How to generate "a, b, c, and d"?

[Terry Reedy]

On 12/15/2011 12:27 PM, MRAB wrote:
> On 15/12/2011 16:48, Roy Smith wrote:
>> I've got a list, ['a', 'b', 'c', 'd']. I want to generate the string,
>> "a, b, c, and d" (I'll settle for no comma after 'c'). Is there some
>> standard way to do this, handling all the special cases?
>>
>> [] ==> ''
>> ['a'] ==> 'a'
>> ['a', 'b'] ==> 'a and b'
>> ['a', 'b', 'c', 'd'] ==> 'a, b, and c'
>>
>> It seems like the kind of thing django.contrib.humanize would handle,
>> but alas, it doesn't.
>
> How about this:
>
> def and_list(items):
> if len(items) <= 2:
> return " and ".join(items)
>
> return ", ".join(items[ : -1]) + ", and " + items[-1]

To avoid making a slice copy,

   last = items.pop()
   return ", ".join(items) + (", and " + last)

I parenthesized the last two small items to avoid copying the long 
string twice with two appends. Even better is

   items[-1] = "and " + items[-1]
   return ", ".join(items)

so the entire output is created in one operation with no copy.

But I would only mutate the list if I started with
   items = list(iterable)
where iterable was the input, so I was mutating a private copy.

-- 
Terry Jan Reedy