python dict update trouble

Question

i'm using Python with MongoDB

why this code is working:

for record in connection.collection.find():
    mydict = dict(record)
    mydict.update({"key": "value"})
    mylist.append(mydict)

result:

{"data": [{"anotherkey": "anothervalue"},{"key": "value"}]}

and this code is not working

for record in connection.collection.find():
    mydict = dict(record).update({"key": "value"})
    mylist.append(mydict)

result:

{"data": [null, null]}

Answer 1

Because dict.update() is in-place, it does not return anything. So when you do -

mydict = dict(record).update({"key": "value"})

mydict is actually None , as if a function does not return anything in python, it by default returns None .

And then when you do - mylist.append(mydict) - you are just appending None (in the second case) .

Answer 2

update is an inplace operation mutating the original object:

update([other])

Update the dictionary with the key/value pairs from other, overwriting existing keys. Return None .

You can use ** to get the behaviour you want:

mydict = dict(record,**{"key": "value"})
 mylist.append(mydict)