Template String Literals

This PEP introduces a new string prefix, t, to define template string literals. These literals resolve to a new type, Template, found in a new top-level standard library module, templatelib.

The following code creates a Template instance:

from templatelib import Template
template = t"This is a template string."
assert isinstance(template, Template)

Template string literals support the full syntax of PEP 701. This includes the ability to nest template strings within interpolations, as well as the ability to use all valid quote marks (', ", ''', and """). Like other string prefixes, the t prefix must immediately precede the quote. Like f-strings, both lowercase t and uppercase T prefixes are supported. Like f-strings, t-strings may not be combined with the b or u prefixes. Additionally, f-strings and t-strings cannot be combined, so the ft prefix is invalid as well. t-strings may be combined with the r prefix; see the Raw Template Strings section below for more information.

The Template Type

Template strings evaluate to an instance of a new type, templatelib.Template:

class Template:
    args: Sequence[str | Interpolation]

    def __init__(self, *args: str | Interpolation):
        ...

The args attribute provides access to the string parts and any interpolations in the literal:

name = "World"
template = t"Hello {name}"
assert isinstance(template.args[0], str)
assert isinstance(template.args[1], Interpolation)
assert template.args[0] == "Hello "
assert template.args[1].value == "World"

See Interleaving of Template.args below for more information on how the args attribute is structured.

The Template type is immutable. Template.args cannot be reassigned or mutated.

The Interpolation Type

The Interpolation type represents an expression inside a template string. Like Template, it is a new concrete type found in the templatelib module:

class Interpolation:
    value: object
    expr: str
    conv: Literal["a", "r", "s"] | None
    format_spec: str

    __match_args__ = ("value", "expr", "conv", "format_spec")

    def __init__(
        self,
        value: object,
        expr: str,
        conv: Literal["a", "r", "s"] | None = None,
        format_spec: str = "",
    ):
        ...

Like Template, Interpolation is shallow immutable. Its attributes cannot be reassigned.

The value attribute is the evaluated result of the interpolation:

name = "World"
template = t"Hello {name}"
assert template.args[1].value == "World"

The expr attribute is the original text of the interpolation:

name = "World"
template = t"Hello {name}"
assert template.args[1].expr == "name"

We expect that the expr attribute will not be used in most template processing code. It is provided for completeness and for use in debugging and introspection. See both the Common Patterns Seen in Processing Templates section and the Examples section for more information on how to process template strings.

The conv attribute is the optional conversion to be used, one of r, s, and a, corresponding to repr(), str(), and ascii() conversions. As with f-strings, no other conversions are supported:

name = "World"
template = t"Hello {name!r}"
assert template.args[1].conv == "r"

If no conversion is provided, conv is None.

The format_spec attribute is the format specification. As with f-strings, this is an arbitrary string that defines how to present the value:

value = 42
template = t"Value: {value:.2f}"
assert template.args[1].format_spec == ".2f"

Format specifications in f-strings can themselves contain interpolations. This is permitted in template strings as well; format_spec is set to the eagerly evaluated result:

value = 42
precision = 2
template = t"Value: {value:.{precision}f}"
assert template.args[1].format_spec == ".2f"

If no format specification is provided, format_spec defaults to an empty string (""). This matches the format_spec parameter of Python’s format() built-in.

Unlike f-strings, it is up to code that processes the template to determine how to interpret the conv and format_spec attributes. Such code is not required to use these attributes, but when present they should be respected, and to the extent possible match the behavior of f-strings. It would be surprising if, for example, a template string that uses {value:.2f} did not round the value to two decimal places when processed.

Processing Template Strings

Developers can write arbitrary code to process template strings. For example, the following function renders static parts of the template in lowercase and interpolations in uppercase:

from templatelib import Template, Interpolation

def lower_upper(template: Template) -> str:
    """Render static parts lowercased and interpolations uppercased."""
    parts: list[str] = []
    for arg in template.args:
        if isinstance(arg, Interpolation):
            parts.append(str(arg.value).upper())
        else:
            parts.append(arg.lower())
    return "".join(parts)

name = "world"
assert lower_upper(t"HELLO {name}") == "hello WORLD"

There is no requirement that template strings are processed in any particular way. Code that processes templates has no obligation to return a string. Template strings are a flexible, general-purpose feature.

See the Common Patterns Seen in Processing Templates section for more information on how to process template strings. See the Examples section for detailed working examples.

