This is a similar question to Extracting numbers from string field using Python Parser of ArcMap Field Calculator?
I have a similar issue in ArcMap 10.6. I have a field named 'SIZE' that holds number values and size unit (e.g. '200 mm' or '12"').
I am trying first to extract the numbers and deal with the units after. I created a new string field named 'SIZE2', while using the same code, but when I run it the values in 'SIZE2' stay NULL.
def makestr(SIZE): numlist =  for s in SIZE: if s.isdigit(): numlist.append(s) return ''.join(numlist)