[MRG + 1] Clarified error msg in plot_partial_dependence (scikit-lear…

…n#7673)

* Clarified error msg in plot_partial_dependence

* Changed err msg for feature[i] out of range. Updated docs.

* Error message shows invalid value.

sklearn/ensemble/partial_dependence.py

@@ -183,10 +183,14 @@ def plot_partial_dependence(gbrt, X, features, feature_names=None,
  A fitted gradient boosting model.
  X : array-like, shape=(n_samples, n_features)
  The data on which ``gbrt`` was trained.
- features : seq of tuples or ints
+ features : seq of ints, strings, or tuples of ints or strings
  If seq[i] is an int or a tuple with one int value, a one-way
  PDP is created; if seq[i] is a tuple of two ints, a two-way
  PDP is created.
+ If feature_names is specified and seq[i] is an int, seq[i]
+ must be < len(feature_names).
+ If seq[i] is a string, feature_names must be specified, and
+ seq[i] must be in feature_names.
  feature_names : seq of str
  Name of each feature; feature_names[i] holds
  the name of the feature with index i.
@@ -306,8 +310,9 @@ def convert_feature(fx):
  l.append(feature_names[i])
  names.append(l)
  except IndexError:
- raise ValueError('features[i] must be in [0, n_features) '
- 'but was %d' % i)
+ raise ValueError('All entries of features must be less than '
+ 'len(feature_names) = {0}, got {1}.'
+ .format(len(feature_names), i))
 
  # compute PD functions
  pd_result = Parallel(n_jobs=n_jobs, verbose=verbose)(