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From: Peter Otten <__peter__@web.de>
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Subject: Re: Slice equivalent to dict.get
Date: Thu, 31 Mar 2016 17:24:50 +0200
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Xref: csiph.com comp.lang.python:106167

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