《简明python教程》总结（五）-- 面向对象编程，输入/输出，异常
在python中一切皆为对象。例： int为一个类，int i 中的 i为 int 类的对象/一个实例类的属性： 方法和域。 域分两种类型：属于每个对象和属于类本身。分别称为 实例变量和 类变量。 类使用class关键字创建。类的域和方法被列在一个缩进块中。 self类的方法与普通的函数只有一个特别的区别——它们必须有一个额外的第一个参数名称，但是在调用这个方法的时候不为这个参数赋值，Python会提供这个值。这个特别的变量指对象本身，按照惯例它的名称是self。（相当于C++中的this指针） 调用时不需为给self赋值以及为何不需要给它赋值 原理：假如有一个类称为MyClass和这个类的一个实例MyObject。当调用这个对象的方法MyObject.method(arg1, arg2)的时候，这会由Python自动转为MyClass.method(MyObject, arg1, arg2)——这就是self的原理了。 提醒：定义一个不需要参数的方法，同样还是得给这个方法定义一个self参数。 __int__方法__init__方法在类的一个对象被建立时，马上运行。这个方法可以用来对对象做一些 初始化 。注意，这个名称的开始和结尾都是双下划线。（相当于C++中的构造函数） 两种类型的域类的变量和对象的变量，它们根据是类还是对象 拥有 这个变量而区分。类的变量 由一个类的所有对象（实例）共享使用。对象的变量 由类的每个对象/实例拥有。因此每个对象有自己对这个域的一份拷贝，即它们不是共享的，在同一个类的不同实例中，虽然对象的变量有相同的名称，但是是互不相关的。 __del__方法同C++中的析构函数作用 Python中所有的类成员（包括数据成员）都是 公共的 ，所有的方法都是有效的 。只有一个例外：如果你使用的数据成员名称以 双下划线前缀 比如__privatevar，Python的名称管理体系会有效地把它作为私有变量。惯例：可以将欲作为私有变量的成员前加单下划线前缀，其他名称作为公共的#!/usr/bin/python
# Filename: objvar.py
class Person:
'''Represents a person.'''
population = 0
def __init__(self, name):
'''Initializes the person's data.'''
self.name = name
print '(Initializing %s)' % self.name
# When this person is created, he/she
# adds to the population
Person.population += 1
def __del__(self):
'''I am dying.'''
print '%s says bye.' % self.name
Person.population -= 1
if Person.population == 0:
print 'I am the last one.'
else:
print 'There are still %d people left.' % Person.population
def sayHi(self):
'''Greeting by the person.
Really, that's all it does.'''
print 'Hi, my name is %s.' % self.name
def howMany(self):
'''Prints the current population.'''
if Person.population == 1:
print 'I am the only person here.'
else:
print 'We have %d persons here.' % Person.population
swaroop = Person('Swaroop')
swaroop.sayHi()
swaroop.howMany()
kalam = Person('Abdul Kalam')
kalam.sayHi()
kalam.howMany()
swaroop.sayHi()
swaroop.howMany()Person.__doc__()Person.howMany().__doc()输出$ python objvar.py(Initializing Swaroop)Hi, my name is Swaroop.I am the only person here.(Initializing Abdul Kalam)Hi, my name is Abdul Kalam.We have 2 persons here.Hi, my name is Swaroop.We have 2 persons here.Abdul Kalam says bye.There are still 1 people left.Swaroop says bye.I am the last one.Represents a person.Prints the current population. 分析：population 属于类的域，为各实例共享。 类的域，使用时通过 className.field调用，实例的域在函数定义时，通过self.field调用。使用Person.__doc__和Person.sayHi.__doc__来分别访问类与方法的文档字符串 继承面向对象的编程带来的主要好处之一是代码的重用，实现这种重用的方法之一是通过 继承 机制。示例：#!/usr/bin/python
# Filename: inherit.py
class SchoolMember:
'''Represents any school member.'''
def __init__(self, name, age):
self.name = name
self.age = age
print '(Initialized SchoolMember: %s)' % self.name
def tell(self):
'''Tell my details.'''
print 'Name:"%s" Age:"%s"' % (self.name,self.age),
class Teacher(SchoolMember):
'''Represents a teacher.'''
def __init__(self, name, age, salary):
SchoolMember.__init__(self, name, age)
self.salary = salary
print '(Initialized Teacher: %s)' % self.name
def tell(self):
SchoolMember.tell(self)
print 'Salary: "%d"' % self.salary
class Student(SchoolMember):
'''Represents a student.'''
