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I don't see how this prevents calling your returned f-string with unsensitized inputs.

    evil = "<script>alert('evil')</script>"
    sanitized = Sanitized(evil)
    whoops = f"<p>{evil}</p>"

▲ ratorx 9 days ago | parent [-]

I’m not sure you understood my example. The f-string is within a function. The function argument only accepts sanitised input type.

If you create a subclass of str which has an init function that sanitises, then you can’t create a Sanitised type by casting right?

And even if you could, there is also nothing stopping you from using a different function to “html” that just returns the string without sanitising. They are on the same relative level of safety.

▲ itishappy 9 days ago | parent [-]

Oh, I'm pretty sure I didn't understand your example and am probably missing something obvious. That's why I'm here asking dumb questions!

I think I'm following more, and I see how you can accomplish this by encapsulating the rendering, but I'm still not seeing how this is possible with user facing f-strings. Think you can write up a quick example?

▲ ratorx 9 days ago | parent [-]

Added example to parent comment.

▲ itishappy 9 days ago | parent [-]

Thanks mate! (BTW: Indenting code with four spaces makes HN format it like code.)

So the thing I'm still not getting from your example is allowing the template itself to be customized.

   evil = "<script>alert('evil')</script>"
   template1 = t"<p>{evil}</p>"
   template2 = t"<h1>{evil}</h1>"
   html(template1)
   html(template2)

▲

ratorx 9 days ago | parent [-]

template1 is a function that takes in a parameter evil (with a SanitisedString type that wraps a regular str) and returns the fully expanded str. It is implemented by just returning an f-string equivalent to the t-string in your example. Same with template2.

Using the SanitisedString type forces the user to explicitly call a sanitiser function that returns a SanitisedString and prevents them from passing in an unsanitised str.

	▲	itishappy 8 days ago \| parent [-]
		You're just handing off responsibility for sanitization to the user instead of the library author. With t-strings the rendering function is responsible for sanitization, and users can pass unrendered templates to it. With f-strings there's no concept of an unrendered template, it just immediately becomes a string. Whoever is creating the template therefore has to be careful what they put in it.