pyexiv2-developers team mailing list archive

[Damien Moore <damienlmoore@xxxxxxxxx>]

to be clear... when I said:

"This is a break from dictionary syntax, but I guess it makes sense
that the user needs
to be explicit about adding new keys."

I meant

"This is a break from [the old] dictionary syntax, but I guess it
makes sense that the user needs
to be explicit about adding new keys."


On Wed, Feb 17, 2010 at 2:54 PM, Damien Moore <damienlmoore@xxxxxxxxx> wrote:
> Sorry for all of the questions. As you've probably gathered I'm a
> little clueless about the XMP spec...
>
>> meta[key] always returns a {Exif,Iptc,Xmp}Tag object. That's a bit more code
>> to write for application developers, but that's also much more flexible: a
>> tag object contains more information than just a value.
>
> I understand that evaluating meta[key] (i.e. using __getitem__),
> should return a *Tag object. But...
> (thinking out loud) I'm not sure why meta[key]=value (i.e.
> __setitem__) shouldn't be interpretable as
> meta[key]=*Tag(key,value). For example:
>
> def __setitem__(self,key,value):
>    try:
>        #try setting the key assuming that value is a *Tag type
>    except TypeError:
>        tag_class=lookup_tag_type(key)
>        if value.__class__ != tag_class:
>            value=tag_class(key,value)
>            #try setting again
>
> Is the issue that lookup_tag_type is expensive?
>
> I agree that direct support for this is probably no big deal because I
> can do what I need in helper functions.
>
>> should I assume that the user wants
>> to set the value, or the raw_value?
>
> I guess always assume a value because that is how the *Tag __init__
> call works right?
> If someone wants to do something more complicated they can use the full idiom.
>
>> if key in meta.xmp_keys:
>>    meta[key].value = value
>> else:
>>    meta[key] = XmpTag(key, value)
>
> I gather this is also true for exif and iptc keys? This is a break
> from dictionary syntax, but I guess it makes sense that the user needs
> to be explicit about adding new keys. (Again, not a big deal, but
> something that wasn't immediately obvious). I guess I don't like the
> redundancy of having to refer to the key twice when adding it (invites
> someone to try: meta[key1]=XmpTag(key2,value)). A simple alternative
> would be:
>
> meta.add(XmpTag(key,value))
>
> btw, setting the value property will update raw_value and vice versa right?
>
>
>>> {'x-default': ['test2', 'test3']}
>>> #works as expected
>>
>> Actually, it shouldn't. According to the XMP specification, the type of
>> Xmp.dc.subject is "bag Text", meaning it expects a list of values, not a
>> dictionary (it's not a LangAlt).
>
> So for key=="Xmp.dc.subject" this should work:
> meta[key]=XmpTag(key,['test2','test3'])
>
> Please don't interpret these comments/questions as a request to change
> anything. It's just taking a little bit to get my head around the new
> API.
>
> cheers,
> Damien
>