I have built this data structure countless times. So I am in favor.

> Why can’t you just subclass dict and override that?  

Because TypeError: multiple bases have instance lay-out conflict is one of my least favorite errors.

Perhaps `__missing__` could be a first class part of the getitem of protocol, instead of a `dict` specific feature.  So that
```
r = x[key]
```
means:
```
try:
  r = x.__getitem__(key)
except KeyError as e: # should we also catch IndexError?
  try:
    missing = x.__missing__
  except AttributeError:
    raise e from None
  r = missing(key)

```

Obviously this would come at some performance cost for non dict mappings so I don't know if this would fly.

So instead maybe there could have standard decorator to get the same behavior?
```
def usemissing(getitem):
  @wraps(getitem)
  def wrapped(self, key):
    try:
      return getitem(self, key)
    except KeyError as e:
      try:
        missing = self.__missing__
      except AttributeError:
        raise e from None
    return missing(key)
  return wrapped    

```
Alternatively, it could be implemented as part of one of the ABCs maybe something like:
```
class MissingMapping(Mapping):
  # Could also give MissingMapping its own metaclass
  # and do the modification of __getitem__ there.
  def __init_subclass__(cls, **kwargs):
    super().__init_subclass__(**kwargs)
    cls.__getitem__ = usemissing(cls.__getitem__)
  
  @abstractmethod
  def __missing__(self, key): pass

```

Caleb Donovick

On Fri, Apr 10, 2020 at 6:39 PM Steven D'Aprano <steve@pearwood.info> wrote:
On Fri, Apr 10, 2020 at 06:02:25PM -0700, Andrew Barnert via Python-ideas wrote:

> (Keep in mind that defaultdict
> was added somewhere around 2.4 or 2.5, while __missing__ has only been
> there since somewhere around 2.7/3.3. I’ll bet it would be different
> if it were invented today.)


Both `__missing__` and `defaultdict` were added in version 2.5.

https://docs.python.org/2/library/stdtypes.html#dict
https://docs.python.org/2/library/collections.html#defaultdict-objects


--
Steven
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