具有多列数据帧的值计数的单列

我想用熊猫将多列的频率求和.列的数量可以在2至15列之间变化.这是仅3列的示例:

I would like to sum the frequencies over multiple columns with pandas. The amount of columns can vary between 2-15 columns. Here is an example of just 3 columns:

code1    code2    code3
27       5        56
534      27       78
27       312      55
89       312      27

我想得到以下结果:

code    frequency
5       1
27      4
55      1
56      2
78      1
312     2
534     1

要对一列内的值进行计数不是问题，只需要一个数据帧中所有频率的总和即可显示一个值，而不管列数如何.

To count values inside one column is not the problem, just need a sum of all frequencies in a dataframe a value can appear, no matter the amount of columns.