Sklearn pipeline throws ValueError: too many values to unpack (expected 2)

Question

I am trying to create a sklearn pipeline which will first extract the average word length in a text, and then standardize it using StandardScaler.

custom transformer

class AverageWordLengthExtractor(BaseEstimator, TransformerMixin):

    def __init__(self):
        pass
    def average_word_length(self, text):
        return np.mean([len(word) for word in text.split( )])
    def fit(self, x, y=None):
        return self
    def transform(self, x , y=None):
        return pd.DataFrame(pd.Series(x).apply(self.average_word_length))

My goal is to achieve this. X is a pandas series with text values. This works.

    extractor=AverageWordLengthExtractor()
    print(extractor.transform(X[:10]))
    sc=StandardScaler()
    print(sc.fit_transform(extractor.transform(X[:10])))

The pipeline I created for this is.

pipeline = Pipeline([('text_length', AverageWordLengthExtractor(), 'scale', StandardScaler())])

But the pipeline.fit_transform() producing below error.

Traceback (most recent call last):
  File "custom_transformer.py", line 48, in <module>
    main()
  File "custom_transformer.py", line 43, in main
    'scale', StandardScaler())])
  File "/opt/conda/lib/python3.6/site-packages/sklearn/pipeline.py", line 114, in __init__
    self._validate_steps()
  File "/opt/conda/lib/python3.6/site-packages/sklearn/pipeline.py", line 146, in _validate_steps
    names, estimators = zip(*self.steps)
ValueError: too many values to unpack (expected 2)

Answer 1

Your brackets are in the wrong place / you are missing brackets when creating the Pipeline, should be a list of tuples:

pipeline = Pipeline([
   ('text_length', AverageWordLengthExtractor()), 
   ('scale', StandardScaler())
])