import requests

url = 'http://web.expasy.org/cgi-bin/peptide_mass/peptide-mass.pl'
payload = dict(
    protein='P35225',
    mandatory='',
    reagents='nothing+%28in+reduced+form%29',
    mplus='mh',
    masses='monoisotopic',
    enzyme='Trypsin',
    MC='0',
    minmass='500',
    maxmass='unlimited',
    order='mass',
    modification='on'
)

data = requests.post(url,data=payload).text
print data