Re: Using a class as a structure/container

En Wed, 06 Feb 2008 00:59:48 -0200, <david.car7@xxxxxxxxx> escribi�:

Is it appropriate to use a class as a simple container in order to
access attributes using a series of dot operators? Is their a more
Pythonic way of doing this? For instance, I have a "container" class
which is never instantiated directly, but only through another class.
Access to certain attributes would be something like:

main_class.a.b.x

where row and v1 are instances of the container class which are
instantianted by main_class. I know I can use dictionaries, but this
syntax is a little clearer as long as the number of dot operators is
not too lengthy. Some code would be something like:

class container(object):
def __init__(self):
pass

You can omit the __init__ method if it's empty

class main_class(object):
def __init__(self):
self.a = container()
settatr(self.a, 'b', container())
settatr(self.a.b, 'x', 2)

Usually written as:
self.a.b = container()
self.a.b.x = 2

Nothing in the language forbids doing what you do, but I'd ask if the attributes may be split logically between the various containers, or it is an artificial division and they really should be attributes of the main_class.
There are documentation problems too: such generic container cannot tell which attributes are valid or not; introspection tools cannot tell very much about the class; perhaps some code completion tools get confused. I don't like a long sequence of dots either.
Using such a deep structure you are violating the Law of Demeter "Don't talk to strangers" http://en.wikipedia.org/wiki/Law_of_Demeter (altough I don't follow it strictly, things like xobj.parent.connection.user.name look wrong to me)

Another problem of such generic container is that you don't get any meaningful information when printing it. You may find the NamedTuples recipe useful; look into the Python Cookbook http://www.activestate.com/ASPN/Python/Cookbook/ - or perhaps using this class:

class Record(object):
def __init__(self, **kw):
self.__dict__.update(kw)

def __repr__(self):
values = ['%s=%r' % item
for item in sorted(self.__dict__.iteritems())]
return '%s(%s)' % (self.__class__.__name__, ','.join(values))

__str__ = __repr__


x = Record(a=1, b=2.0, c="three")
print x
# output: Record(a=1,b=2.0,c='three')



--
Gabriel Genellina

.

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