Universally create Derived class from Base in python

Question

Found some questions on SO but still have no answer... There is a data class

class Proxy(object):
    def __init__(self, ip, port):
        self.ip = ip
        self.port = port

There is a getter class which is supposed to read these data from different sources

class FileProxyGetter(ProxyGetter):
    def __init__(self, fname = "d:\\proxies.txt"):
        self.fileName = fname 

    def Get(self):
        proxies = []

        f = open(self.fileName)

        for l in f.xreadlines(): 
            proxies.append(Proxy.fromstring(l[:-1]))

        f.close()

        return proxies

    def Update(self):
        return []

I need to have a Proxy class with more options like

class SecureProxy(Proxy):
    def __init__(self, ip, port):
        super(SecureProxy, self).__init__(ip, port)
        self.transparent = None

Now I want to improve FileProxyGetter as follows:

class FileSecureProxyGetter(FileProxyGetter):
    def Get(self):
        proxies = super(FileProxyGetter, self).Get()
        secureProxies = []
        for proxy in proxies:
            # Create or Cast Proxy to SecureProxy.
            # The transparent should be initialized to None or any other value that I may need
            secureProxies.append(SecureProxy(proxy))

        return secureProxies

So how do I cast or create an instance of derived class from base class in Python universally. It would be better if no changes to the classes needed.

Or can you suggest more pythonic way of developing such relationships and architecture?

Answer 1

You can use inheritance:

class FileProxyGetter(ProxyGetter):
    ...
    def MakeProxy(self, *args, **kwargs):
        return Proxy.fromstring(*args, **kwargs)
    def Get(self):
        ...
           proxies.append(self.MakeProxy(l[:-1]))
        ...
    ...
class FileSecureProxyGetter(FileProxyGetter):
    def MakeProxy(self, *args, **kwargs):
        return SecureProxy.fromstring(*args, **kwargs)

but it's probably more useful in this case to use composition.

class FileProxyGetter(ProxyGetter):
    def __init__(self, proxyclass, fname = "d:\\proxies.txt"):
        self.proxyClass = proxyclass
        self.fileName = fname
    ...
    def Get(self):
        ...
            proxies.append(self.proxyclass.fromstring(l[:-1]))
        ...
    ...

# use this as such
FileProxyGetter(Proxy, "proxies.txt")
FileProxyGetter(SecureProxy, "secure_proxies.txt")

EDIT: A dirty trick in python to switch the type of an object:

>>> class A(object):
...     def foo(self):
...         print 'hello A'
... 
>>> class B(object):
...     def foo(self):
...         print 'hello B'
... 
>>> a = A()
>>> a.foo()
hello A
>>> a.__class__
<class '__main__.A'>
>>> a.__class__ = B
>>> a.foo()
hello B

Another dirty trick for two objects of different types to share the same state:

>>> class B(object):
...     def rename(self, name):
...         self.name = name
... 
>>> class A(object):
...     def say(self):
...         print 'Hello', self.name
... 
>>> a, b = A(), B()
>>> a.__dict__ = b.__dict__
>>> b.rename('john')
>>> a.say()
Hello john
>>> a.rename('mary')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'A' object has no attribute 'rename'
>>> b.say()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'B' object has no attribute 'say'

However, these tricks, while possible in Python, I would not call them pythonic nor a good OO design.

Another possibility in Python 3.x and up, which had removed "unbound method" in place of using regular function:

>>> class A(object):
...     def say(self):
...         print('Hello', self.name)
... 
>>> class B(object):
...     def rename(self, name):
...         self.name = name + name
... 
>>> a = A()
>>> B.rename(a, 'josh')
>>> a.say()
Hello joshjosh