pyexiv2-developers team mailing list archive

[Olivier Tilloy <olivier@xxxxxxxxxx>]

Hey Damien,

Thanks for your questions. Please find my answers inline, and don't 
hesitate to ask if you have more doubts/requests!

On 2010-02-17, Damien Moore <damienlmoore@xxxxxxxxx> wrote:
> > although there is no bug report to track it, could you please file one?
>
> will do.

Thanks!

> Now I'm having some minor issues with Xmp. I doubt it matters, but I
> was working on this image:
> http://farm5.static.flickr.com/4050/4346878811_b0c030a8fa_o.jpg
>
> in python
> > > > import pyexiv2
> > > > meta=pyexiv2.ImageMetadata("test.jpg")
> > > > meta.read()
> > > > meta.xmp_keys
> []
> > > > key = 'Xmp.dc.title'
> > > > value = {'x-default': 'Sunset', 'fr': 'Coucher de soleil'}
> > > > meta[key]=pyexiv2.XmpTag(key,value)
> > > > meta[key]
> > > > meta[key]="Sunset"
> <Xmp.dc.title [Lang Alt] = {'x-default': 'snow', 'fr': 'neige'}>
> #Seems to work fine, but why can't I just use: meta[key]=value  (i.e.
> what can you do in XmpTag constructor that you can't do in
> __setitem__?). Also, using a dict is kind of verbose if I just want to
> set x-default, no?

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.
Allowing to set the value directly with an idiom such as 
meta[key]="Sunset" would be possible, but would break consistency, and 
imply defining possibly non-intuitive default behaviours. E.g., should I 
assume that the user wants to set the value, or the raw_value?

LangAlt tags always translate to dictionary-like values (because that's 
the very definition of a LangAlt in the XMP specification). Again, 
that's more work for the application developer, but provides a 
consistent API. You can still write some helper functions that either 
accept a dict or a single value, and set a dict with x-default with this 
single value.

> > > > meta["Xmp.dc.subject"]=["abc","def","ghi"]
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "pyexiv2/metadata.py", line 275, in __setitem__
>     return getattr(self, '_set_%s_tag' % family)(tag)
>   File "pyexiv2/metadata.py", line 229, in _set_xmp_tag
>     raise TypeError('Expecting an XmpTag')
> TypeError: Expecting an XmpTag
> #this is straight from your examples.py so I'm surprised this doesn't work

Where in the examples.py script did you find that?
Again, the [] operator on the ImageMetadata object only accepts 
{Exif,Iptc,Xmp}Tag objects. If you want to set the value of an existing 
tag, you can do: meta[key].value = value
If you don't know in advance whether the metadata already contains the 
tag, the following conditional code is what you want to use:

if key in meta.xmp_keys:
    meta[key].value = value
else:
    meta[key] = XmpTag(key, value)

> > > > key="Xmp.dc.subject"
> > > > value={'x-default':['snow','home']}
> > > > meta[key]=pyexiv2.XmpTag(key,value)
> > > > print meta[key]
> {'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).

> Some additional questions:
> * In the value, what is 'x-default'? (does this write to whatever the
> users locale is or just represents the absence of a locale?) Are there
> other possibilities here besides language codes?

See the "Languages Alternatives" section in the XMP specification (page 
18 of the PDF document). I'm partially quoting it:

  XMP uses the "x-default" language code to denote the default value for
  a language alternative.

  The language qualifier is a property qualifier, [...] and its value is
  a string that conforms to RFC 3066 notation.

> * Can all Xmp tags be set with the meta[key]=XmpTag(key,value) idiom?

Yes they can. If you don't want to write the conditional code suggested 
above, you can still set (or override) the value of a tag using this idiom.

> If so is value always a dict or can it be something else? The dict
> always refers to the languages? What about non-text types like dates
> or numbers, such as the Xmp.exif.DateTimeOriginal (or are they just
> stored as text too?)

As stated in the documentation (which by the way I finally completed and 
committed yesterday), the python type of the value depends on the XMP 
type of the tag. There is a types translation table in the 
documentation. For instance, LangAlt translates to dict, Date translates 
to datetime.date, ...

> * Is it ever possible to use meta[key]=value? (I gather from the
> example that it should work for Xmp.dc.subject)

No it's not allowed, it will always raise a TypeError.

> * Is there a way to discover from within the lib what keys can be set?

Not currently. It may be possible to implement depending on what 
libexiv2 exposes on this side, though.


I hope this helps.

Olivier