np.unique()

np.unique(ar,return_index=False,return_inverse=False,return_counts=False,axis=None,)

• np.nan is included in np.unique()
  

Series.unique()

• Series.unique() contains missing values
  

Series.nunique(), DataFrame.nunique()

• Series.nunique(), DataFrame.nunique() do not contain missing values ​​by default, you can choose whether to include missing values ​​through the parameter dropna