Dynamically creating attributes and methods

[user]

Hi,

I'm writing a DBI for my company's database.
I will have one class for each of the tables
in the database.

I would then like each class, once instantiated,
to be able to read the field list from it's table,
and create properties based on each field, with
get/set methods created for accessing.

The get/set methods could be the same for every
field, provided that I can determine the name
of the field(property) being accessed.

Given a field name such as 'phone', how can I
give the class a home property, and then attach
that property to the get/set methods on the fly?

Thanks,

Toby

 

[John Roth]

Hi,

I'm writing a DBI for my company's database.
I will have one class for each of the tables
in the database.

I would then like each class, once instantiated,
to be able to read the field list from it's table,
and create properties based on each field, with
get/set methods created for accessing.

The get/set methods could be the same for every
field, provided that I can determine the name
of the field(property) being accessed.

Given a field name such as 'phone', how can I
give the class a home property, and then attach
that property to the get/set methods on the fly?

Thanks,

Toby

The easiest method to understand is to build
the desired method in a string and use exec() to
create a function object. Then use setattr() to
insert the function object into the class object.
Then you can create the property object and
insert it into the class the same way.

John Roth

 

[Alex Martelli]

Hi,

I'm writing a DBI for my company's database.
I will have one class for each of the tables
in the database.

I would then like each class, once instantiated,
to be able to read the field list from it's table,
and create properties based on each field, with
get/set methods created for accessing.

OK -- I think you can find already-made Python
modules that do such kinds of jobs, but still,
it's probably OK to do the job yourself, too.

It is not clear to me if you want to regenerate
all the classes and their attributes/properties
dinamycally every time your program starts, and
why -- generating Python source code once, then
importing it on each run (regenerating the source,
perhaps automatically, if and when you determine
the database's schema has changed.

I assume the source-generation approach is clear
to you and will sketch how attack the "rebuild
dynamically each time the program starts" one.

The get/set methods could be the same for every
field, provided that I can determine the name
of the field(property) being accessed.

Sure, and that's the key idea -- use a closure.

Given a field name such as 'phone', how can I
give the class a home property, and then attach
that property to the get/set methods on the fly?

I'm not sure what "a home property" is. In
context it would seem to make more sense to ask
for "a property named 'phone'".

E.g., a trivial example...:

def add_property(cls, name):
def get(self):
return getattr(self, '_'+name)
def set(self, value):
setattr(self, '_'+name, value)
setattr(cls, name, property(get, set))

this shows how to add a property that's a pair
of trivial accessors which just map get and set
to an underlying data attribute which has the
same name plus a leading underscore; of course,
in your case the get and set will be way more
elaborate, but I hope this might have all the
elements you might have been missing -- setattr,
getattr, closures, the property descriptor type
and how it's used...


Alex

 

[Sean Ross]

Hi,

I'm writing a DBI for my company's database.
I will have one class for each of the tables
in the database.

I would then like each class, once instantiated,
to be able to read the field list from it's table,
and create properties based on each field, with
get/set methods created for accessing.

The get/set methods could be the same for every
field, provided that I can determine the name
of the field(property) being accessed.

Given a field name such as 'phone', how can I
give the class a home property, and then attach
that property to the get/set methods on the fly?

Thanks,

Toby

Hi.
You may find this relevant:
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/157768

HTH,
Sean