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# pandas.Index.factorize

Index.factorize(sort=False, use_na_sentinel=True)

  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.

    use_na_sentinel bool, default True

      If True, the sentinel -1 will be used for NaN values. If False, NaN values will be encoded as non-negative integers and will not drop the NaN from the uniques of the values.

      Added in version 1.5.0.

  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.

  Notes

  Reference the user guide for more examples.

  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(np.array(['b', 'b', 'a', 'c', 'b'], dtype="O"))
      >>> 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(np.array(['b', 'b', 'a', 'c', 'b'], dtype="O"),
      ...                               sort=True)
      >>> codes
      array([1, 1, 0, 2, 1])
      >>> uniques
      array(['a', 'b', 'c'], dtype=object)

  When `use_na_sentinel=True` (the default), missing values are indicated in the codes with the sentinel value `-1` and missing values are not included in uniques.

      >>> codes, uniques = pd.factorize(np.array(['b', None, 'a', 'c', 'b'], dtype="O"))
      >>> 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 `use_na_sentinel=False`.

      >>> values = np.array([1, 2, 1, np.nan])
      >>> codes, uniques = pd.factorize(values)  # default: use_na_sentinel=True
      >>> codes
      array([ 0,  1,  0, -1])
      >>> uniques
      array([1., 2.])

      >>> codes, uniques = pd.factorize(values, use_na_sentinel=False)
      >>> codes
      array([0, 1, 0, 2])
      >>> uniques
      array([ 1.,  2., nan])