def __init__(self, name, age, marks):
SchoolMember.__init__(self, name, age)
self.marks = marks
print '(Initialized Student: %s)' % self.name
def tell(self):
SchoolMember.tell(self)
print 'Marks: "%d"' % self.marks
t = Teacher('Mrs. Shrividya',40, 30000)
s = Student('Swaroop',22, 75)
print # prints a blank line
members = [t, s]
for member in members:
member.tell() # works for both Teachers and Students输出$ python inherit.py(Initialized SchoolMember: Mrs. Shrividya)(Initialized Teacher: Mrs. Shrividya)(Initialized SchoolMember: Swaroop)(Initialized Student: Swaroop)Name:"Mrs. Shrividya" Age:"40" Salary: "30000"Name:"Swaroop" Age:"22" Marks: "75" SchoolMember类被称为 基本类 或 超类 。而Teacher和Student类被称为导出类 或 子类 。 输入/输出文件与储存器 文件 （创建一个file类的对象来打开一个文件，分别使用file类的read、readline或write方法来恰当地读写文件。'w'--写，'r'--读，'a'--追加）#!/usr/bin/python
# Filename: using_file.py
poem = '''\
Programming is fun
When the work is done
if you wanna make your work also fun:
use Python!
'''
f = file('poem.txt','w') # open for 'w'riting
f.write(poem) # write text to file
f.close() # close the file
f = file('poem.txt')
# if no mode is specified, 'r'ead mode is assumed by default
while True:
line = f.readline()
if len(line) == 0: # Zero length indicates EOF
break
print line,
# Notice comma to avoid automatic newline added by Python
f.close() # close the file输出$ python using_file.pyProgramming is funWhen the work is doneif you wanna make your work also fun: use Python! 储存器Python提供一个标准的模块，称为pickle。可以在一个文件中储存任何Python对象，之后又可以把它完整无缺地取出来。这被称为 持久地 储存对象。还有另一个模块称为cPickle，它的功能和pickle模块完全相同，只不过它是用C语言编写的，因此要快得多（比pickle快1000倍）。 示例（储存与取储存）#!/usr/bin/python
# Filename: pickling.py
import cPickle as p
#import pickle as p
shoplistfile = 'shoplist.data'
# the name of the file where we will store the object
shoplist = ['apple','mango', 'carrot']
# Write to the file
f = file(shoplistfile, 'w')
p.dump(shoplist, f) # dump the object to a file
f.close()
del shoplist # remove the shoplist
# Read back from the storage
f = file(shoplistfile)
storedlist = p.load(f)
print storedlist 输出$ python pickling.py['apple', 'mango', 'carrot'] 分析：import cPickle as p 将模块cPickle命名为 p 优点：1. 方便更换模块，只需修改cPickle 2.方便方法调用 p.dump(),而非 cPickle.dump() 异常try..except引发异常使用raise语句 引发 异常。需指明错误/异常的名称和伴随异常 触发的 异常对象。可以引发的错误或异常应该分别是一个Error或Exception类的直接或间接导出类。#!/usr/bin/python
# Filename: raising.py
class ShortInputException(Exception):
'''A user-defined exception class.'''
def __init__(self, length, atleast):
Exception.__init__(self)
self.length = length
self.atleast = atleast
try:
s = raw_input('Enter something --> ')
if len(s) < 3:
raise ShortInputException(len(s),3)
# Other work can continue as usual here
except EOFError:
print '\nWhy did you do an EOF on me?'
except ShortInputException, x:
print 'ShortInputException: The input was of length %d, \
was expecting at least %d' % (x.length, x.atleast)
else:
print 'No exception was raised.'输出$ python raising.pyEnter something -->Why did you do an EOF on me?$ python raising.pyEnter something --> abShortInputException: The input was of length 2, was expecting at least 3$ python raising.pyEnter something --> abcNo exception was raised. try..finally假如你在读一个文件的时候，希望在无论异常发生与否的情况下都关闭文件，该怎么做呢？这可以使用finally块来完成。示例：#!/usr/bin/python
# Filename: finally.py
import time
try:
f = file('poem.txt')
while True: # our usual file-reading idiom
line = f.readline()
if len(line) == 0:
break
time.sleep(2)
print line,
finally:
f.close()
print 'Cleaning up...closed the file'输出：正常运行$ python finally.pyProgramming is funWhen the work is doneCleaning up...closed the file 运行中 ctrl+c中断/取消程序 $ python finally.pyProgramming is funWhen the work is done^Cclose the fileTraceback (most recent call last): File "finally.py", line 12, in ? time.sleep(2)KeyboardInterrupt 分析：观察到KeyboardInterrupt异常被触发，程序退出。但是在程序退出之前，finally从句仍然被执行，把文件关闭