核心区别

1.Python2 默认编码方式ACSII，Python3默认编码utf-8

导致Python3的string只支持unicode的string，Python2支持str和unicode

2.Python3采用绝对路径方式进行import

避免Python2中导入标准库与绝对路径下的文件重名引发的问题

3.Python3只采用新式类，Python2有经典类和新式类

新式类：采用就近原则

class NewStyleClassA(object):
    var = 'New Style Class A'


class NewStyleClassB(NewStyleClassA):
    pass


class NewStyleClassC(object):
    var = 'New Style Class C'


class SubNewStyleClass(NewStyleClassB, NewStyleClassC):
    pass


if __name__ == '__main__':
    print(SubNewStyleClass.mro())
    print(SubNewStyleClass.var)

[<class '__main__.SubNewStyleClass'>, <class '__main__.NewStyleClassB'>, <class '__main__.NewStyleClassA'>, <class '__main__.NewStyleClassC'>, 'object'>]
New Style Class A

经典类：采用 广度优先原则

class NewStyleClassA(object):
    var = 'New Style Class A'


class NewStyleClassB(NewStyleClassA):
    pass


class NewStyleClassC(NewStyleClassA)
    var = 'New Style Class C'


class SubNewStyleClass(NewStyleClassB, NewStyleClassC):
    pass


if __name__ == '__main__':
    print(SubNewStyleClass.mro())
    print(SubNewStyleClass.var)

[<class '__main__.SubNewStyleClass'>, <class '__main__.NewStyleClassB'>, <class '__main__.NewStyleClassC'>, <class '__main__.NewStyleClassA'>, 'object'>]
New Style Class C

4.Python3中使用更加严格的缩进

Python2中一个tab和8个空格是等价的，Python3要么都使用tab要么都是四个空格

参考链接

https://blog.csdn.net/pangzhaowen/article/details/80650478

https://www.cnblogs.com/blackmatrix/p/5630515.html

https://www.runoob.com/python/python-2x-3x.html