Python expression of str(me.digits()) inside a string

Hey Guys, recently I was trying to use Select TOP to pick the right channel for Midi CC.

In my Midimap, the their expression be like “ch1ctrl1” or “ch1n36”… so on and so forth.
What if I want all the “ch1*” stuff using me.digits() expression?
That’s the time I found my python expression dosen’t work:

‘ch’ + str(me.digits()) + “ctrl*”

below is the screen shot of it. Thanks!

digits is a member op the OP Class, not a method.
This mean you can only call it as me.digits , not like me.digits()