Review User API Type Hints

Current Situation

ReactPy can generate type hinting errors generated (from Pylance).

For example, the fact that reactpy.component can ingest a ComponentType, but can only return a Component. Or the fact that use_state can be set to None initially and changed later.

Proposed Actions

Review user API type hints and fix where needed.

I think idom.component is typed correctly. It's input function is a render function which may return ComponentType | VdomDict | None and its output is a constructor for a Component object. The distinction between Component and ComponentType is that the latter is a protocol, an abstract interface. Component on the other hand is a concrete implementation of ComponentType. ContextProvider is another example of a ComponentType.

For use_state, if its value is initially None but may also be some other type (e.g. a str) then you must explicitly declare this upfront.

initial_value: str | None = None
value, set_value = use_state(initial_value)

# Or cast initial value

from typing import cast
value, set_value = use_state(cast(str | None, None))

# Or declare state, set_state upfront

value: str | None
set_value: Callable[[str | None], None]
value, set_value = use_state(None)

Admittedly, while these solutions work, I think they all feel a bit awkward. Here are two alternatives:

# Use a named tuple
state: State[str | None] = use_state(None)
value, set_value = state
state.value
state.set_value

# Or do some weird typing magic to make this possible
value, set_value = use_state[str | None](None)

I haven't seen the latter pattern used very many places so it might surprise people, but it looks nice.

On thinking further, use_state is complicated by the fact that Python 3.9 broke generic NamedTuples which we'd need to get this working. The best we'd be able to do is a generic tuple till this is fixed in Python 3.11.

Mypy 0981 includes support for generic NamedTuples.

Also, presently the @component hides the type annotations of the underlying rendering function. This is because @component adds the special key parameter that is not consumed by the rendering function. Typing this correctly is blocked on concatenating keyword-only arguments with ParamSpec

Currently getting type hint errors on the component decorator, if str() is directly returned within a component. Or if a component is directly returned within a component.

VDOM constructor type hints currently don't like None.

In the case a None child is passed?

Yeah, for example a conditional to return a component or None within a div tag.

It's also still very annoying that I can't directly return a component within a component without type hint errors.

a conditional to return a component or None within a div tag.

Can you share an example. I'm not quite sure I understand what you're describing.

I can't directly return a component within a component

Can you share the error you're getting? The type hint for the allowable return type for component functions should allow for this.

Argument of type "None" cannot be assigned to parameter "attributes_and_children" of type "VdomAttributesAndChildren" in function "__call__"
  Type "None" cannot be assigned to type "VdomAttributesAndChildren"
    Type "None" cannot be assigned to type "Mapping[str, Any]"
    "__iter__" is not present
    "key" is not present
    "type" is not present
    "render" is not present
    "should_render" is not present
    Type "None" cannot be assigned to type "VdomDict"
  ...Pylance[reportGeneralTypeIssues](https://github.com/microsoft/pylance-release/blob/main/DIAGNOSTIC_SEVERITY_RULES.md#diagnostic-severity-rules)

@component
def component_1():
   return html.div(
      None
   )

Argument of type "(*args: Unknown, **kwargs: Unknown) -> (Component | Unknown)" cannot be assigned to parameter "function" of type "(...) -> (ComponentType | VdomDict | None)" in function "component"
  Type "(*args: Unknown, **kwargs: Unknown) -> (Component | Unknown)" cannot be assigned to type "(...) -> (ComponentType | VdomDict | None)"
    Function return type "Component | Unknown" is incompatible with type "ComponentType | VdomDict | None"
      Type "Component | Unknown" cannot be assigned to type "ComponentType | VdomDict | None"
        Type "Component" cannot be assigned to type "ComponentType | VdomDict | None"
          "Component" is incompatible with protocol "ComponentType"
          "Component" is incompatible with "VdomDict"
          Type cannot be assigned to type "None"Pylance[reportGeneralTypeIssues](https://github.com/microsoft/pylance-release/blob/main/DIAGNOSTIC_SEVERITY_RULES.md#diagnostic-severity-rules)

@component
def component_2():
   return html.div()

@component
def component_1():
   return component_2()

Can you elaborate on what those issues are?

Seems like the main MyPy issue is _Type is not correctly inferred for state and set_state.

Basically every usage of state/set_state result errors similar to
Argument 1 has incompatible type "bool"; expected "Union[_Type, Callable[[_Type], _Type]]"
or
Argument 2 to "Mutation" has incompatible type "_Type"; expected "bool"

You can see this in django_idom.hooks.

I've been noticing some weird behavior with my MyPy cache. Does the error persist if you clear the cache first?

Clearing cache fixed the issues.

How strange... Maybe we can see if sqlite_cache resolves this issue?

Yeah, I have no idea what's going on. I've been meaning to try and write up a bug report.

The issue kept returning in my environment.

Changing sqlite_cache and cache_fine_grained did not resolve the issue.

However, it seems like changing incremental to false does resolve the issue.

That's good to know, I've been manually deleting the cache on every run.

Note to self:

The type hints around reactpy.html elements is too strict. Frankly, we may as well just accept Any for children and have the end-user handle incompatibilities.

VdomAttributes should be rewritten to dict[str, Any] since type hinters really don't giving any auto completion for MutableMapping.

The Context type created by create_context() currently suggests that a list of elements can be provided. This is unfortunately false, and will result in rendering failures.

Either remove this type hint, or begin official support for lists in Context.

None type is currently an accepted type for Context, but is rendered as a literal string None when used as a child element.

VdomDict and VdomChild are currently incompatible types, despite being the same thing. We should remove one of them.

Layout currently only accepts ComponentType as an arg, but Context should be allowed as well.

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Review User API Type Hints #794

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rmorshea commented on Jul 30, 2022

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