Template String Concatenation

Template strings support explicit concatenation using +. Concatenation is supported for two Template instances as well as for a Template instance and a str:

name = "World"
template1 = t"Hello "
template2 = t"{name}"
assert template1 + template2 == t"Hello {name}"
assert template1 + "!" == t"Hello !"
assert "Hello " + template2 == t"Hello {name}"

Concatenation of templates is “viral”: the concatenation of a Template and a str always results in a Template instance.

Python’s implicit concatenation syntax is also supported. The following code will work as expected:

name = "World"
template = t"Hello " "World"
assert template == t"Hello World"
template2 = t"Hello " t"World"
assert template2 == t"Hello World"

The Template type implements the __add__() and __radd__() methods roughly as follows:

class Template:
    def __add__(self, other: object) -> Template:
        if isinstance(other, str):
            return Template(*self.args[:-1], self.args[-1] + other)
        if not isinstance(other, Template):
            return NotImplemented
        return Template(*self.args[:-1], self.args[-1] + other.args[0], *other.args[1:])

    def __radd__(self, other: object) -> Template:
        if not isinstance(other, str):
            return NotImplemented
        return Template(other + self.args[0], *self.args[1:])

Special care is taken to ensure that the interleaving of str and Interpolation instances is maintained when concatenating. (See the Interleaving of Template.args section for more information.)

Template and Interpolation Equality

Two instances of Template are defined to be equal if their args attributes contain the same strings and interpolations in the same order:

assert t"I love {stilton}" == t"I love {stilton}"
assert t"I love {stilton}" != t"I love {roquefort}"
assert t"I " + t"love {stilton}" == t"I love {stilton}"

The implementation of Template.__eq__() is roughly as follows:

class Template:
    def __eq__(self, other: object) -> bool:
        if not isinstance(other, Template):
            return NotImplemented
        return self.args == other.args

Two instances of Interpolation are defined to be equal if their value, expr, conv, and format_spec attributes are equal:

class Interpolation:
    def __eq__(self, other: object) -> bool:
        if not isinstance(other, Interpolation):
            return NotImplemented
        return (
            self.value == other.value
            and self.expr == other.expr
            and self.conv == other.conv
            and self.format_spec == other.format_spec
        )

No Support for Ordering

The Template and Interpolation types do not support ordering. This is unlike all other string literal types in Python, which support lexicographic ordering. Because interpolations can contain arbitrary values, there is no natural ordering for them. As a result, neither the Template nor the Interpolation type implements the standard comparison methods.

Support for the debug specifier (=)

The debug specifier, =, is supported in template strings and behaves similarly to how it behaves in f-strings, though due to limitations of the implementation there is a slight difference.

In particular, t'{expr=}' is treated as t'expr={expr}':

name = "World"
template = t"Hello {name=}"
assert template.args[0] == "Hello name="
assert template.args[1].value == "World"

Raw Template Strings

Raw template strings are supported using the rt (or tr) prefix:

trade = 'shrubberies'
t = rt'Did you say "{trade}"?\n'
assert t.args[0] == r'Did you say "'
assert t.args[2] == r'"?\n'

In this example, the \n is treated as two separate characters (a backslash followed by ‘n’) rather than a newline character. This is consistent with Python’s raw string behavior.

As with regular template strings, interpolations in raw template strings are processed normally, allowing for the combination of raw string behavior and dynamic content.

Interpolation Expression Evaluation

Expression evaluation for interpolations is the same as in PEP 498:

The expressions that are extracted from the string are evaluated in the context where the template string appeared. This means the expression has full access to its lexical scope, including local and global variables. Any valid Python expression can be used, including function and method calls.

Template strings are evaluated eagerly from left to right, just like f-strings. This means that interpolations are evaluated immediately when the template string is processed, not deferred or wrapped in lambdas.

Exceptions

Exceptions raised in t-string literals are the same as those raised in f-string literals.

Interleaving of Template.args

In the Template type, the args attribute is a sequence that will always alternate between string literals and Interpolation instances. Specifically:

• Even-indexed elements (0, 2, 4, …) are always of type str, representing the literal parts of the template.
• Odd-indexed elements (1, 3, 5, …) are always Interpolation instances, representing the interpolated expressions.

