无法在Python中更新字典

Question

I'm trying to add some records into a dictionary.

Initially I was doing it this way

licenses = [dict(licenseid=row[0], client=row[1], macaddress=row[2], void=row[18]) for row in db]

But I've since realized I need to do some processing to filter records from db, so I tried changing the code to:

for rec in db:
    if rec['deleted'] == False:
       licenses.update(dict(licenseid=row[0], client=row[1], macaddress=row[2], void=row[18])

That code runs without exceptions, but I only end up with the last db record in licenses, which is confusing me.

Answer 1

I think licenses is a list:

licenses = []
...

and you should append to it new dictionaries:

licenses.append(dict(...))

Answer 2

If I understand correctly, you want to add multiple records in a single dictionary, right ? Instead of making a list of dictionaries, why wouldn't you make a dictionary of lists instead?

• Start by building a list of the keys you'll need (so that you always access them in the same order).

   keys = ["licenses", "client", "macaddress", "void"] 

• Construct an empty dictionary:

   licences = dict((k,[]) for k in keys] 

• Recursively add entries to your dictionary:

   for (k,item) in row: dict[k].append(item) 

Of course, it might be easier to build a list of all your records first, and then construct a dictionary at the very end.

Answer 3

Quoth the dict.update() documentation:

update([other]) Update the dictionary with the key/value pairs from other, overwriting existing keys. Return None.

Which explains why the last update "wins". licences cannot be a list as there is no update method for lists.

Answer 4

If the code in your post is your genuine code, then you might consider replacing row with rec in the last line (the one with the update), because there are chances you're updating your dictionary with always the same values !

Edit: There's obviously something very wrong in this code, from the other answer I see that I overlooked the fact that licenses was declared as a list: so the only explanation for not having an exception is either the snippets you show are not the genuine one or all your record are so that rec['deleted'] is True (so that the update method is never called).

Answer 5

After responses, I've amended my code:

licenses = []

for row in db:
        if row.deleted == False:
            licenses.append(dict(licenseid=row[0], client=row[1], macaddress=row[2], void=row[18]))

Which now works perfectly. Thanks for spotting my stupidity! ;)