Q:

Fastai issue with TextLMDataBunch and language_model_learner

Tried to replicate this code using fastai lib, but I'm running in two major issues. 

This par of code:
data_lm = TextLMDataBunch.from_df('data', train_df, valid_df, text_cols='idea')

Gives this kind of error:
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-39-74ec5bcc1e2a> in <module>
----> 1 data_lm = TextLMDataBunch.from_df('data', train_df, valid_df, text_cols='idea')

~\Anaconda3\lib\site-packages\fastai\text\data.py in from_df(cls, path, train_df, valid_df, test_df, tokenizer, vocab, **kwargs)
    325         k_names = ['max_vocab', 'min_freq', 'n_labels', 'txt_cols', 'label_cols', 'clear_cache']
    326         txt_kwargs, kwargs = extract_kwargs(k_names, kwargs)
--> 327         train_ds = TextDataset.from_df(path, train_df, tokenizer, 'train', vocab=vocab, **txt_kwargs)
    328         datasets = [train_ds, TextDataset.from_df(path, valid_df, tokenizer, 'valid', vocab=train_ds.vocab, **txt_kwargs)]
    329         if test_df is not None: datasets.append(TextDataset.from_df(path, test_df, tokenizer, 'test', vocab=train_ds.vocab, **txt_kwargs))

~\Anaconda3\lib\site-packages\fastai\text\data.py in from_df(cls, folder, df, tokenizer, name, **kwargs)
    150         tokenizer = ifnone(tokenizer, Tokenizer())
    151         chunksize = 1 if (type(df) == DataFrame) else df.chunksize
--> 152         return cls(folder, tokenizer, df=df, create_mtd=TextMtd. DF, name=name, chunksize=chunksize, **kwargs)
    153 
    154     @classmethod

~\Anaconda3\lib\site-packages\fastai\text\data.py in __init__(self, path, tokenizer, vocab, max_vocab, chunksize, name, df, min_freq, n_labels, txt_cols, label_cols, create_mtd, classes, clear_cache)
     35         os.makedirs(self.path, exist_ok=True)
     36         if clear_cache: self.clear()
---> 37         if not self.check_toks(): self.tokenize()
     38         if not self.check_ids():  self.numericalize()
     39 

~\Anaconda3\lib\site-packages\fastai\text\data.py in tokenize(self)
     86             df = next(dfs) if (type(dfs) == pd.io.parsers. TextFileReader) else self.df
     87             lbl_type = np.float32 if len(self.label_cols) > 1 else np.int64
---> 88             lbls = df[self.label_cols].values.astype(lbl_type) if (len(self.label_cols) > 0) else []
     89             self.txt_cols = ifnone(self.txt_cols, list(range(len(self.label_cols),len(df.columns))))
     90             texts = f'{FLD} {1} ' + df[self.txt_cols[0]].astype(str)

~\Anaconda3\lib\site-packages\pandas\core\frame.py in __getitem__(self, key)
   2680         if isinstance(key, (Series, np.ndarray, Index, list)):
   2681             # either boolean or fancy integer index
-> 2682             return self._getitem_array(key)
   2683         elif isinstance(key, DataFrame):
   2684             return self._getitem_frame(key)

~\Anaconda3\lib\site-packages\pandas\core\frame.py in _getitem_array(self, key)
   2724             return self._take(indexer, axis=0)
   2725         else:
-> 2726             indexer = self.loc._convert_to_indexer(key, axis=1)
   2727             return self._take(indexer, axis=1)
   2728 

~\Anaconda3\lib\site-packages\pandas\core\indexing.py in _convert_to_indexer(self, obj, axis, is_setter)
   1325                 if mask.any():
   1326                     raise KeyError('{mask} not in index'
-> 1327                                    .format(mask=objarr[mask]))
   1328 
   1329                 return com._values_from_object(indexer)

KeyError: '[0] not in index'

and this part of code:
learn = language_model_learner(data_lm, pretrained_model=URLs. WT103, drop_mult=0.5)
learn.fit_one_cycle(1, 1e-2)

Running in this error:
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-37-7b61575a202a> in <module>
----> 1 learn = language_model_learner(data_lm, pretrained_model=URLs. WT103, drop_mult=0.5)
      2 learn.fit_one_cycle(1, 1e-2)

NameError: name 'language_model_learner' is not defined

I had issues to install fastai, but I have no clue what's the issue. 

A:

Well, your question was 2 months ago and the library went through a lot of changes since then.  It seems to me that your first error is because you don't specify a column with labels, self.labels_cols is set to [0] and as a result, it is not in the index of your Dataframe.  I believe this behaviour has changed since your post and that today, not specifying label_cols will work as intended. 
Concerning your second issue, languague_model_learner used to be call differently before a refactor.  Were you on the latest version of fastai when trying to use it.  Feel free to test that again with the latest version of fastai and see if you get the same errors.