For example, the following assertions hold:

name = "World"
template = t"Hello {name}"
assert len(template.args) == 3
assert template.args[0] == "Hello "
assert template.args[1].value == "World"
assert template.args[2] == ""

These rules imply that the args attribute will always have an odd length. As a consequence, empty strings are added to the sequence when the template begins or ends with an interpolation, or when two interpolations are adjacent:

a, b = "a", "b"
template = t"{a}{b}"
assert len(template.args) == 5
assert template.args[0] == ""
assert template.args[1].value == "a"
assert template.args[2] == ""
assert template.args[3].value == "b"
assert template.args[4] == ""

Most template processing code will not care about this detail and will use either structural pattern matching or isinstance() checks to distinguish between the two types of elements in the sequence.

The detail exists because it allows for performance optimizations in template processing code. For example, a template processor could cache the static parts of the template and only reprocess the dynamic parts when the template is evaluated with different values. Access to the static parts can be done with template.args[::2].

Interleaving is an invariant maintained by the Template class. Developers can take advantage of it but they are not required to themselves maintain it. Specifically, Template.__init__() can be called with str and Interpolation instances in any order; the constructor will “interleave” them as necessary before assigning them to args.

Example: Implementing f-strings with t-strings

It is easy to “implement” f-strings using t-strings. That is, we can write a function f(template: Template) -> str that processes a Template in much the same way as an f-string literal, returning the same result:

name = "World"
value = 42
templated = t"Hello {name!r}, value: {value:.2f}"
formatted = f"Hello {name!r}, value: {value:.2f}"
assert f(templated) == formatted

The f() function supports both conversion specifiers like !r and format specifiers like :.2f. The full code is fairly simple:

from templatelib import Template, Interpolation

def convert(value: object, conv: Literal["a", "r", "s"] | None) -> object:
    if conv == "a":
        return ascii(value)
    elif conv == "r":
        return repr(value)
    elif conv == "s":
        return str(value)
    return value


def f(template: Template) -> str:
    parts = []
    for arg in template.args:
        match arg:
            case str() as s:
                parts.append(s)
            case Interpolation(value, _, conv, format_spec):
                value = convert(value, conv)
                value = format(value, format_spec)
                parts.append(value)
    return "".join(parts)

Example: Structured Logging

Structured logging allows developers to log data in both a human-readable format and a structured format (like JSON) using only a single logging call. This is useful for log aggregation systems that process the structured format while still allowing developers to easily read their logs.

We present two different approaches to implementing structured logging with template strings.

message = StructuredMessage("user action", {
    "action": "traded",
    "amount": 42,
    "item": "shrubs"
})
logging.info(message)

# Outputs:
# user action >>> {"action": "traded", "amount": 42, "item": "shrubs"}

import json
from templatelib import Interpolation, Template
from typing import Mapping

class TemplateMessage:
    def __init__(self, template: Template) -> None:
        self.template = template

    @property
    def message(self) -> str:
        # Use the f() function from the previous example
        return f(self.template)

    @property
    def values(self) -> Mapping[str, object]:
        return {
            arg.expr: arg.value
            for arg in self.template.args
            if isinstance(arg, Interpolation)
        }

    def __str__(self) -> str:
        return f"{self.message} >>> {json.dumps(self.values)}"

_ = TemplateMessage  # optional, to improve readability
action, amount, item = "traded", 42, "shrubs"
logging.info(_(t"User {action}: {amount:.2f} {item}"))

# Outputs:
# User traded: 42.00 shrubs >>> {"action": "traded", "amount": 42, "item": "shrubs"}

import json
from logging import Formatter, LogRecord
from templatelib import Interpolation, Template
from typing import Any, Mapping


class MessageFormatter(Formatter):
    def message(self, template: Template) -> str:
        # Use the f() function from the previous example
        return f(template)

    def format(self, record: LogRecord) -> str:
        msg = record.msg
        if not isinstance(msg, Template):
            return super().format(record)
        return self.message(msg)


class ValuesFormatter(Formatter):
    def values(self, template: Template) -> Mapping[str, Any]:
        return {
            arg.expr: arg.value
            for arg in template.args
            if isinstance(arg, Interpolation)
        }

    def format(self, record: LogRecord) -> str:
        msg = record.msg
        if not isinstance(msg, Template):
            return super().format(record)
        return json.dumps(self.values(msg))

import logging
import sys

logger = logging.getLogger(__name__)
message_handler = logging.StreamHandler(sys.stdout)
message_handler.setFormatter(MessageFormatter())
logger.addHandler(message_handler)

values_handler = logging.StreamHandler(sys.stderr)
values_handler.setFormatter(ValuesFormatter())
logger.addHandler(values_handler)

action, amount, item = "traded", 42, "shrubs"
logger.info(t"User {action}: {amount:.2f} {item}")

