@classmethod question

[Scott SA]

Hi,

I'm using the @classemethod decorator for some convenience methods and for some reason, either mental block or otherwise, can't seem to figure out howto elegantly detect if the call is from an instance or not.

Here's the problem: Within the class definition, 'isinstance' has nothing to compare to because the class does not appear to exist.

This is NOT a great example, but it outlines the the code:

class RecipieClass:
def __init__(self):
pass

@classmethod
def get_ingrendients(self, recipie_list=None):

if isinstnace(self,RecipieClass):
return self.do_something_interesting()
else:
return do_something_boring(recipie_list)

Yes, I can test to see if the param exists, but that makes the call exclusive i.e. I can _only_ call it as an instance or with a parameter.

Why am I doing this?

It is a series of convenience methods, in this case I'm interacting with a database via an ORM (object-relational model). I want the ability to call aclass-ojbect and get related values, or pass some criteria and get relatedvalues for them without collecting the records first as instances, then iterating them. I need to call this from several places so I want to be DRY (don't repeat yourself).

The easiest way to describe this as an analogy would be like having a recipie for cookies and wanting to know all of the ingredients ahead of time. Then, at another time, wanting to know what all the ingredients would be to make cookies, cake and bread (i.e. complete shopping list).

cookie_recipie = RecipieClass.get_recipie('cookies')
cookie_recipie.get_ingredients()
2C Flour
0.5 C Sugar
...

RecipieClass.get_ingrendients(['cookies','cake','bread'])
8C Flour
2C Sugar
...

Of course any suggestions on how this might be better approached would be interesting too.

TIA,

Scott

[Ivan Illarionov]

On 24 ÁÐÒ, 07:27, Scott SA <[e-mail]:

Hi,

I'm using the @classemethod decorator for some convenience methods and forsome reason, either mental block or otherwise, can't seem to figure out howto elegantly detect if the call is from an instance or not.

Here's the problem: Within the class definition, 'isinstance' has nothing to compare to because the class does not appear to exist.

This is NOT a great example, but it outlines the the code:

class RecipieClass:
def __init__(self):
pass

@classmethod
def get_ingrendients(self, recipie_list=None):

if isinstnace(self,RecipieClass):
return self.do_something_interesting()
else:
return do_something_boring(recipie_list)

Yes, I can test to see if the param exists, but that makes the call exclusive i.e. I can _only_ call it as an instance or with a parameter.

Why am I doing this?

It is a series of convenience methods, in this case I'm interacting with adatabase via an ORM (object-relational model). I want the ability to call aclass-ojbect and get related values, or pass some criteria and get related values for them without collecting the records first as instances, then iterating them. I need to call this from several places so I want to be DRY (don't repeat yourself).

The easiest way to describe this as an analogy would be like having a recipie for cookies and wanting to know all of the ingredients ahead of time. Then, at another time, wanting to know what all the ingredients would be to make cookies, cake and bread (i.e. complete shopping list).

cookie_recipie = RecipieClass.get_recipie('cookies')
cookie_recipie.get_ingredients()
2C Flour
0.5 C Sugar
...

RecipieClass.get_ingrendients(['cookies','cake','bread'])
8C Flour
2C Sugar
...

Of course any suggestions on how this might be better approached would be interesting too.

TIA,

Scott

Hi,
It would make sense to separate instance-level and class-level
behaviour with additional 'objects' namespace. e.g.
cookie_recipie.get_ingredients() to get ingredients only for cookie
recipie and RecipieClass.objects.get_ingrendients([....]) to get all
the ingredients.

The elegant solution (AFAIK used by Django) would be to use metaclass
and the object with custom descriptor for class-object/table level
stuff.

Something like this:

class RecipieMetaclass(type):
def __new__(cls, bases, attrs):
new_cls = type.__new__(cls, name, bases, attrs)
new_cls.objects = IngredientsDescriptor(IngredientsManager())
return new_cls

class RecipieClass(object):
__metaclass__ = RecipieMetaclass
def get_ingredients(self, recipie_list=None):
return self.do_something_interesting(recipie_list)

class IngredientsManager(object):
def get_ingredients(self, recipie_list=None):
return do_something_boring(recipie_list)

class IngredientsDescriptor(object):
def __init__(self, ingredients_manager):
self.ingredients_manager = ingredients_manager
def __get__(self, instance, type=None):
if instance is not None:
raise AttributeError, "Access via %s instances is not
allowed" % type.__name__
return self.ingredients_manager

Then, "at another time, wanting to know what all the ingredients would
be to make cookies, cake and bread" you would call:
RecipieClass.objects.get_ingrendients(['cookies','cake','bread'])

Both Django and Google Apps Engine API use similar concepts and you
can learn much more interesting looking in their source code.

Regards,

--
Ivan

[Bruno Desthuilliers]

Scott SA a écrit :

Hi,

I'm using the @classemethod decorator for some convenience methods
and for some reason, either mental block or otherwise, can't seem to
figure out how to elegantly detect if the call is from an instance or
not.

Well, the point is that a classmethod *always* receive the class as
first argument, wether it's called on the class or an instance. If
that's not what you want, then you don't use the right tool.

Here's the problem: Within the class definition, 'isinstance' has

nothing to compare to because the class does not appear to exist.

>
This is NOT a great example, but it outlines the the code: >
class RecipieClass:
def __init__(self):
pass

@classmethod
def get_ingrendients(self, recipie_list=None):

if isinstnace(self,RecipieClass):
return self.do_something_interesting()
else:
return do_something_boring(recipie_list)

Yes, I can test to see if the param exists, but that makes the call
exclusive i.e. I can _only_ call it as an instance or with a parameter.

Why am I doing this?

It is a series of convenience methods, in this case I'm interacting
with a database via an ORM (object-relational model).

out of curiosity : which one ?

I want the ability
to call a class-ojbect and get related values, or pass some criteria and
get related values for them without collecting the records first as
instances, then iterating them. I need to call this from several places
so I want to be DRY (don't repeat yourself).

The easiest way to describe this as an analogy would be like having a
recipie for cookies and wanting to know all of the ingredients ahead of
time. Then, at another time, wanting to know what all the ingredients
would be to make cookies, cake and bread (i.e. complete shopping list).

cookie_recipie = RecipieClass.get_recipie('cookies')
cookie_recipie.get_ingredients()
2C Flour
0.5 C Sugar
...

RecipieClass.get_ingrendients(['cookies','cake','bread'])
8C Flour
2C Sugar
...

Of course any suggestions on how this might be better approached

would be interesting too.

Why do you want the same method to do two different things ? You clearly
have two distincts methods doing different things here, and as a user of
your code I'd find your API confusing. May I suggest a much simpler
approach:
class Recipies(object):
@property
def ingredients(self):
return <dict of ingredient:qty for self>

@classmethod
def get_ingredients_for(cls, *id_recipies):
return <dict of ingredient:summed_qty for all id_recipies>
print Recipie.get('cookie').ingredients
print Recipies.get_ingredients_for('cookie', 'cake', 'bread')

My 2 cents...

[Arnaud Delobelle]

Scott SA <py***@rscorp.ab.cawrites:

A side note

class RecipieClass:

Recipe is a more widespread spelling, I believe. Moreover it is the
convention in python that only class names are capitalized, so you
don't need to append a 'Class'.

class Recipe:
...
clafoutis = Recipe('eggs', 'spam')

--
Arnaud