[Python-ideas] getitem() (was Re: [Python-Dev] PEP 463: Exception-catching expressions)

[Eric Snow]

(in response to
https://mail.python.org/pipermail/python-dev/2014-February/132648.html)

On Fri, Feb 21, 2014 at 9:25 PM, Yury Selivanov <yselivanov.ml at gmail.com> wrote:
> On 2/21/2014, 10:42 PM, Chris Angelico wrote:
>> That fixes it for the list. Square brackets notation works for any
>> sequence. Are you proposing adding magic to the language so that any
>> class that defines a __getitem__ automatically has a .get() with these
>> semantics? Or, conversely, that defining __getitem__ requires that you
>> also explicitly define get()?
>
>
> No, I proposed to consider adding 'list.get()' and
> 'collections.abc.Sequence.get()' (or a better name),
> if catching IndexError is such a popular pattern.
>
> We can also fix stdlib. Authors of the python libraries/
> frameworks will likely follow.
>
> No magic in the language, no requirements on __getitem__,
> obviously.

We could also add a companion to getattr: getitem().

  def getitem(seq, index, default=_NOT_SET):
      try:
          return seq[index]
      except IndexError:
          if default is _NOT_SET:
              raise
          return default

Of course, one might expect getitem() to work for mappings too (or
we'd have 2 distinct functions for mappings and sequences).  Having
getitem() work for both would not be too hard.

Keep in mind that I still think that the PEP 463 syntax is worth
adding more than any getitem() function.  However, I think the
getitem() route would be better than adding another method to
sequences (including list).

-eric