# Outputs to sys.stdout:
# User traded: 42.00 shrubs

# At the same time, outputs to sys.stderr:
# {"action": "traded", "amount": 42, "item": "shrubs"}

Example: HTML Templating

This PEP contains several short HTML templating examples. It turns out that the “hypothetical” html() function mentioned in the Motivation section (and a few other places in this PEP) exists and is available in the pep750-examples repository. If you’re thinking about parsing a complex grammar with template strings, we hope you’ll find it useful.

Structural Pattern Matching

Iterating over the Template.args with structural pattern matching is the expected best practice for many template function implementations:

from templatelib import Template, Interpolation

def process(template: Template) -> Any:
    for arg in template.args:
        match arg:
            case str() as s:
                ... # handle each string part
            case Interpolation() as interpolation:
                ... # handle each interpolation

Processing code may also commonly sub-match on attributes of the Interpolation type:

match arg:
    case Interpolation(int()):
        ... # handle interpolations with integer values
    case Interpolation(value=str() as s):
        ... # handle interpolations with string values
    # etc.

Memoizing

Template functions can efficiently process both static and dynamic parts of templates. The structure of Template objects allows for effective memoization:

source = template.args[::2]  # Static string parts
values = [i.value for i in template.args[1::2]]  # Dynamic interpolated values

This separation enables caching of processed static parts, while dynamic parts can be inserted as needed. Authors of template processing code can use the static source as cache keys, leading to significant performance improvements when similar templates are used repeatedly.

Parsing to Intermediate Representations

Code that processes templates can parse the template string into intermediate representations, like an AST. We expect that many template processing libraries will use this approach.

For instance, rather than returning a str, our theoretical html() function (see the Motivation section) could return an HTML Element defined in the same package:

@dataclass(frozen=True)
class Element:
    tag: str
    attributes: Mapping[str, str | bool]
    children: Sequence[str | Element]

    def __str__(self) -> str:
        ...


def html(template: Template) -> Element:
    ...

Calling str(element) would then render the HTML but, in the meantime, the Element could be manipulated in a variety of ways.

Context-sensitive Processing of Interpolations

Continuing with our hypothetical html() function, it could be made context-sensitive. Interpolations could be processed differently depending on where they appear in the template.

For example, our html() function could support multiple kinds of interpolations:

attributes = {"id": "main"}
attribute_value = "shrubbery"
content = "hello"
template = t"<div {attributes} data-value={attribute_value}>{content}</div>"
element = html(template)
assert str(element) == '<div id="main" data-value="shrubbery">hello</div>'

Because the {attributes} interpolation occurs in the context of an HTML tag, and because there is no corresponding attribute name, it is treated as a dictionary of attributes. The {attribute_value} interpolation is treated as a simple string value and is quoted before inclusion in the final string. The {content} interpolation is treated as potentially unsafe content and is escaped before inclusion in the final string.

Nested Template Strings

Going a step further with our html() function, we could support nested template strings. This would allow for more complex HTML structures to be built up from simpler templates:

name = "World"
content = html(t"<p>Hello {name}</p>")
template = t"<div>{content}</div>"
element = html(template)
assert str(element) == '<div><p>Hello World</p></div>'

Because the {content} interpolation is an Element instance, it does not need to be escaped before inclusion in the final string.

One could imagine a nice simplification: if the html() function is passed a Template instance, it could automatically convert it to an Element by recursively calling itself on the nested template.

We expect that nesting and composition of templates will be a common pattern in template processing code and, where appropriate, used in preference to simple string concatenation.

Approaches to Lazy Evaluation

Like f-strings, interpolations in t-string literals are eagerly evaluated. However, there are cases where lazy evaluation may be desirable.

If a single interpolation is expensive to evaluate, it can be explicitly wrapped in a lambda in the template string literal:

name = "World"
template = t"Hello {(lambda: name)}"
assert callable(template.args[1].value)
assert template.args[1].value() == "World"

This assumes, of course, that template processing code anticipates and handles callable interpolation values. (One could imagine also supporting iterators, awaitables, etc.) This is not a requirement of the PEP, but it is a common pattern in template processing code.

