ArcMap Field Calculator - find and add last number

I’m trying to get a part of a string (numbers after letter) in a new table (field Description) using field calculator, but I need it only from a field next to it (field MUID) containing the first letter "P" while the rest, that contains something else will be null.

That means if MUID = P14 then Desc = 14 or MUID = P1 then Desc = 1 and if MUID = HT9 then Desc = 0.

Is there a way to write it in the Python parser?

Will need it in the future to implement it via ModelBuilder.

I tried to use this, but it keeps getting Error signs

def nums(textfield):
    if textfield[0]=='P': #If first letter is P
        newval = ''.join([n for n in textfield if n.isnumeric()]) #List all numbers and join them together to a string
        try: #If there is no integer this will prevent the code from breaking
            int(newval)
            return newval
        except ValueError: #Dont return / return NULL if there is no number
            pass