Asymmetric difference of two pandas Dataframes

pandas_diff.py

def diff_df(df1, df2, how="left"):
    """
        Find Difference of rows for given two dataframes
        this function is not symmetric, means
              diff(x, y) != diff(y, x)
        however
              diff(x, y, how='left') == diff(y, x, how='right')
                
        Ref: https://stackoverflow.com/questions/18180763/set-difference-for-pandas/40209800#40209800
    """
    if (df1.columns != df2.columns).any():
        raise ValueError("Two dataframe columns must match")

    if df1.equals(df2):
        return None
    elif how == 'right':
        return pd.concat([df2, df1, df1]).drop_duplicates(keep=False)
    elif how == 'left':
        return pd.concat([df1, df2, df2]).drop_duplicates(keep=False)
    else:
        raise ValueError('how parameter supports only "left" or "right keywords"')