Bollinger bands calculation

def bolliger_band(df):

    df['mean'] = df['close'].rolling(20).mean()
    df['std'] = df['close'].rolling(20).std()

    df['upperband'] = df['mean'] + (df['std'] * 2)
    df['lowerband'] = df['mean'] - (df['std'] * 2)
    df = df.drop(['mean', 'std'], axis=1)
    return df