Setdefault bypasses __setitem__

[Ron Garret]

Is this a bug or a feature?

class mydict(dict):
def __setitem__(sel f, key, val):
print 'foo'
dict.__setitem_ _(self, key, val)
[color=blue][color=green][color=darkred]
>>> d=mydict()
>>> d[1]=2[/color][/color][/color]
foo[color=blue][color=green][color=darkred]
>>> d.setdefault(2, 3)[/color][/color][/color]
3

rg

[Diez B. Roggisch]

Re: Setdefault bypasses __setitem__

Ron Garret wrote:[color=blue]
> Is this a bug or a feature?
>
> class mydict(dict):
> def __setitem__(sel f, key, val):
> print 'foo'
> dict.__setitem_ _(self, key, val)
>
>[color=green][color=darkred]
>>>>d=mydict( )
>>>>d[1]=2[/color][/color]
>
> foo
>[color=green][color=darkred]
>>>>d.setdefaul t(2,3)[/color][/color][/color]


Feature. If it wouldn't bypass __setitem__, how exactly would you make a
default-item? Using __setitem__ implies a key. So if setdefault
was implemented as

def setdefault(self , v):
self["SOME_DEFAULT_K EY_NAME"] = v

and later on one writes e.g. a HTML-page with a form input field named
"SOME_DEFAULT_K EY_NAME" that gets stored in a dict - it would overwrite
the default value.

So it has to bypass __setitem__, as otherwise it can't distinguish
between "real" and the default value - the latter one is not allowed to
have a key that is in any imaginable way used by the user.

Diez

[Peter Otten]

Re: Setdefault bypasses __setitem__

Diez B. Roggisch wrote:
[color=blue]
> Ron Garret wrote:[color=green]
>> Is this a bug or a feature?
>>
>> class mydict(dict):
>> def __setitem__(sel f, key, val):
>> print 'foo'
>> dict.__setitem_ _(self, key, val)
>>
>>[color=darkred]
>>>>>d=mydict ()
>>>>>d[1]=2[/color]
>>
>> foo
>>[color=darkred]
>>>>>d.setdefau lt(2,3)[/color][/color]
>
>
> Feature. If it wouldn't bypass __setitem__, how exactly would you make a
> default-item? Using __setitem__ implies a key. So if setdefault
> was implemented as
>
> def setdefault(self , v):
> self["SOME_DEFAULT_K EY_NAME"] = v
>
> and later on one writes e.g. a HTML-page with a form input field named
> "SOME_DEFAULT_K EY_NAME" that gets stored in a dict - it would overwrite
> the default value.
>
> So it has to bypass __setitem__, as otherwise it can't distinguish
> between "real" and the default value - the latter one is not allowed to
> have a key that is in any imaginable way used by the user.[/color]

The implementation is certainly a design decision. setdefault() could be
implemented in terms of __set/getitem__() as

def setdefault(self , key, value=None):
try:
return self[key]
except KeyError:
self[key] = value
return self[key]

I guess it's not done for performance reasons.

Peter

[Diez B. Roggisch]

Re: Setdefault bypasses __setitem__

> The implementation is certainly a design decision. setdefault() could be[color=blue]
> implemented in terms of __set/getitem__() as
>
> def setdefault(self , key, value=None):
> try:
> return self[key]
> except KeyError:
> self[key] = value
> return self[key]
>
> I guess it's not done for performance reasons.[/color]

Nope. What if you changed your default value? Then you'd have to update
the whole dictionary - but without keeping track of the keys you placed
the default value under that isn't possible. Which strikes me as
more-than-marginal overhead - without any advantage (as using
__setitem__ for the default value isn't something I consider being a
missing feature...)

