Mongo_MultiKeyUpdate.py

from pymongo import InsertOne, DeleteOne, ReplaceOne
from pymongo.errors import BulkWriteError
from pymongo import MongoClient
client = MongoClient('localhost', 27017)
update_test = client['UpdateOne']
table = update_test.table

# drops the table to start fresh
def drop_testdb():
    client.drop_database(update_test)
    
# Example document
doc_in = {
    'append': ['one'],
    'append2' : ['a'],
    'uid': '1'
}

# Drop the table, insert the doc, query and print the result.
drop_testdb()
result = table.insert_one(doc_in)
doc_out = table.find({'uid' : '1'})
print(list(doc_out))

# Example document 2
doc_in2 = {
    'append': ['two', 'three', 'four'],
    'append2': ['b', 'c', 'd'],
    'uid': '1'}

# Update_one, but appending to multiple key in the same operation
# The append3 line appends to a key that does not exist yet.
# $push gives an error if you try to append to a key that exists but is not a list.
# I'm looking for a way to handle the situation where we have a key with a single value, 
# and we want to convert it to a list of values and then append.
table.update_one({'uid' : '1'}, {'$push' : 
                                 { 'append' : {'$each' : doc_in2['append']}, 
                                  'append2' : {'$each' : doc_in2['append2']},
                                  'append3' : {'$each' : doc_in2['append2']}
                                 }
                                },
                 upsert=True)

doc_out = table.find({'uid' : '1'})

print(list(doc_out))