__getattr__ on non-instantiated class

[Fredp]

Hi
I was wondering if it is possible to have the various magic methods,
mainly __getattr__ and __setattr__, and @property attributes called
when accessing the attribute of a non-intantiated class.

Imagin something like this:
#####
class MyClass:
@property
def prop(self):
print "Accessed"
return "ABCD"

print MyClass.prop
#####
having it printing:
#####
Accessed
ABCD
#####

Thanks very much

 

[Larry Bates]

Hi
I was wondering if it is possible to have the various magic methods,
mainly __getattr__ and __setattr__, and @property attributes called
when accessing the attribute of a non-intantiated class.

Imagin something like this:
#####
class MyClass:
@property
def prop(self):
print "Accessed"
return "ABCD"

print MyClass.prop
#####
having it printing:
#####
Accessed
ABCD
#####

Thanks very much

Looks like you want Python to execute a method on an uninstantiated
class. I can't imagine how you would use such a thing. Can you
give us a real-life "use case"?

This produces the output you want:

m=MyClass()
print m.prop()

-Larry Bates

 

[Fredp]

Larry Bates ha scritto:

Looks like you want Python to execute a method on an uninstantiated
class. I can't imagine how you would use such a thing. Can you
give us a real-life "use case"?

This produces the output you want:

m=MyClass()
print m.prop()

-Larry Bates

I have something like a simple ORM which objects haven't a fixed number
of fields, and I need to have properties (or methods) for each of them,
but currently it is more comfortable for me to use uninstantiaded
classes (as someway SQLObject does).
I guess I'd better taking another approach to him, maybe using
something from ASPN cookbook :-\

 

[bruno at modulix]

Looks like you want Python to execute a method on an uninstantiated
class.

s/uninstantiated//

Python's classes are objects, and as such can have attributes and
methods (read about staticmethod or classmethod).

I can't imagine how you would use such a thing.

I do - that's something I do quite frequently.

This produces the output you want:

m=MyClass()
print m.prop()

But this is not what the OP asked for !-)

 

[bruno at modulix]

Fredp wrote:
(snip)

I have something like a simple ORM which objects haven't a fixed number
of fields, and I need to have properties (or methods) for each of them,

dumbiest possible example, but this should het you started

class Field(object):
# dummy
def __init__(self, **kw):
self.__dict__.update(kw)

class SomeORMObject(object):
# this is a class property
fields = {'toto' : Field(type='int', primary=True),
'tata' : Field(type='varchar', maxlen=255, default=''),
}

# this is a class method,
# the first param is the class object
@classmethod
def get_field(cls, fldname):
return cls.fields.get(fldname, None)


SomeORMObject.get_field('toto')

Properties won't probably cut it, but you can write custom descriptors
(google or search python.org for a description of what descriptors are -
for the record, properties are a kind of descriptor). I did use this
kind of stuff for an 'object/ldap mapper', using descriptors for ldap
attributes and some classmethod for building queries etc...

but currently it is more comfortable for me to use uninstantiaded
classes (as someway SQLObject does).
I guess I'd better taking another approach to him, maybe using
something from ASPN cookbook :-\

Your approach is quite sensible IMHO - minus one detail : you should
avoid reinventing the Square Wheel(tm). There already are some Python
orms. I'm not found of SQLObject, but I've played a bit with SQLAlchemy
and it seems quite promising.

HTH

 

[Fredp]

Well, actually I need only a tiny subset of a ORM. Or perhaps what I
need isn't exactly an ORM, because I have only one kind of Object with
a various number of Fields on a separate Table, so I was looking to
create something that could fit this scheme and nothing more, but I see
in any case it needs much work.
SQLObject is a bit unflexible for me (well, I didn't go too deep in
it), so I'll give a look to SqlAlchemy, that seems very interesting -
though I don't like its "look'n'feel" very much.

Thank you very much

-Federico Pelloni