Slice equivalent to dict.get

[Terry Reedy]

On 3/31/2016 11:24 AM, Peter Otten wrote:
> Steven D'Aprano wrote:
>
>> Sometimes people look for a method which is equivalent to dict.get, where
>> they can set a default value for when the key isn't found:
>>
>>
>> py> d = {1: 'a', 2: 'b'}
>> py> d.get(999, '?')
>> '?'
>>
>>
>> The equivalent for sequences such as lists and tuples is a slice. If the
>> slice is out of range, Python returns a empty sequence:
>>
>> py> L = [2, 4, 8, 16]
>> py> L[5]  # out of range, raises IndexError
>> Traceback (most recent call last):
>>    File "<stdin>", line 1, in <module>
>> IndexError: list index out of range
>> py> L[5:6]  # out of range slice return empty list
>> []
>>
>> To get a default:
>>
>> py> L[5:6] or -1
>> -1
>>
>>
>> This is short and simple enough to use in place, but we can also wrap this
>> into a convenient helper function:
>>
>> def get(seq, index, default=None):
>>      return (seq[index:index+1] or [default])[0]
>>
>>
>>
>> py> get(L, 2, -1)
>> 8
>> py> get(L, 200, -1)
>> -1
>
> But note:
>
>>>> def get(seq, index, default=None):
> ...     return (seq[index:index+1] or [default])[0]
> ...
>>>> get("abc", -1, "default")
> 'default'
>
> God old try...except to the rescue:
>
>>>> def get(seq, index, default=None):
> ...     try: return seq[index]
> ...     except IndexError: return default

Replace IndexError with (IndexError, KeyError) and the functions works 
with any subscriptable that raises either exception.

> ...
>>>> get("abc", -1, "default")
> 'c'
>
>


-- 
Terry Jan Reedy