inheritance and how to use it

Dave Angel davea at
Fri Feb 15 19:03:49 CET 2013

On 02/15/2013 12:23 PM, Bob Brusa wrote:
> Am 15.02.2013 18:11, schrieb Dave Angel:
>> On 02/15/2013 11:59 AM, Bob Brusa wrote:
>>> Hi,
>>> I use a module downloaded from the net. Now I want to build my own
>>> class, based
>>> on the class SerialInstrument offered in this module - and in my class
>>> I would
>>> like to initialize a few things, using e. g. the method clear()
>>> offered by
>>> SerialInstrument. Hence I type:
>>> class myClass(SerialInstrument)
>>>       self.clear(self)
>>>       def f1(self, str1, str2)
>>>  something etc.
>>> I then get the message "self not know" from the statement
>>> self.clear(self). I
>>> have tried many other notations - none worked. What works is however the
>>> following code - specifying myClass without the self.clear(self) in it:
>>> x = myClass("argument")
>>> x.clear()
>>> How can I integrate this call into the definition of myClass? Thanks
>>> for advice.
>>> Bob
>> By initialize, I'll assume you want this code to execute when your class
>> is instantiated.  The way to do that is with a method called __init__().
>>   Notice the double underscore at begin and end.
>> class myClass(SerialInstrument):
>>      def __init__(self):
>>          self.val1 = 42
>>          self.val2 = 31
>>      #...   also initialize the base class
>>          self.clear()
>>      def f1(self, str1, str2):
>>              ....
>> You should also call the __init__() method of the base class.  But I
>> don't know whether you're using Python2 or Python3, so I won't write
>> that call
>> This is without knowing anything about your base class, so there may be
>> many other adjustments to be made.
> I defined (which should clear visa-buffers when instantiating the class):
> class myvisa(visa.SerialInstrument):
>      def __init__ (self):
>          self.clear()

I still don't see the call to the superclass __init__().  Get that code 
from Bob Brusa's message.

>      def io (self, printstr, askstr):
> ...cut
> when I run (python 2.7) a program using this class I get this:
> C:\Projekte\TDSsw\mypython>python
> Traceback (most recent call last):
>    File "", line 8, in <module>
>      from myvisa import *
>    File "C:\Projekte\TDSsw\mypython\", line 15, in <module>
>      class myvisa(visa.SerialInstrument):
>    File "C:\Projekte\TDSsw\mypython\", line 121, in myvisa
>      visa.Instrument.clear()
> TypeError: unbound method clear() must be called with Instrument
> instance as first argument (got nothing instead)
> would it help to define instead:
> class myvisa(visa.SerialInstrument):
>      def __init__ (self):
>      x = SerialInstrument(self)
>          x.clear()    #and then forget about this x?
>      def io (self, printstr, askstr):
> ...cut

Besides being indented wrong (did you even try it ?), that code doesn't 
begin to be what you want.  You're calling clear on some other instance, 
then throwing that instance away, and not clearing the one you just created.

But as I said before, you haven't said word-one about what the base 
class looks like, or how it's supposed to be used, nor when clear() is 
supposed to be called.

You also aren't showing us the code which got the error, so I can't see 
how we could help.  What does the code around line 115 look like?  Is it 
part of the same class definition?


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