A small shell for checking Steam Market prices. Commands are `watch <url>` (add item to watch), `clear` (remove all watches), `get` (get prices), `quit`.

SteamMarket.py

import json, re, shlex, urllib.request

watch = []

def getPrice(item):
    listings = json.loads(urllib.request.urlopen(makeUrl(item)).read().decode("utf-8"))["listinginfo"]
    lowest = None
    for id, obj in list(listings.items()):
        try:
            price = obj["converted_price"]
        except KeyError:
            price = obj["price"]
        if not lowest or price < lowest:
            lowest = price
    return "£{:0,.2f}".format(lowest / 100)

def getPrices():
    for item in watch:
        print(item["name"] + ":", end=" ")
        print(getPrice(item))
    
def processUrl(url):
    match = re.search("([0-9]+)/([0-9]+)-([^\/?#]+)", url)
    if not match:
        return
    groups = match.groups()
    return {
        "app": groups[0],
        "set": groups[1],
        "name": urllib.request.unquote(groups[2])
    }

def makeUrl(item):
    return "http://steamcommunity.com/market/listings/" + item["app"] + "/" + item["set"] + "-" + urllib.request.quote(item["name"]) + "/render/?query=&start=0&count=100"

def addWatch(url):
    newItem = processUrl(url)
    if not newItem:
        return print("Couldn't decode the URL.  Paste in a Market link to watch.")
    for item in watch:
        match = True
        for prop in ["app", "set", "name"]:
            match &= item[prop] == newItem[prop]
        if match:
            return print("Already watching " + newItem["name"] + ".")
    watch.append(newItem)
    print("Added " + newItem["name"] + ".")

if __name__ == "__main__":
    while True:
        cmd = shlex.split(input("~> "))
        if len(cmd):
            if cmd[0] in ["watch", "add", "w", "a"]:
                for url in cmd[1:]:
                    addWatch(cmd[1])
                if len(cmd) == 1:
                    print("Paste in a Market link to watch.")
            elif cmd[0] in ["clear", "empty", "e"]:
                watch = []
            elif cmd[0] in ["check", "get", "c", "g"]:
                getPrices()
            elif cmd[0] in ["quit", "exit", "q", "x"]:
                break
            else:
                print("Unknown command " + cmd[0] + ".  Available: watch, clear, get, quit.")

how do you ensure that the price is always in GBP?