In general, we hope that the community will develop best practices for lazy evaluation of interpolations in template strings and that, when it makes sense, common libraries will provide support for callable or awaitable values in their template processing code.

Approaches to Asynchronous Evaluation

Closely related to lazy evaluation is asynchronous evaluation.

As with f-strings, the await keyword is allowed in interpolations:

async def example():
    async def get_name() -> str:
        await asyncio.sleep(1)
        return "Sleepy"

    template = t"Hello {await get_name()}"
    # Use the f() function from the f-string example, above
    assert f(template) == "Hello Sleepy"

More sophisticated template processing code can take advantage of this to perform asynchronous operations in interpolations. For example, a “smart” processing function could anticipate that an interpolation is an awaitable and await it before processing the template string:

async def example():
    async def get_name() -> str:
        await asyncio.sleep(1)
        return "Sleepy"

    template = t"Hello {get_name}"
    assert await aformat(template) == "Hello Sleepy"

This assumes that the template processing code in aformat() is asynchronous and is able to await an interpolation’s value.

Approaches to Template Reuse

If developers wish to reuse template strings multiple times with different values, they can write a function to return a Template instance:

def reusable(name: str, question: str) -> Template:
    return t"Hello {name}, {question}?"

template = reusable("friend", "how are you")
template = reusable("King Arthur", "what is your quest")

This is, of course, no different from how f-strings can be reused.

Arbitrary String Literal Prefixes

Inspired by JavaScript tagged template literals, an earlier version of this PEP allowed for arbitrary “tag” prefixes in front of literal strings:

my_tag'Hello {name}'

The prefix was a special callable called a “tag function”. Tag functions received the parts of the template string in an argument list. They could then process the string and return an arbitrary value:

def my_tag(*args: str | Interpolation) -> Any:
    ...

This approach was rejected for several reasons:

• It was deemed too complex to build in full generality. JavaScript allows for arbitrary expressions to precede a template string, which is a significant challenge to implement in Python.
• It precluded future introduction of new string prefixes.
• It seemed to needlessly pollute the namespace.

Use of a single t prefix was chosen as a simpler, more Pythonic approach and more in keeping with template strings’ role as a generalization of f-strings.

Delayed Evaluation of Interpolations

An early version of this PEP proposed that interpolations should be lazily evaluated. All interpolations were “wrapped” in implicit lambdas. Instead of having an eagerly evaluated value attribute, interpolations had a getvalue() method that would resolve the value of the interpolation:

class Interpolation:
    ...
    _value: Callable[[], object]

    def getvalue(self) -> object:
        return self._value()

This was rejected for several reasons:

• The overwhelming majority of use cases for template strings naturally call for immediate evaluation.
• Delayed evaluation would be a significant departure from the behavior of f-strings.
• Implicit lambda wrapping leads to difficulties with type hints and static analysis.

Most importantly, there are viable (if imperfect) alternatives to implicit lambda wrapping when lazy evaluation is desired. See the section on Approaches to Lazy Evaluation, above, for more information.

Making Template and Interpolation Into Protocols

An early version of this PEP proposed that the Template and Interpolation types be runtime checkable protocols rather than concrete types.

In the end, we felt that using concrete types was more straightforward.

An Additional Decoded Type

An early version of this PEP proposed an additional type, Decoded, to represent the “static string” parts of a template string. This type derived from str and had a single extra raw attribute that provided the original text of the string. We rejected this in favor of the simpler approach of using plain str and allowing combination of r and t prefixes.

Other Homes for Template and Interpolation

Previous versions of this PEP proposed that the Template and Interpolation types be placed in the types module. This was rejected in favor of creating a new top-level standard library module, templatelib. This was done to avoid polluting the types module with seemingly unrelated types.

Enable Full Reconstruction of Original Template Literal

Earlier versions of this PEP attempted to make it possible to fully reconstruct the text of the original template string from a Template instance. This was rejected as being overly complex.

There are several limitations with respect to round-tripping to the original source text:

• Interpolation.format_spec defaults to "" if not provided. It is therefore impossible to distinguish t"{expr}" from t"{expr:}".
• The debug specifier, =, is treated as a special case. It is therefore not possible to distinguish t"{expr=}" from t"expr={expr}".
• Finally, format specifiers in f-strings allow arbitrary nesting. In this PEP and in the reference implementation, the specifier is eagerly evaluated to set the format_spec in the Interpolation, thereby losing the original expressions. For example:

value = 42
precision = 2
template = t"Value: {value:.{precision}f}"
assert template.args[1].format_spec == ".2f"

We do not anticipate that these limitations will be a significant issue in practice. Developers who need to obtain the original template string literal can always use inspect.getsource() or similar tools.

