[Python-Dev] Add a frozendict builtin type

[Xavier Morel]

On 2012-02-27, at 19:53 , Victor Stinner wrote:

> Rationale
> =========
> 
> A frozendict type is a common request from users and there are various
> implementations. There are two main Python implementations:
> 
> * "blacklist": frozendict inheriting from dict and overriding methods
> to raise an exception when trying to modify the frozendict
> * "whitelist": frozendict not inheriting from dict and only implement
> some dict methods, or implement all dict methods but raise exceptions
> when trying to modify the frozendict
> 
> The blacklist implementation has a major issue: it is still possible
> to call write methods of the dict class (e.g. dict.set(my_frozendict,
> key, value)).
> 
> The whitelist implementation has an issue: frozendict and dict are not
> "compatible", dict is not a subclass of frozendict (and frozendict is
> not a subclass of dict).

This may be an issue at the C level (I'm not sure), but since this would
be a Python 3-only collection, "user" code (in Python) should/would
generally be using abstract base classes, so type-checking would not
be an issue (as in Python code performing `isinstance(a, dict)` checks
naturally failing on `frozendict`)

Plus `frozenset` does not inherit from `set`, it's a whitelist
reimplementation and I've never known anybody to care. So there's
that precedent. And of course there's no inheritance relationship
between lists and tuples.

> * frozendict has not the following methods: clear, __delitem__, pop,
> popitem, setdefault, __setitem__ and update. As tuple/frozenset has
> less methods than list/set.

It'd probably be simpler to define that frozendict is a Mapping (where
dict is a MutableMapping). And that's clearer.

> * Make dict inherits from frozendict

Isn't that the other way around from the statement above? Not that I'd
have an issue with it, it's much cleaner, but there's little gained by
doing so since `isinstance(a, dict)` will still fail if `a` is a
frozendict.

> * Add a frozendict abstract base class to collections?

Why? There's no `dict` ABC, and there are already a Mapping and a
MutableMapping ABC which fit the bill no?