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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: Concerning Dictionaries and += in Python 2.x
Date: Wed, 21 Jan 2015 09:43:31 +0100
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Denis McMahon wrote:

> On Mon, 19 Jan 2015 16:12:57 -0800, Luke Tomaneng wrote:
> 
>> I have been having a bit of trouble with the things mentioned in the
>> title.
> 
> I've uploaded a slightly different approach to your code at:
> 
> http://www.sined.co.uk/tmp/shop.py.txt
> 
> def compute_bill(shopping):
>     """
>     Takes a dictionary of purchases requested in the form {item: quantity}
>     Returns a tuple of:
>         a dictionary of items supplied in the form {item: quantity}; and
>         the cost of the supplied items
>     """
>     # the invoice amount
>     invoice = 0
>     # what we were able to supply
>     sold = {k:0 for k in shopping.keys()}

There is also dict.from_keys()

>     # check each requested item
>     for item in shopping:
> 
>         # try and sell the requested qty
>         for i in range(shopping[item]):

The inner loop is not just inefficient for stock sold in large quantities, 
it will fail for stock sold by weight, volume etc.

>             # if we have stock remaining
>             if stock[item] > 0:
> 
>                 # reduce stock count for item by 1
>                 stock[item] = stock[item] - 1
>                 # add 1 item to the sale
>                 sold[item] += 1
>                 # add item cost to the invoice
>                 invoice += prices[item]
> 
>     # return the items supplied and their cost
>     return sold, invoice

Here is a possible alternative:

    sold = {}

    for item, wanted_quantity in shopping.iteritems(): # items() in Python 3
        available_quantity = stock.get(item, 0)
        sold_quantity = min(wanted_quantity, available_quantity)
        sold[item] = sold_quantity
        stock[item] -= sold_quantity
        invoice += sold_quantity * prices[item]