Passing arguments to subclasses

[John Dann]

May I ask a simple newbie question, which I presume is true, but for
which I can't readily find confirmation:

Let's say I have a parent class with an __init__ method explicitly
defined:

class ParentClass(obj ect):
def __init__(self, keyword1, keyword2):
etc

and I subclass this:

class ChildClass(Pare ntClass):
# No __init__ method explicitly defined

Now I presume that I can instantiate a child object as:

child = ChildClass(arg1 , arg2)

and arg1, arg2 will be passed through to the 'constructor' of the
antecedent ParentClass (there being no overrriding __init__ method
defined for ChildClass) and mapping to keyword1, keyword2 etc.

Have I understood this correctly?

[Bruno Desthuilliers]

Re: Passing arguments to subclasses

John Dann a écrit :

May I ask a simple newbie question, which I presume is true, but for
which I can't readily find confirmation:
>
Let's say I have a parent class with an __init__ method explicitly
defined:
>
class ParentClass(obj ect):
def __init__(self, keyword1, keyword2):
etc
>
and I subclass this:
>
class ChildClass(Pare ntClass):
# No __init__ method explicitly defined
>
Now I presume that I can instantiate a child object as:
>
child = ChildClass(arg1 , arg2)
>
and arg1, arg2 will be passed through to the 'constructor' of the
antecedent ParentClass (there being no overrriding __init__ method
defined for ChildClass) and mapping to keyword1, keyword2 etc.
>
Have I understood this correctly?

Yes. But you could have checked by yourself:

bruno@bruno:~$ python
Python 2.5.1 (r251:54863, Mar 7 2008, 03:41:45)
[GCC 4.1.2 (Ubuntu 4.1.2-0ubuntu4)] on linux2
Type "help", "copyright" , "credits" or "license" for more information.

>>class Parent(object):

.... def __init__(self, kw1, kw2):
.... print self, kw1, kw2
.... self.kw1, self.kw2 = kw1, kw2
....

>>class Child(Parent): pass

....

>>c = Child("arg1", "arg2")

<__main__.Chi ld object at 0xb7ddecccarg1 arg2

>>c.kw1

'arg1'

>>c.kw2

'arg2'

>>>

HTH

[Gary Herron]

Re: Passing arguments to subclasses

John Dann wrote:

May I ask a simple newbie question, which I presume is true, but for
which I can't readily find confirmation:
>
Let's say I have a parent class with an __init__ method explicitly
defined:
>
class ParentClass(obj ect):
def __init__(self, keyword1, keyword2):
etc
>
and I subclass this:
>
class ChildClass(Pare ntClass):
# No __init__ method explicitly defined
>
Now I presume that I can instantiate a child object as:
>
child = ChildClass(arg1 , arg2)
>
and arg1, arg2 will be passed through to the 'constructor' of the
antecedent ParentClass (there being no overrriding __init__ method
defined for ChildClass) and mapping to keyword1, keyword2 etc.
>
Have I understood this correctly?

Yes, but...

The nice things about Python is that you can use the interactive
interpreter to test such things in an instant. (And not have to wait
for a response from python-list. Like this:

>>class ParentClass(obj ect):

.... def __init__(self, keyword1, keyword2):
.... print 'ParentClass.__ init__ called with', keyword1, keyword2
....

>>class ChildClass(Pare ntClass):

.... pass
....

>>child = ChildClass(123, 456)

ParentClass.__i nit__ called with 123 456

>>>

Gary Herron

[John Dann]

Re: Passing arguments to subclasses

Thanks for the responses - they're much appreciated. And I understand
the slight impatience with questions that could possibly be answered
without recourse to a forum - I'm usually in the opposite position of
fielding many newbie questions in a forum in a completely different
field!

But don't be too harsh on this category of questions for a couple of
reasons. First, newbies don't necessarily yet have the same easy
familiarity with the Python interpreter that's implied. - personally
I'd be a little concerned as to whether any significant block of
test/example code that I'd entered into the interpreter was actually
addressing the exact question that I had in mind.

And second, the answer might have been of the 'yes, but' kind. In
other words, it might perhaps have been true in a certain sort of
simple example, but one that failed to provide the complete picture.
So sometimes it's reassuring to be able to get an authoritative
answer.

Thanks again for taking the time to answer.

[Scott David Daniels]

Re: Passing arguments to subclasses

John Dann wrote:

... the answer might have been of the 'yes, but' kind.

Well, if you really care, there is a 'yes, but' answer, but it
only has to do with multiple inheritance, and hence is a bit
esoteric for the issues you are currently addressing.

This is not meant to be a tease; I think it would take pages to
address the issue in a comprehensible way for someone who hasn't
thought about the issues. If you are curious enough to pursue it,
image some issues, design experiments, and see if you can ask a
coherent question; if not, "YAGNI" (You Aint Gonna Need It).

--Scott David Daniels
Scott.Daniels@A cm.Org