[Tutor] Re: How to accomplish Dict setdefault(key, (append to existing value list) or (start new list) )?

[Jeff Shannon]

Kristoffer Erlandsson wrote:

>Appending to a list doesn't return anything, it changes the list
>directly and returns None. So what
>
>OrderDates[OrderDate] = OrderDates.get(OrderDate, []).append(OrderID)
>
>does is to set OrderDates[OrderDate] to the returned value of append,
>which is none:
>  
>

Why, so it does.  You'd think I'd know by now that I shouldn't post 
untested code.  Ah well.

So, this leaves us with a couple of options.  One is to go back to doing 
this in several lines, but making the append() into a separate step:

for OrderDate, OrderID in Orders:
    current = OrderDates.get(OrderDate, [])
    current.append(OrderID)
    OrderDates[OrderDate] = current

Another option is to follow the old maxim that "It's easier to ask 
forgiveness than permission", and to directly catch the KeyError that 
happens when we try to access a nonexistent key:

for OrderDate, OrderID in Orders:
    try:
        OrderDates[OrderDate].append(OrderID)
    except KeyError:
        OrderDates[OrderDate] = [OrderID]

This calls append() directly on the dictionary contents.  If nothing is 
found for that OrderDate, then we get a KeyError exception, and when 
that happens we add a new list to the dictionary that contains the 
current OrderID.

Jeff Shannon
Technician/Programmer
Credit International