[Tutor] best way to dynamically set class variables?

[Avi Gross]

I have been reading the replies and wonder sometimes if we understand the
real question as intended.

Classes in Python can be changed in all kinds of ways even after they have
been defined and the changes take effect on any new instances created
afterward. So can instances in multiple ways. If you want to store the names
of a hundred columns in a variable or even a hundred variables, you have
ways to assign them. You can even change methods on the fly.

If what you want is even more flexibility to design the class later after
receiving more data such as the names and types of the columns in a data
table, you can either write the description as text into a temporary file
and import it, if that makes sense, or make a string to be evaluated in
memory. Both can be dangerous if you do not trust the parts added as the
code is going to be run at runtime and can do malicious things.

Python often has so many ways to do things that various ones may work better
for you. In your case, one example would be to intercept the ability to set
and get (unknown) components of a class or instance by using the right
dunder function such as __getattr__ and have it KNOW about your dynamic
variable names and control access to them. There are many ways to do this,
CAREFULLY, and some work only or differently in new style classes. Heck, you
can put all the important code in an external function called by the above
that can dynamically be made in Python at a later time. One architecture
might be to store your new info in one or more dictionaries and have that
functionality check if a valid request is made and return it. Obviously it
matters where you want the data held as in per instance or per class or
superclass and so on.

Of course, I may misunderstand your issue. But from what it sounds like,
your main request is a way to associate multiple items to be stored after a
class is created but before it is used. There are an amazing number of ways
even before you loom at more advanced methods like decorators.

-----Original Message-----
From: Tutor <tutor-bounces+avigross=verizon.net at python.org> On Behalf Of
Oscar Benjamin
Sent: Wednesday, November 7, 2018 5:33 PM
To: tutor at python.org
Subject: Re: [Tutor] best way to dynamically set class variables?

On Wed, 7 Nov 2018 at 18:35, Alan Gauld via Tutor <tutor at python.org> wrote:
>
> On 07/11/2018 14:48, Albert-Jan Roskam wrote:
>
> > What is the best way to dynamically set class variables?
>
> I think I'm maybe missing the point of your question?

I think you are as well :)

IIUC then the question is: how can I programatically/dynamically create a
class that has some attributes derived from data that is known at runtime?

Am I understanding this correctly Albert?

> > # -------
> > class Parent: pass
> > class_vars = dict(col1='str', col2='int')
> >
> > # approach 1
> > Child = type('Child', (Parent,), class_vars)

This seems fine to me. It may seem cryptic but that's only because it's
unusual to do this. You are creating a "type" and that is the constructor
for type objects.

--
Oscar
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