If so, what does that mean? Does it create a protocol? I would like the PEP to include an explicit list covering all the allowed ways TypedMapping can interact with inheritance: what classes can be base classes of a TypedMapping, and what classes can inherit from a TypedMapping?įor example, is this legal? class Base(TypedMapping): I started looking into implementing TypedMapping in typing-extensions and ran into a few issues, overlapping with what Eric posted above. Perhaps you could give some details of what problems you anticipate encountering? I’m not sure why this would be conceptually problematic, given the language already supports Protocols with a mix of mutable and read-only properties, for instance. It could always be mutated by concurrent code, aliases, methods, callbacks…) In protocols, you can’t ever declare something “immutable”, you can only declare it read-only in the context of your structurally typed method. (Aside: I prefer the term “read-only” rather than “immutable” here, as I think it is less misleading. The types of key1 and key2 can be any type hasher accepts, including subtypes, but that doesn’t mean that any type hasher accepts could be written to key1/ key2, so no mutator method declarations make sense. For instance: def set_hash_field(message): These types are necessary to correctly type hint methods that read some keys and write others, without artificially limiting what types the type checker permits. Conceptually and practically, hybrid mutable/immutable classes are problematic I don’t think it will work to support multiple inheritance between TypedDict and TypedMapping. I’ll try to get to that within the next couple of days. I’m happy to implement the draft PEP in pyright to uncover any remaining edge cases. I presume that TypedMapping can also be used with **kwargs and Unpack, but the **kwargs value will be treated as immutable in this case. The current draft doesn’t say anything about the behavioral intersection with PEP 692. I recommend mentioning this explicitly in the PEP so it’s not overlooked by the implementor(s). The same exception should obviously be made for TypedMapping. The one exception to this rule is Generic, which is now permitted as a base class for a TypedDict as of Python 3.10. Likewise, a TypedMapping that inherits from any type other than another TypedMapping should generate an exception.

Today, a TypedDict that inherits from any type other than another TypedDict generates an exception. I recommend replacing the multiple inheritance section with the statement that says any attempt to create a class that inherits from both TypedDict and TypedMapping will result in a runtime exception.

Conceptually and practically, hybrid mutable/immutable classes are problematic. There’s one area that I feel strongly needs to be changed.

Class inheritance python