urlib usage from Python 3

Question

I get the following error:

TypeError: POST data should be bytes, an iterable of bytes, or a file object. It cannot be of type str.

When making the call below

import urllib.request, urllib.parse, urllib.error, urllib.request, 
urllib.error, urllib.parse
import json

chemcalcURL = 'http://www.chemcalc.org/chemcalc/em'

# Define a molecular formula string
mfRange = 'C0-100H0-100N0-10O0-10'
# target mass
mass = 300

# Define the parameters and send them to Chemcalc
# other options (mass tolerance, unsaturation, etc.
params = {'mfRange': mfRange,'monoisotopicMass': mass}


response = urllib.request.urlopen(chemcalcURL, urllib.parse.urlencode(params))

# Read the output and convert it from JSON into a Python dictionary
jsondata = response.read()
data = json.loads(jsondata)

print(data)

Answer 1

You have to convert your request to bytes which involves the use of bytes() arguement:

response = urllib.request.urlopen(chemcalcURL, bytes(urllib.parse.urlencode(params), encoding="utf-8")

bytes() must take an encoding which for websites is almost always utf-8.