Disallowing String Concatenation

Earlier versions of this PEP proposed that template strings should not support concatenation. This was rejected in favor of allowing concatenation.

There are reasonable arguments in favor of rejecting one or all forms of concatenation: namely, that it cuts off a class of potential bugs, particularly when one takes the view that template strings will often contain complex grammars for which concatenation doesn’t always have the same meaning (or any meaning).

Moreover, the earliest versions of this PEP proposed a syntax closer to JavaScript’s tagged template literals, where an arbitrary callable could be used as a prefix to a string literal. There was no guarantee that the callable would return a type that supported concatenation.

In the end, we decided that the surprise to developers of a new string type not supporting concatenation was likely to be greater than the theoretical harm caused by supporting it. (Developers concatenate f-strings all the time, after all, and while we are sure there are cases where this introduces bugs, it’s not clear that those bugs outweigh the benefits of supporting concatenation.)

While concatenation is supported, we expect that code that uses template strings will more commonly build up larger templates through nesting and composition rather than concatenation.

Arbitrary Conversion Values

Python allows only r, s, or a as possible conversion type values. Trying to assign a different value results in SyntaxError.

In theory, template functions could choose to handle other conversion types. But this PEP adheres closely to PEP 701. Any changes to allowed values should be in a separate PEP.

Removing conv From Interpolation

During the authoring of this PEP, we considered removing the conv attribute from Interpolation and specifying that the conversion should be performed eagerly, before Interpolation.value is set.

This was done to simplify the work of writing template processing code. The conv attribute is of limited extensibility (it is typed as Literal["r", "s", "a"] | None). It is not clear that it adds significant value or flexibility to template strings that couldn’t better be achieved with custom format specifiers. Unlike with format specifiers, there is no equivalent to Python’s format() built-in. (Instead, we include an sample implementation of convert() in the Examples section.)

Ultimately we decided to keep the conv attribute in the Interpolation type to maintain compatibility with f-strings and to allow for future extensibility.

Alternate Interpolation Symbols

In the early stages of this PEP, we considered allowing alternate symbols for interpolations in template strings. For example, we considered allowing ${name} as an alternative to {name} with the idea that it might be useful for i18n or other purposes. See the Discourse thread for more information.

This was rejected in favor of keeping t-string syntax as close to f-string syntax as possible.

A Lazy Conversion Specifier

We considered adding a new conversion specifier, !(), that would explicitly wrap the interpolation expression in a lambda.

This was rejected in favor of the simpler approach of using explicit lambdas when lazy evaluation is desired.

Alternate Layouts for Template.args

During the development of this PEP, we considered several alternate layouts for the args attribute of the Template type. This included:

• Instead of args, Template contains a strings attribute of type Sequence[str] and an interpolations attribute of type Sequence[Interpolation]. There are zero or more interpolations and there is always one more string than there are interpolations. Utility code could build an interleaved sequence of strings and interpolations from these separate attributes. This was rejected as being overly complex.
• args is typed as a Sequence[tuple[str, Interpolation | None]]. Each static string is paired with is neighboring interpolation. The final string part has no corresponding interpolation. This was rejected as being overly complex.
• args remains a Sequence[str | Interpolation] but does not support interleaving. As a result, empty strings are not added to the sequence. It is no longer possible to obtain static strings with args[::2]; instead, instance checks or structural pattern matching must be used to distinguish between strings and interpolations. We believe this approach is easier to explain and, at first glance, more intuitive. However, it was rejected as offering less future opportunty for performance optimization. We also believe that args[::2] may prove to be a useful shortcut in template processing code.

Mechanism to Describe the “Kind” of Template

If t-strings prove popular, it may be useful to have a way to describe the “kind” of content found in a template string: “sql”, “html”, “css”, etc. This could enable powerful new features in tools such as linters, formatters, type checkers, and IDEs. (Imagine, for example, black formatting HTML in t-strings, or mypy checking whether a given attribute is valid for an HTML tag.) While exciting, this PEP does not propose any specific mechanism. It is our hope that, over time, the community will develop conventions for this purpose.