如何改变dataframe中的列顺序？

df = pd.DataFrame(np.arange(20).reshape(-1, 5), columns=list('abcde'))

print(df)
# 交换col1和col2
def switch_columns(df, col1=None, col2=None):
    colnames = df.columns.tolist()
    i1, i2 = colnames.index(col1), colnames.index(col2)
    colnames[i2], colnames[i1] = colnames[i1], colnames[i2]
    return df[colnames]

df1 = switch_columns(df, 'a', 'c')
print(df1)

#>	    a   b   c   d   e
	0   0   1   2   3   4
	1   5   6   7   8   9
	2  10  11  12  13  14
	3  15  16  17  18  19
#>	    c   b   a   d   e
	0   2   1   0   3   4
	1   7   6   5   8   9
	2  12  11  10  13  14
	3  17  16  15  18  19