Diez

[Peter Otten]

Re: Setdefault bypasses __setitem__

Diez B. Roggisch wrote:
[color=blue][color=green]
>> The implementation is certainly a design decision. setdefault() could be
>> implemented in terms of __set/getitem__() as
>>
>> def setdefault(self , key, value=None):
>> try:
>> return self[key]
>> except KeyError:
>> self[key] = value
>> return self[key]
>>
>> I guess it's not done for performance reasons.[/color]
>
> Nope. What if you changed your default value? Then you'd have to update
> the whole dictionary - but without keeping track of the keys you placed
> the default value under that isn't possible. Which strikes me as
> more-than-marginal overhead - without any advantage (as using
> __setitem__ for the default value isn't something I consider being a
> missing feature...)[/color]

Are we talking about the same setdefault()?

setdefault(...)
D.setdefault(k[,d]) -> D.get(k,d), also set D[k]=d if k not in D

There is no per-instance default value just on per call:
[color=blue][color=green][color=darkred]
>>> d = {}
>>> d.setdefault("a ", 1)[/color][/color][/color]
1[color=blue][color=green][color=darkred]
>>> d.setdefault("a ", 42)[/color][/color][/color]
1

I'm sure there is a misunderstandin g in our conversation, I'm just not able
to nail it...

Peter

[Fredrik Lundh]

Re: Setdefault bypasses __setitem__

Peter Otten wrote:
[color=blue]
> Are we talking about the same setdefault()?
>
> setdefault(...)
> D.setdefault(k[,d]) -> D.get(k,d), also set D[k]=d if k not in D[/color]

note that it might be spelled "setdefault ", but it should be pronounced
"get or set".

</F>

[Duncan Booth]

Re: Setdefault bypasses __setitem__

Diez B. Roggisch wrote:
[color=blue]
> So if setdefault
> was implemented as
>
> def setdefault(self , v):
> self["SOME_DEFAULT_K EY_NAME"] = v[/color]

if setdefault was implemented that way then all current uses of setdefault
would throw an exception.

setdefault takes *three* parameters: self, key, value. Once you include the
key parameter your entire argument implodes.

[Diez B. Roggisch]

Re: Setdefault bypasses __setitem__

[color=blue]
> Are we talking about the same setdefault()?
>
>
> D.setdefault(k[,d]) -> D.get(k,d), also set D[k]=d if k not in D
>
> There is no per-instance default value just on per call:[/color]

Oh. You're right. I was somehow under the impression that setdefault is
per-instance, so that I can avoid

d.get(key, default)

and write

d[key]

instead, for all keys, and get no more KeyErrors. But then you are
right of course.

Regards,

Diez

[Diez B. Roggisch]

Re: Setdefault bypasses __setitem__

Duncan Booth wrote:[color=blue]
> Diez B. Roggisch wrote:
>
>[color=green]
>>So if setdefault
>>was implemented as
>>
>>def setdefault(self , v):
>> self["SOME_DEFAULT_K EY_NAME"] = v[/color]
>
>
> if setdefault was implemented that way then all current uses of setdefault
> would throw an exception.
>
> setdefault takes *three* parameters: self, key, value. Once you include the
> key parameter your entire argument implodes.[/color]

Yup. It does implode, leaving me thunderstruck because of my dumbness.

I rarely find things in python strange or named incorrectly, but this is
IMHO such a case - setdefault led me to think that using it would set a
default value to return for _future_ lookups of non-existant keys. That
semantics is known in e.g. ruby or java.

I think a better name would be getdefault, or even get_setdefault - in
oppposition to the get(key, d) form.

But now that this became clear to me... I guess I can live with the name :)

Diez

[Fredrik Lundh]

Re: Setdefault bypasses __setitem__

Diez B. Roggisch wrote:
[color=blue]
> I rarely find things in python strange or named incorrectly, but this is
> IMHO such a case - setdefault led me to think that using it would set a
> default value to return for _future_ lookups of non-existant keys. That
> semantics is known in e.g. ruby or java.
>
> I think a better name would be getdefault, or even get_setdefault - in
> oppposition to the get(key, d) form.
>
> But now that this became clear to me... I guess I can live with the name :)[/color]

as long as you pronounce it correctly (see my earlier post).

</F>