python学习——基础（六）

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python能够改变变量作用域的代码段有：lamaba、def、class

if/elif/else、for/while、try/except/finally不能改变变量的作用域范围，在这些代码段中的变量，外部也能访问；

变量的访问路径：

local：比如function中的变量所在的作用域就是local的。简称L
enclosing：某个function被嵌套后的上一层function的作用域。简称E
global：当前模块中的全局作用域。简称G
built-in：python内置作用域.简称B
对某个变量搜索的顺序是：local>enclosing>global>built-in。也就是LEGB

def片段：

a =100

def change():
    a = 200
    print a

if __name__ == ‘__main__‘:
    print a
    change()

结果：
100
200

结论：def代码段的a与全局的a是不同的作用域，互不干扰

a =100

def change():
    print a
    a = 200
    print a

if __name__ == ‘__main__‘:
    print a
    change()

UnboundLocalError: local variable ‘a‘ referenced before assignment

本地变量在分配前被调用：在def函数未被运行时先进行预编译；def片段中的第一个print a在分配之前就先访问，所以报错

想要避免这种问题：

a =100

def change():
    global a
    print a
    a = 200
    print a

if __name__ == ‘__main__‘:
    print a
    change()

结果：
100
100
200

def change():
    a = 200
    print a

if __name__ == ‘__main__‘:
    print a
    change()

NameError: global name ‘a‘ is not defined

类变量和实例变量：

实例变量访问：self.propertyName

类变量访问：className.propertyName

class Person:
    name = "class global name"

    def __init__(self, newPersionName):
        self.name = newPersionName

    def sayYourName(self):
        print ‘My name is %s‘%(self.name); # -> class global name
        print ‘name within class Person is actually the global name: %s‘%(name); #-> whole global name
        print "only access Person‘s name via Person.name=%s"%(Person.name); # -> class global name

def selfAndInitDemo():
    ApersionInstance = Person("instance name crifan")
    ApersionInstance.sayYourName()
    BpersionInstance = Person("instance name simith")
    BpersionInstance.sayYourName()
    print "whole global name is %s"%(name); # -> whole global name

if __name__=="__main__":
    selfAndInitDemo()

My name is instance name crifan
name within class Person is actually the global name: whole global name
only access Person‘s name via Person.name=class global name
My name is instance name simith
name within class Person is actually the global name: whole global name
only access Person‘s name via Person.name=class global name
whole global name is whole global name

实例变量新增不会改变类变量，而类变量的变化会直接影响到实例变量

class people:
    country = ‘china‘

p1 = people()

people.age = 10 # 类变量增加age
p1.name = ‘syc‘ # 实例变量增加name
p1.country = ‘japan‘ # 实例变量country进行修改
print p1.country # japan
print people.country # china
print people.age # 10
print p1.age # 10
print people.name # AttributeError: class people has no attribute ‘name‘
print p1.name # syc

关于python面向对象编程的链接：

http://www.cnblogs.com/dolphin0520/archive/2013/03/29/2986924.html

python学习——基础（六）

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原文地址：http://my.oschina.net/u/257801/blog/520515