pandas.Index.factorize   Index.factorize(sort=False, na_sentinel=- 1)[source]
 
Encode the object as an enumerated type or categorical variable. This method is useful for obtaining a numeric representation of an array when all that matters is identifying distinct values. factorize is available as both a top-level function pandas.factorize(), and as a method Series.factorize() and Index.factorize().  Parameters 
 
sort:bool, default False


Sort uniques and shuffle codes to maintain the relationship.  
na_sentinel:int or None, default -1


Value to mark “not found”. If None, will not drop the NaN from the uniques of the values.  Changed in version 1.1.2.     Returns 
 
codes:ndarray


An integer ndarray that’s an indexer into uniques. uniques.take(codes) will have the same values as values.  
uniques:ndarray, Index, or Categorical


The unique valid values. When values is Categorical, uniques is a Categorical. When values is some other pandas object, an Index is returned. Otherwise, a 1-D ndarray is returned.  Note Even if there’s a missing value in values, uniques will not contain an entry for it.       See also  cut

Discretize continuous-valued array.  unique

Find the unique value in an array.    Examples These examples all show factorize as a top-level method like pd.factorize(values). The results are identical for methods like Series.factorize(). 
>>> codes, uniques = pd.factorize(['b', 'b', 'a', 'c', 'b'])
>>> codes
array([0, 0, 1, 2, 0]...)
>>> uniques
array(['b', 'a', 'c'], dtype=object)
  With sort=True, the uniques will be sorted, and codes will be shuffled so that the relationship is the maintained. 
>>> codes, uniques = pd.factorize(['b', 'b', 'a', 'c', 'b'], sort=True)
>>> codes
array([1, 1, 0, 2, 1]...)
>>> uniques
array(['a', 'b', 'c'], dtype=object)
  Missing values are indicated in codes with na_sentinel (-1 by default). Note that missing values are never included in uniques. 
>>> codes, uniques = pd.factorize(['b', None, 'a', 'c', 'b'])
>>> codes
array([ 0, -1,  1,  2,  0]...)
>>> uniques
array(['b', 'a', 'c'], dtype=object)
  Thus far, we’ve only factorized lists (which are internally coerced to NumPy arrays). When factorizing pandas objects, the type of uniques will differ. For Categoricals, a Categorical is returned. 
>>> cat = pd.Categorical(['a', 'a', 'c'], categories=['a', 'b', 'c'])
>>> codes, uniques = pd.factorize(cat)
>>> codes
array([0, 0, 1]...)
>>> uniques
['a', 'c']
Categories (3, object): ['a', 'b', 'c']
  Notice that 'b' is in uniques.categories, despite not being present in cat.values. For all other pandas objects, an Index of the appropriate type is returned. 
>>> cat = pd.Series(['a', 'a', 'c'])
>>> codes, uniques = pd.factorize(cat)
>>> codes
array([0, 0, 1]...)
>>> uniques
Index(['a', 'c'], dtype='object')
  If NaN is in the values, and we want to include NaN in the uniques of the values, it can be achieved by setting na_sentinel=None. 
>>> values = np.array([1, 2, 1, np.nan])
>>> codes, uniques = pd.factorize(values)  # default: na_sentinel=-1
>>> codes
array([ 0,  1,  0, -1])
>>> uniques
array([1., 2.])
  
>>> codes, uniques = pd.factorize(values, na_sentinel=None)
>>> codes
array([0, 1, 0, 2])
>>> uniques
array([ 1.,  2., nan])