Parent context refactor

[tbrlpld]

Description

This PR refactors how parent_context is handled between the render_html and get_context_data methods.

Previously, the render_html method was accepting the parent_context to be optional, defaulting to None. If the parent_context is None it is overridden to be an empty Context object. That context object is then used to call get_context_data.

Usually, the render_html method is called from the component template tag. The template tag always passes a Context object as the parent_context. It never passes None.

The get_context_data method required a parent_context to be passed, it does not have a default value for the argument. But, the base implementation in Component.get_context_data just ignores the parent_context object and returns an empty dict.

This setup has the effect that when child classes override the get_context_data that their
super().get_context_data(parent_context) call require the parent_context argument. But they only get an empty dict. This seems confusing and unnecessary, since we ignore the parent_context anyways. The only reason it's there is so that child components can make use of it.

Also, by always passing a Context object as the parent_context from render_html to the get_context_data, this suggest that this would really be a parent context, which typically includes things like the request object. However, we cannot rely on the request to be in the parent_context because render_html might have been called with None and just gives us an empty Context.

This inconsistency between the parent_context that is passed to render_html vs the one passed to get_context_data could easily lead to confusion in the case where we call render_html directly and don't pass a parent_context (as opposed it being called by the component tag which always passes the parent context).

This PR tries to make the parent_context between render_html and get_context_data more consistent. To do so, the parent_context argument of get_context_data now defaults to None. We still just return an empty dict. We don't override the parent_context in the render_html method anymore.

This means that all decisions about the parent_context can now be made in the get_context_data method. This feels like the appropriate place for all handling about context data, as the method is accordingly named.

The only case where this refactor would require a change in usage (of which there is not much at this point) is when the render_html method is called directly without passing a parent context and get_context_data expects to pull some data out of the parent_context. This case would need to guard against the parent_context being None.

I have checked the Wagtail code base and found no usage conflicting with this change. The Wagtail test suite also passes just fine. Therefore, I consider this change as "Wagtail-compatible", which is the only "backward-compatibility" I am concerned with at this stage.

Checklist

• CHANGELOG.md has been updated.
• README.md has been updated.
• Checked Wagtail compatibility.
• Self code reviewed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (d9fe305) to head (9e4edba).

Additional details and impacted files

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##              main       #24   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           13        13           
  Lines          255       254    -1     
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- Hits           255       254    -1     

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