trap attributeError from inside a dict ?

[GrelEns]

hello,

i would like if this behaviour can be obtained from python : trap an
attributeError from inside a subclassing dict class... (here is a silly
examples to explain my question)

class Test(dict):
"""subclass ing a dict and each key will permit access to a tuple of fized
size"""
def __init__(self):
pass
def load(self, adict):
"""actually it will be done from a text file with fixed size fields, each
tuple will have the same size"""
for k in adict:
self[k] = tuple(adict[k])

t = Test()
t.load({1: ('a', 'aa'), 2: ('(b', 'bb')})

here is what i would have :[color=blue][color=green][color=darkred]
>>> t[1].get('first')[/color][/color][/color]
'a'

without having to define tuple as an object or subclassing tuple, of course
now i had :[color=blue][color=green][color=darkred]
>>> t[1].get('first')[/color][/color][/color]

Traceback (most recent call last):
File "<pyshell#1 5>", line 1, in -toplevel-
t['A'].get('first')
AttributeError: 'tuple' object has no attribute 'get'

is it possible ?

btw my question is something like is there a way from inside Test code to
say to python that when it first get t[1] it should not go further, trap the
things one ask to do on t[key] and then calling a function such as

(inside Test)
def __get(self, key, readable):
mapping = {'first' : 1, 'second' : 2}
return self[key][mapping[readable]]

(maybe it is a silly design...but i need this because i do want some kind of
readable object access without having to create all these objects)

[Peter Otten]

Re: trap attributeError from inside a dict ?

GrelEns wrote:
[color=blue]
> hello,
>
> i would like if this behaviour can be obtained from python : trap an
> attributeError from inside a subclassing dict class... (here is a silly
> examples to explain my question)
>
> class Test(dict):
> """subclass ing a dict and each key will permit access to a tuple of fized
> size"""
> def __init__(self):
> pass
> def load(self, adict):
> """actually it will be done from a text file with fixed size fields,
> each
> tuple will have the same size"""
> for k in adict:
> self[k] = tuple(adict[k])
>
> t = Test()
> t.load({1: ('a', 'aa'), 2: ('(b', 'bb')})
>
> here is what i would have :[color=green][color=darkred]
>>>> t[1].get('first')[/color][/color]
> 'a'
>
> without having to define tuple as an object or subclassing tuple, of
> course now i had :[color=green][color=darkred]
>>>> t[1].get('first')[/color][/color]
>
> Traceback (most recent call last):
> File "<pyshell#1 5>", line 1, in -toplevel-
> t['A'].get('first')
> AttributeError: 'tuple' object has no attribute 'get'
>
> is it possible ?
>
> btw my question is something like is there a way from inside Test code to
> say to python that when it first get t[1] it should not go further, trap
> the things one ask to do on t[key] and then calling a function such as
>
> (inside Test)
> def __get(self, key, readable):
> mapping = {'first' : 1, 'second' : 2}
> return self[key][mapping[readable]]
>
> (maybe it is a silly design...but i need this because i do want some kind
> of readable object access without having to create all these objects)[/color]

Here's one way to do it:
[color=blue][color=green][color=darkred]
>>> class Dict(dict):[/color][/color][/color]
.... translate = {"first": 0, "second": 1}
.... def __getitem__(sel f, key):
.... value = dict.__getitem_ _(self, key[0])
.... return value[self.translate[key[1]]]
....[color=blue][color=green][color=darkred]
>>> d = Dict([(1,("a", "aa")), (2, ("b", "bb"))])
>>> d[1, "first"][/color][/color][/color]
'a'[color=blue][color=green][color=darkred]
>>> d[2, "second"][/color][/color][/color]
'bb'

Here's another:
[color=blue][color=green][color=darkred]
>>> class Dict(dict):[/color][/color][/color]
.... def __getitem__(sel f, key):
.... return Wrap(dict.__get item__(self, key))
....[color=blue][color=green][color=darkred]
>>> class Wrap(object):[/color][/color][/color]
.... def __init__(self, data): self.data = data
.... first = property(lambda self: self.data[0])
.... second = property(lambda self: self.data[1])
....[color=blue][color=green][color=darkred]
>>> d = Dict([(1,("a", "aa")), (2, ("b", "bb"))])
>>> d[1].first[/color][/color][/color]
'a'[color=blue][color=green][color=darkred]
>>> d[2].second[/color][/color][/color]
'bb'[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

Note that you might end with *more* objects as every call to __getitem__()
creates a new Wrap instance. A cache could help, but if you did it properly
with a custom class for the values the problem wouldn't arise.
Personally, I would put custom objects into a normal dictionary instead of
playing one of these tricks.

Peter

[GrelEns]

Re: trap attributeError from inside a dict ?


"Peter Otten" <__peter__@web. de> a écrit dans le message de news:
c2mp83$b4l$02$1 @news.t-online.com...
[color=blue]
> Personally, I would put custom objects into a normal dictionary instead of
> playing one of these tricks.[/color]

you convince me thanks i will do it this way,

best regards