Creating Classes

[Steve Holden]

Dave Angel wrote:
> seafoid wrote:
>> Hey Guys,
>>
>> I have started to read over classes as a brief respite from my parsing
>> problem.
>>
>> When a class is defined, how does the class access the data upon which
>> the
>> class should act?
>>
>> Example:
>>
>> class Seq:                                                               
>>     def __init__(self, data, alphabet = Alphabet.generic_alphabet):
>>         self.data = data         self.alphabet = alphabet
>>
>>     def
>> tostring(self):                                                  
>>         return self.data                                                 
>>     def tomutable(self):
>>         return MutableSeq(self.data, self.alphabet)
>>         def count(self, item):
>>         return len([x for x in self.data if x == item])
>>
>> I know what it should do, but have no idea how to feed it the data.
>>
>> Methinks I need to invest in actual computing books as learning from
>> biologists is hazy!
>>
>> Kind regards,
>> Seafoid.
>>             
> Steve's message was good, but I feel he kind of jumped in the middle.  A
> class is a description of a type of object, and the behaviors and data
> that each instance of the object supports.
> 
> You create the object by using the class name like a function call.  The
> arguments to that "call" are passed to the __init__ method.
> 
> So  obj = Seq("abcd")    or   obj = Seq("defg", "abcdefg")    would each
> create an object of the class.  But generally, many objects will exist,
> each with different data.
> 
> The data in the object is accessed in what appears to be the "self"
> namespace.  The name self is just a convention, but it's the first
> argument of each method of the class.  So when somebody calls the
> count() method, they pass 'item' a value, but self is used to refer to
> that particular object's data.
> 
> So how does 'self' get assigned?  That's what Steve was describing. 
> When you use the syntax:
>     obj.count("value")
> 
> you actually call the count method with self referring to "obj" and item
> referring to "value".  obj does double-duty here, both defining which
> class' count() method will be called, and also supplying the first
> parameter to the call, the "self" parameter.
> 
> There are more complex things that can go on, like creating "bound"
> function objects, but  I think this should get you pretty far.
> 
> One other point:  you should always derive a class from some other
> class, or 'object' by default.  So you should being the class definition
> by:
> 
> class Seq(object):
> 
> Why?  It mainly has to do with super().  But in any case if you omit the
> 'object' it's an "old style" class, and that's not even supported in
> 3.x, so it's better to just get in the habit before it matters.
> 
With respect, unless you have to expound on the differences between the
old-style and the new-style classes (which aren't relevant here) you are
just introducing a red herring by even mentioning it. The average Python
user won't need to use super() in their first year as a Python programmer.

And, since you brought up Python 3, it's not necessary to explicitly
inherit from object to get new-style classes because, as you correctly
point out, old-style classes don't exist in Python 3.

I have no idea why you think "you should always derive a class from some
other class". That's pretty unnecessary.

regards
 Steve
-- 
Steve Holden           +1 571 484 6266   +1 800 494 3119
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