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...
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