Training from .key files with only subject labels fails

[osma]

As pointed out in #303, it's impossible to train from a directory with .txt and .key files which contain only subject labels but no URIs. Example traceback with tfidf backend:

Traceback (most recent call last):
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/bin/annif", line 11, in <module>
    load_entry_point('annif', 'console_scripts', 'annif')()
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/flask/cli.py", line 569, in main
    return super(FlaskGroup, self).main(*args, **kwargs)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/flask/cli.py", line 419, in decorator
    return __ctx.invoke(f, *args, **kwargs)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/local/oisuomin/git/Annif/annif/cli.py", line 154, in run_train
    proj.train(documents)
  File "/home/local/oisuomin/git/Annif/annif/project.py", line 197, in train
    self._create_vectorizer(corpus)
  File "/home/local/oisuomin/git/Annif/annif/project.py", line 186, in _create_vectorizer
    self._vectorizer.fit((subj.text for subj in subjectcorpus.subjects))
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/sklearn/feature_extraction/text.py", line 1631, in fit
    X = super().fit_transform(raw_documents)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/sklearn/feature_extraction/text.py", line 1058, in fit_transform
    self.fixed_vocabulary_)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/sklearn/feature_extraction/text.py", line 989, in _count_vocab
    raise ValueError("empty vocabulary; perhaps the documents only"
ValueError: empty vocabulary; perhaps the documents only contain stop words

With fasttext backend:

Traceback (most recent call last):
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/bin/annif", line 11, in <module>
    load_entry_point('annif', 'console_scripts', 'annif')()
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/flask/cli.py", line 569, in main
    return super(FlaskGroup, self).main(*args, **kwargs)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/flask/cli.py", line 419, in decorator
    return __ctx.invoke(f, *args, **kwargs)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/local/oisuomin/git/Annif/annif/cli.py", line 154, in run_train
    proj.train(documents)
  File "/home/local/oisuomin/git/Annif/annif/project.py", line 198, in train
    self.backend.train(corpus, project=self)
  File "/home/local/oisuomin/git/Annif/annif/backend/fasttext.py", line 108, in train
    self._create_model()
  File "/home/local/oisuomin/git/Annif/annif/backend/fasttext.py", line 103, in _create_model
    self._model = fastText.train_supervised(trainpath, **params)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-G8ShVyyO/lib/python3.5/site-packages/fastText/FastText.py", line 343, in train_supervised
    fasttext.train(ft.f, a)
ValueError: Empty vocabulary. Try a smaller -minCount value.

The problem seems to be that subject labels are not being converted to URIs internally, although they should.

[osma]

This has now been fixed, thanks to @mo-fu for pointing it out!