enhancing/wrapping an existing instance of a duck

[Neville Dempsey]

Basically I have an existing (maybe a rather large and complicated
(existing) instance) that
I want to add new member to.

Cheers
N

Hacks/attempts follow:

from math import sqrt

############ try2 ############
duck_obj = [ i*i for i in range(25) ] # OR a large sparse matrix

# I "want" to an a useful property, eg length, and retain the ducks
existing properties.
# I COULD try...
setattr(duck_obj,"length",lambda: sqrt(sum(*duck_obj)))
print duck_obj.length() # returns 70
duck_obj[0]=70+71
print duck_obj.length() # returns 71

############ try2 ############
# **BUT** I'd rather encapsulate a the original instance somehow.

# I presume that I could define a class to do this somehow?
duck_obj = [ i*i for i in range(25) ] # OR a LargeSparseMatrix()

dec = Vec(duck_obj) ???
print dec.length() # returns 70
duck_obj[0]=70+71 # original "large and complicated duck instance"
print dec.length() # returns 71

Any hints on how I need to define Vec so that any kind of duck_obj can
be decorated/wrapped/encapsulated.

 

[Bruno Desthuilliers]

Neville Dempsey a écrit :

Basically I have an existing (maybe a rather large and complicated
(existing) instance) that
I want to add new member to.

I suppose you mean "attributes" ?

Cheers
N

Hacks/attempts follow:

from math import sqrt

############ try2 ############
duck_obj = [ i*i for i in range(25) ] # OR a large sparse matrix

# I "want" to an a useful property, eg length, and retain the ducks
existing properties.
# I COULD try...
setattr(duck_obj,"length",lambda: sqrt(sum(*duck_obj)))

Won't work on a list.

print duck_obj.length() # returns 70
duck_obj[0]=70+71
print duck_obj.length() # returns 71

You obviously didn't try the above code.

Traceback (most recent call last):
File "<stdin>", line 2, in <module>
AttributeError: 'list' object has no attribute 'length'



Also and FWIW, in Python, the "sizeable" protocol is implemented using a
__len__ method, that will get called by the generic len(sizeable) function.

############ try2 ############
# **BUT** I'd rather encapsulate a the original instance somehow.
>
# I presume that I could define a class to do this somehow?
duck_obj = [ i*i for i in range(25) ] # OR a LargeSparseMatrix()

dec = Vec(duck_obj) ???
print dec.length() # returns 70
duck_obj[0]=70+71 # original "large and complicated duck instance"
print dec.length() # returns 71

Any hints on how I need to define Vec so that any kind of duck_obj can
be decorated/wrapped/encapsulated.

Depends on the type of duck_obj. In the above case, I'd inherit from
list and override __len__ :
.... def __len__(self):
.... return math.sqrt(sum(self))
....

>>> Vec(range(10)) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> len(Vec(range(10)))

6