 
 
       def __init__(self, name): 
           self.name = name    # instance variable unique to each instance 
 
   >>> d = Dog('Fido') 
   >>> e = Dog('Buddy') 
   >>> d.kind                  # shared by all dogs 
   'canine' 
   >>> e.kind                  # shared by all dogs 
   'canine' 
   >>> d.name                  # unique to d 
   'Fido' 
   >>> e.name                  # unique to e 
   'Buddy' 
 

   class Dog: 
 
       tricks = []             # mistaken use of a class variable 
 
       def __init__(self, name): 
           self.name = name 
 
       def add_trick(self, trick): 
           self.tricks.append(trick) 
 
   >>> d = Dog('Fido') 
   >>> e = Dog('Buddy') 
   >>> d.add_trick('roll over') 
   >>> e.add_trick('play dead') 
   >>> d.tricks                # unexpectedly shared by all dogs 
   ['roll over', 'play dead'] 
 

   class Dog: 
 
       def __init__(self, name): 
           self.name = name 
           self.tricks = []    # creates a new empty list for each dog 
 
       def add_trick(self, trick): 
           self.tricks.append(trick) 
 
   >>> d = Dog('Fido') 
   >>> e = Dog('Buddy') 
   >>> d.add_trick('roll over') 
   >>> e.add_trick('play dead') 
   >>> d.tricks 
   ['roll over'] 
   >>> e.tricks 
   ['play dead'] 
 
 

9.4. Random Remarks

9.4. 补充说明

   >>> class Warehouse: 
           purpose = 'storage' 
           region = 'west' 
 
   >>> w1 = Warehouse() 
   >>> print(w1.purpose, w1.region) 
   storage west 
   >>> w2 = Warehouse() 
   >>> w2.region = 'east' 
   >>> print(w2.purpose, w2.region) 
   storage east 
 

   # Function defined outside the class 
   def f1(self, x, y): 
       return min(x, x+y) 
 
   class C: 
       f = f1 
 
       def g(self): 
           return 'hello world' 
 
       h = g 
 

   class Bag: 
       def __init__(self): 
           self.data = [] 
 
       def add(self, x): 
           self.data.append(x) 
 
       def addtwice(self, x): 
           self.add(x) 
           self.add(x) 
 

9.5. Inheritance

9.5. 继承

   class DerivedClassName(BaseClassName): 
        
       . 
       . 
       . 
        
 

   class DerivedClassName(modname.BaseClassName): 
 

   class DerivedClassName(Base1, Base2, Base3): 
        
       . 
       . 
       . 
        
 

   class Mapping: 
       def __init__(self, iterable): 
           self.items_list = [] 
           self.__update(iterable) 
 
       def update(self, iterable): 
           for item in iterable: 
               self.items_list.append(item) 
 
       __update = update   # private copy of original update() method 
 
   class MappingSubclass(Mapping): 
 
       def update(self, keys, values): 
           # provides new signature for update() 
           # but does not break __init__() 
           for item in zip(keys, values): 
               self.items_list.append(item) 
 

   class Employee: 
       pass 
 
   john = Employee()  # Create an empty employee record 
 
   # Fill the fields of the record 
   john.name = 'John Doe' 
   john.dept = 'computer lab' 
   john.salary = 1000 
 

   for element in [1, 2, 3]: 
       print(element) 
   for element in (1, 2, 3): 
       print(element) 
   for key in {'one':1, 'two':2}: 
       print(key) 
   for char in "123": 
       print(char) 
   for line in open("myfile.txt"): 
       print(line, end='') 
 

   >>> s = 'abc' 
   >>> it = iter(s) 
   >>> it 
    
   >>> next(it) 
   'a' 
   >>> next(it) 
   'b' 
   >>> next(it) 
   'c' 
   >>> next(it) 
   Traceback (most recent call last): 
     File "", line 1, in  
       next(it) 
   StopIteration 
 

   class Reverse: 
       """Iterator for looping over a sequence backwards.""" 
       def __init__(self, data): 
           self.data = data 
           self.index = len(data) 
 
       def __iter__(self): 
           return self 
 
       def __next__(self): 
           if self.index == 0: 
               raise StopIteration 
           self.index = self.index - 1 
           return self.data[self.index] 
 
   >>> rev = Reverse('spam') 
   >>> iter(rev) 
   <__main__.Reverse object at 0x00A1DB50> 
   >>> for char in rev: 
   ...     print(char) 
   ... 
   m 
   a 
   p 
   s 
 
 

   def reverse(data): 
       for index in range(len(data)-1, -1, -1): 
           yield data[index] 
 
   >>> for char in reverse('golf'): 
   ...     print(char) 
   ... 
   f 
   l 
   o 
   g 
 

   >>> sum(i*i for i in range(10))                 # sum of squares 
   285 
 
   >>> xvec = [10, 20, 30] 
   >>> yvec = [7, 5, 3] 
   >>> sum(x*y for x,y in zip(xvec, yvec))         # dot product 
   260 
 
   >>> unique_words = set(word for line in page  for word in line.split()) 
 
   >>> valedictorian = max((student.gpa, student.name) for student in graduates) 
 
   >>> data = 'golf' 
   >>> list(data[i] for i in range(len(data)-1, -1, -1)) 
   ['f', 'l', 'o', 'g']