Python - inheriting from old-style classes

Question

I am trying to connect via telnet to a laboratory instrument. I'd like to extend the Telnet class from the telnetlib module in the standard library, to include functions specific to our instrument:

import telnetlib
class Instrument(telnetlib.Telnet):
    def __init__(self, host=None, port=0, timeout=5):
        super(Instrument,self).__init__(host, port, timeout)

All I am trying to do in this code is inherit the __init__ method from the parent class (telnetlib.Telnet) and pass on the standard arguments, so I can add things to __init__ later. This formula has worked for me on other occasions; this time it gives me an error at the super() statement when I try to instantiate:

TypeError: must be type, not classobj

I looked at the source code for telnetlib, and Telnet appears to be an old-style class (it doesn't inherit from object) - I'm wondering if this could be the source of my problem? If so, how can it be overcome? I've seen some code examples where the derived class inherits both from the superclass and object, though I'm not entirely sure if this is a response to the same problem as me.

Full disclosure: I have also tried using telnetlib.Telnet in place of super(), and from telnetlib import Telnet with Telnet in place of super(). The problem persists in these cases.

Thanks!

Solution 1

You need to call the constructor like this:

telnetlib.Telnet.__init__(self, host, port, timeout)

You need to add the explicit self since telnet.Telnet.__init__ is not a bound method but rather an unbound method, i.e. witout an instance assigned. So when calling it you need to pass the instance explicitely.

>>> Test.__init__
<unbound method Test.__init__>
>>> Test().__init__
<bound method Test.__init__ of <__main__.Test instance at 0x7fb54c984e18>>
>>> Test.__init__()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unbound method __init__() must be called with Test instance as first argument (got nothing instead)

Solution 2

You have to inherit from object, and you must put it after the old-style class you are trying to inherit from (so that object's methods aren't found first):

>>> class Instrument(telnetlib.Telnet,object):
...     def __init__(self, host=None, port=0, timeout=5):
...         super(Instrument,self).__init__(host, port, timeout)
...
>>> Instrument()
<__main__.Instrument object at 0x0000000001FECA90>

Inheriting from object gives you a new-style class which works with super.