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The template can then be combined with functions that operate on the template’s structure to produce a str or a string-like result. For example, sanitizing input:

template = t"<p>{evil}</p>"
assert html(template) == "<p>&lt;script&gt;alert('evil')&lt;/script&gt;</p>"

As another example, generating HTML attributes from data:

template = t"<img {attributes}>"
assert html(template) == '<img src="/api/flow.js?q=https%3A%2F%2Fgithub.com%2Fpython%2Fcpython%2Fissues%2Fshrubbery.jpg" alt="looks nice" />'

Compared to using an f-string, the html function has access to template attributes containing the original information: static strings, interpolations, and values from the original scope. Unlike existing templating approaches, t-strings build from the well-known f-string syntax and rules. Template systems thus benefit from Python tooling as they are much closer to the Python language, syntax, scoping, and more.

In PEP-750, the author assumed that you had defined the function html:

For example, imagine we want to generate some HTML. Using template strings, we can define an html() function that allows us to automatically sanitize content:

template = t"<p>{evil}</p>"
assert html(template) == "<p>&lt;script&gt;alert('evil')&lt;/script&gt;</p>"

Likewise, our hypothetical html() function can make it easy for developers to add attributes to HTML elements using a dictionary:

template = t"<img {attributes} />"
assert html(template) == '<img src="/api/flow.js?q=https%3A%2F%2Fgithub.com%2Fpython%2Fcpython%2Fissues%2Fshrubbery.jpg" alt="looks nice" />'

However, in the document, the operation "define" was gone, so that some users will mistakenly think that there is a function html that has been defined.

I think that it is better to explain that the function html should be defined by user, like this:

The template can then be combined with functions that operate on the template’s structure to produce a str or a string-like result. For example, sanitizing input (assuming that you have defined a function html that can deal with the input to html):