from urllib.request import urlopen

id = "P42212"
data = urlopen("http://www.uniprot.org/uniprot/" + id + ".txt").read().decode()
result = data.split('SQ   ')[-1] #here all the info you need
mw = int(result.split(';')[1].strip().split()[0]) #extract the molecular weigth
print(mw)