python中的类定义语法

Question

I am very new to python.I am working with some python code.I am trying to map the python object oriented concepts to those of C++ which I think is a good way to learn.I can across two types of class definitions.

class SourcetoPort(Base):
    """"""
    __tablename__ = 'source_to_port'
    id = Column(Integer, primary_key=True)
    port_no        = Column(Integer)
    src_address    = Column(String)

    #----------------------------------------------------------------------
    def __init__(self, src_address,port_no):
        """"""
        self.src_address = src_address    
    self.port_no     = port_no

and the second one.

class Tutorial (object):
  def __init__ (self, connection):
    print "calling Tutorial __init__"
    self.connection = connection
    connection.addListeners(self)
    self.mac_to_port = {} 
    self.matrix={} 

I want to know what is the difference between the Base in SourcetoPort and object in Tutorial?

Answer 1

In Python 2.2 new style classes were introduced which should have object as a parent. Without having object as a (grand)parent it'll be an old style class. In Python 3 all classes are "new".

Inheritance from the object gives many nice features including descriptors, properties etc. Even if you're not going to use them, it's good idea to inherit object anyway.

Answer 2

You seem to be using SQLAlchemy in the first case. You definitely could not miss the difference in declaration (or, rather, execution).

Beside the fact that Python classes are rather different from classes in static languages , your SourcePort class depends on a metaclass.

A metaclass is essentially a function that can alter or dynamically generate class contents. It's somehow reminiscent of C++ templates, but acting at runtime (in Python, everything happens at runtime).

So that strange Base class, or some of its parents, has a metaclass bound to it. After the class SourcePort... statement is executed, the contents of SourcePort class are modified by the metaclass. The metaclass reads the initial attributes that explain table name, columns, etc, and adds to SourcePort various methods to access database fields by name as if they were SourcePort 's fields, getters that might lazily load column contents (if initially declared so), setters that change the 'dirty' state of a SourcePort instance, all the mechanics that binds ORM objects to database sessions, etc.

So yes, there's a serious difference.

A bit of unsolicited advice: to better understand Python classes, stop trying to make analogies to C++ classes. They share a few traits, but have a sea of differences. Just learn about Python classes as if these were a totally alien concept.