Hi Ed,
This is a longish response because you are
raising some
very interesting (and deep) issues from a
computer science
point of view.
> Well, self-modifying isn't inherently
necessary. What I guess I
> really need is persistent classes as well as persistent objects.
> really need is persistent classes as well as persistent objects.
Python classes can be persistent just like
any other object,
theres nothing intrinsically different between a class object
and any other kind of object (except that classes as objects
are kind of mind bending as a concept!)
> I always tend to think of classes as
templates for objects rather
> than factories.
They are both.
> In my mind, object methods are just
calls to the class which
> are evaluated every time they are
called.
The methjods are function attributes of a
class which are selected
by the class dispatch mechanism in response
to messages sent to
instances. In Python that means the
getattr() methods look up
the right method in response to receipt of
a message.
> Objects should be strict instances of
classes that can't be
> modified except for the values of
their attributes.
Attributes in a pure OOP language include
both the contained data
and functions. In other workds methods can
be added/removed and
modified as well as the data. In fact many
would argue that the
internal data not only cannot be modified
externally but should not
even be seen - the Data Hiding school of
thought.
> Class Page(object):
> def print(self):
> printPage(self)
> Class Page(object):
> def print(self):
> printPage(self)
What does that gain you?
> And have all my methods call functions (passing on parameters as
> necessary). That way if I change a function, it will be changed for
> every instance of every object of that class.
You mean like this:
>>> class C:
... def m(self): print 'm'
...
>>> def f(o): print 'other m'
...
>>> c = C()
>>> c.m()
m
>>> C.m = f
>>> c.m()
other m
>>>
... def m(self): print 'm'
...
>>> def f(o): print 'other m'
...
>>> c = C()
>>> c.m()
m
>>> C.m = f
>>> c.m()
other m
>>>
> And couldn't I write a function that
would add functions or attributes
> to classes and objects?
> to classes and objects?
Of course, just like I did
there.
> Am I trying to use the wrong language
for this? I love Python but I
> seem to keep coming up against lots of practical issues with it
> seem to keep coming up against lots of practical issues with it
Python is a pretty good language for this
kind of thing,
its just that this kind of thing is intrinsically difficult.
The only lanuages I know of which
are better than Python
at it are Lisp and Smalltalk (and even
Smalltalk
has some limitations compared to Python).
Do you have
experience of a language which is more flexible in this regard?
I'm curious because you seem to assume that its almost
normal to be able to do these things,
whereas in my
experience its usually impossible rather
than just difficult...
> define the behaviours I want without
having to bother with how the
> computer is actually going to handle them.
> computer is actually going to handle them.
Believe it or not thats been the holy grail
of language
designers since the days of COBOL - The Common Business
Oriented Language - which allowed business people to
write theitr own programmes in
"standard, everyday English",
like:
PROGRAM ID - HELLO
PROCEDURE DIVISION
MAIN-LINE-MODULE
MOVE 0 TO COUNTER
OPEN
INPUT IN-FILE
PERFORM READ IN-FILE UNTIL END-OF-FILE = "YES"
ADD 1
TO COUNTER
DISPLAY "LINE NUMBER ", COUNTER
CLOSE
IN-FILE
STOP
RUN.
At the time it was considered revolutionary
and programmers
trembled at the prospect of instant
redundancy. As it happened
your average businessman found COBOL about
as readable as pigin
English! But that hasn't stopped us
trying...
One thing we have discovered is that the
more natural the
language seems to get the less flexible the language gets.
Python is the closest I've seen yet to a general purpose
language that is truly readable,
but thats still a long
way from being plain English! But I digress
:-)
Basically I just wonder what your
background is that you
seem to expect these kind of dynamic facilities? I'd certainly
be interested in investigating it further.
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