calc.py

import csv
import requests


json = requests.get('https://petition.parliament.uk/petitions/171928.json').json()
petition = json['data']['attributes']['signatures_by_constituency']

with open('electorate.tsv') as f:
    r = csv.DictReader(f, delimiter='\t')
    constituencies = [x for x in r]

sigs = []
for x in petition:
    for y in constituencies:
        if x['name'].lower() == y['Constituency'].lower():
            population = y['Electorate (2015)']
            sigs.append({
                'constituency': x['name'],
                'signatures': x['signature_count'],
                'population': population,
                'signatures by population': x['signature_count'] / float(population.replace(',','')),
            })

sigs = sorted(sigs, key=lambda x: x['signatures by population'], reverse=True)

with open('out.csv', 'w') as f:
    fieldnames = [
        'constituency',
        'signatures',
        'population',
        'signatures by population'
    ]
    writer = csv.DictWriter(f, fieldnames=fieldnames)
    writer.writeheader()
    for x in sigs:
        _ = writer.writerow(x)

electorate.tsv

out.csv