code: def nunique(self, dropna=True):
          
          uniqs = self.unique()
          n = len(uniqs)
          if dropna and isna(uniqs).any():
              n -= 1
          return n
docs: Return number of unique elements in the object.
  
          Excludes NA values by default.
  
          Parameters
          ----------
          dropna : bool, default True
              Don't include NaN in the count.
  
          Returns
          -------
          int
  
          See Also
          --------
          DataFrame.nunique: Method nunique for DataFrame.
          Series.count: Count non-NA/null observations in the Series.
  
          Examples
          --------
          >>> s = pd.Series([1, 3, 5, 7, 7])
          >>> s
          0    1
          1    3
          2    5
          3    7
          4    7
          dtype: int64
  
          >>> s.nunique()
          4