Tag Strings

Welcome to tag strings, a planned enhancement for Python 3.13. Here you will find examples, specifications, and sample libraries.

About Tag Strings

This PEP introduces tag strings for custom, repeatable string processing. Tag strings are an extension to f-strings, with a custom function – the “tag” – in place of the f prefix. This function can then provide rich features such as safety checks, lazy evaluation, DSLs such as web templating, and more.

Tag strings are similar to JavaScript tagged templates and similar ideas in other languages. See the work-in-progress PEP. for more detail.

Example

In this tiny example, a greet function is defined and used to “tag” strings:

def greet(*args):
    """Uppercase and add exclamation."""
    salutation = args[0].upper()
    return f"{salutation}!"

With a string that obeys f-string semantics, we can then “tag” it:

>>> print(greet"Hello")
HELLO!

Tag strings are usually much richer and process the arguments Python assembles when calling:

def greet(*args: Decoded | Interpolation) -> str:
    """More about the interpolation."""
    result = []
    for arg in args:
        match arg:
            case str():
                result.append(arg)
            case getvalue, raw, conversion, formatspec:
                gv = f"gv: {getvalue()}"
                r = f"r: {raw}"
                c = f"c: {conversion}"
                f = f"f: {formatspec}"
                result.append(", ".join([gv, r, c, f]))

    return f"{''.join(result)}!"