Python 将列名分配给dataframe中的变量时访问列数据

我有一个dataframe df，它的列名以字符串的形式存储在变量col中，现在如何按col变量中存储的列名访问列

index = 4

# This col variable has column name (str)
col = df.columns[index]

# how to access column of dataframe by column name stored in this col variable
# I am also trying to convert column data to list.
item = df[col].to_list()