Round

#ENCODING
import io
import sys
sys.stdout = io.TextIOWrapper(sys.stdout.buffer, encoding='utf-16')
#MONGO
from pymongo import MongoClient
client = MongoClient()
client.progzoo.authenticate('scott','tiger')
db = client['progzoo']
#PRETTY
import pprint
pp = pprint.PrettyPrinter(indent=4)
#CODE
from bson.code import Code

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Rounding is easy inside a MapReduce as it is possible to use the Math object provided by JavaScript.
More information on Math can be found here.

At the time of writing the aggregation() method has no rounding functions, though it is still doable with $mod and $multiply, as shown here by the lead project manager at MongoDB
.

MapReduce

temp = db.world.map_reduce(
        query={"name":"United Kingdom"},
        map=Code("function(){emit(this.name, this.population)}"), 
        reduce=Code("function(key, values){return values}"),
        finalize=Code("function(key,values){return Math.round(values/1000000)}"),
        out={"inline":1},
)
pp.pprint(
   temp["results"]
)

Aggregate

pp.pprint(list(
    db.world.aggregate([
        {"$match": {"name":"United Kingdom"}},
        {"$project":{
            "_id":1, "name":1, "population":{"$divide":["$population",1000000]}
        }},
        {"$project":{
            "_id":0, "name":1, "population in millions":{"$divide":[
                {"$subtract":[
                    {"$multiply":['$population',100]},
                    {"$mod":[{"$multiply":['$population',100]}, 1]}
                    ]},100
            ]}
        }}
    ])
))