It's easier to ask for forgiveness than permission!
#check whether int conversion will raise an error
if not isinstance(s, str) or not s.isdigit:
return None
elif len(s) > 10: # nonsense in Python !
return None
else:
return int(s)
Better:
try:
return int(s)
except (TypeError, ValueError, OverflowError): #int conversion failed
return None