Python3.0与2.X版本的区别实例分析
本文通过列举出一些常见的实例来分析Python3.0与2.X版本的区别，是作者经验的总结，对于Python程序设计人员来说有不错的参考价值。具体如下：
做为一个前端开发的码农，最近通过阅读最新版的《A byte of Python》并与老版本的《A byte of Python》做对比后，发现Python3.0在某些地方还是有些改变的。之后再查阅官方网站的文档，总结出一下区别：
1. 如果你下载的是最新版的Python，就会发现所有书中的Hello World例子将不再正确。
Python2.X代码如下:
print "Hello World!" #打印字符串
Python3.0代码如下:
print("Hello World!")
将字符串放到括号中print出来，这种写法对于我这种学习Java出身的人来说，很是亲切啊~O(∩_∩)O~
2.
Python2.X代码如下:
guess = int(raw_input('Enter an integer : ')) #读取键盘输入的方法
Python3.0代码如下:
guess = int(input('Enter an integer : '))
方法名变得更加容易记！
3.
加入了一个新的nonlocal statement，非局部变量，它的范围介于global和local之间，主要用于函数嵌套，用法如下：
#!/usr/bin/python
# Filename: func_nonlocal.py
def func_outer():
x = 2
print('x is', x)
def func_inner():
nonlocal x
x = 5
func_inner()
print('Changed local x to', x)
func_outer()
4.
VarArgs parameters，不知道这个翻译成什么比较妥当？先看下面这个例子：
#!/usr/bin/python
# Filename: total.py
def total(initial=5, *numbers, **keywords):
count = initial
for number in numbers:
count += number
for key in keywords:
count += keywords[key]
return count
print(total(10, 1, 2, 3, vegetables=50, fruits=100))
当在参数前面使用*标识的时候，所有的位置参数（1，2，3）作为一个list传递。
当在参数前面使用**标识的时候，所有的关键参数（vegetables=50, fruits=100）作为一个dictionary传递。
5.
关于Packages的话题，个人理解有限。感兴趣的读者可以查阅相关文档。
6.
在数据结构中，多了一种类型:set
Set是一种无序的简单对象的集合，当我们关心一个对象是否在一个集合中存在，而顺序和出现的次数是次要的时候，可以使用set。
7.
关于os.sep方法，（set是separator，分隔符的缩写）
来看看作者的一个很晕菜的例子：
Python2.X代码如下:
target_dir = '/mnt/e/backup/'
target = target_dir + time.strftime('%Y%m%d%H%M%S') + '.zip'
Python3.0代码如下:
target_dir = 'E:\\Backup'
target = target_dir + os.sep + time.strftime('%Y%m%d%H%M%S') + '.zip'
os.sep的功能是自动辨别操作系统，给出不同的分隔符，Windows上是\\，Linux上是/，原理是明白了，功能也很不错，但是作者的例子。只有一处使用了os.sep，其他的地方还是老的写法啊（E:\\）
8.
可以使用@修饰符声明一个类方法：
@classmethod
def howMany(klass):
'''Prints the current population.'''
print('We have {0:d} robots.'.format(Robot.population))
9.
可以将以个类用Metaclasses的方式声明为抽象类抽象方法
from abc import *
class SchoolMember(metaclass=ABCMeta):
'''Represents any school member.'''
def __init__(self, name, age):
self.name = name
self.age = age
print('(Initialized SchoolMember: {0})'.format(self.name))
@abstractmethod
def tell(self):
'''Tell my details.'''
print('Name:"{0}" Age:"{1}"'.format(self.name, self.age), end=" ")
#pass
10.
文件读写的模式又增加了两种：文本本件('t')二进制文件('b')。
11.将打开文件的操作放到使用with语句修饰的方法中，书上说好处是让我们更专注于文件操作，让代码看起来不凌乱，本文还不能完全体会with的好处。现给出示例代码供大家参考：
#!/usr/bin/python
# Filename: using_with.py
from contextlib import context
@contextmanager
def opened(filename, mode="r")
f = open(filename, mode)
try:
yield f
finally:
f.close()
with opened("poem.txt") as f:
for line in f:
print(line, end='')
12.python3.0中添加了logging module，给我的感觉类似于Java中的log4j，直接看代码：
import os, platform, logging
if platform.platform().startswith('Windows'):
logging_file = os.path.join(os.getenv('HOMEDRIVE'),
os.getenv('HOMEPATH'), 'test.log')
else:
logging_file = os.path.join(os.getenv('HOME'), 'test.log')
logging.basicConfig(
level=logging.DEBUG,
format='%(asctime)s : %(levelname)s : %(message)s',
filename = logging_file,
filemode = 'w',
)
logging.debug("Start of the program")
logging.info("Doing something")
logging.warning("Dying now")
希望本文所述能对大家理解Python3.0与Python2.X一些区别性的用法有所帮助。
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