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

Rounding is easy inside a MapReduce as it is possible to use the Math object provided by JavaScript. At the time of writing the aggregation() method has no rounding functions, though it is still doable.

from bson.code import Code
temp = db.world.map_reduce(
        query={"population":{"$ne":None}},
        limit=1,
        map=Code("function(){emit(this.name, this.population)}"), 
        reduce=Code("function(key, values){return Math.round(values/1000000)}"),
        out={"inline":1},
)

pp.pprint(
   temp["results"]
)