PEP: 750 Title: Template Strings Author: Jim Baker , Guido van Rossum , Paul Everitt , Koudai Aono , Lysandros Nikolaou , Dave Peck Discussions-To: https://discuss.python.org/t/pep-750-tag-strings-for-writing-domain-specific-languages/60408 Status: Draft Type: Standards Track Created: 08-Jul-2024 Python-Version: 3.14 Post-History: `09-Aug-2024 `__, `17-Oct-2024 `__, `21-Oct-2024 `__ Abstract ======== This PEP introduces template strings for custom string processing. Template strings are a generalization of f-strings, using a ``t`` in place of the ``f`` prefix. Instead of evaluating to ``str``, t-strings evaluate to a new type, ``Template``: .. code-block:: python template: Template = t"Hello {name}" Templates provide developers with access to the string and its interpolated values *before* they are combined. This brings native flexible string processing to the Python language and enables safety checks, web templating, domain-specific languages, and more. Relationship With Other PEPs ============================ Python introduced f-strings in Python 3.6 with :pep:`498`. The grammar was then formalized in :pep:`701` which also lifted some restrictions. This PEP is based on PEP 701. At nearly the same time PEP 498 arrived, :pep:`501` was written to provide "i-strings" -- that is, "interpolation template strings". The PEP was deferred pending further experience with f-strings. Work on this PEP was resumed by a different author in March 2023, introducing "t-strings" as template literal strings, and built atop PEP 701. The authors of this PEP consider it to be a generalization and simplification of the updated work in PEP 501. (That PEP has also recently been updated to reflect the new ideas in this PEP.) Motivation ========== Python f-strings are easy to use and very popular. Over time, however, developers have encountered limitations that make them `unsuitable for certain use cases `__. In particular, f-strings provide no way to intercept and transform interpolated values before they are combined into a final string. As a result, incautious use of f-strings can lead to security vulnerabilities. For example, a user executing a SQL query with :mod:`python:sqlite3` may be tempted to use an f-string to embed values into their SQL expression, which could lead to a `SQL injection attack `__. Or, a developer building HTML may include unescaped user input in the string, leading to a `cross-site scripting (XSS) `__ vulnerability. More broadly, the inability to transform interpolated values before they are combined into a final string limits the utility of f-strings in more complex string processing tasks. Template strings address these problems by providing developers with access to the string and its interpolated values. 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: .. code-block:: python evil = "" template = t"

{evil}

" assert html(template) == "

<script>alert('evil')</script>

" Likewise, our hypothetical ``html()`` function can make it easy for developers to add attributes to HTML elements using a dictionary: .. code-block:: python attributes = {"src": "shrubbery.jpg", "alt": "looks nice"} template = t"" assert html(template) == 'looks nice' Neither of these examples is possible with f-strings. By providing a mechanism to intercept and transform interpolated values, template strings enable a wide range of string processing use cases. Specification ============= 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: .. code-block:: python 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``: .. code-block:: python 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: .. code-block:: python 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: .. code-block:: python 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: .. code-block:: python name = "World" template = t"Hello {name}" assert template.args[1].value == "World" The ``expr`` attribute is the *original text* of the interpolation: .. code-block:: python 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 :ref:`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: .. code-block:: python 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 :ref:`format specification `. As with f-strings, this is an arbitrary string that defines how to present the value: .. code-block:: python 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: .. code-block:: python 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 :func:`python: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: .. code-block:: python 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``: .. code-block:: python 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: .. code-block:: python 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: .. code-block:: python 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: .. code-block:: python 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: .. code-block:: python 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: .. code-block:: python 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}'``: .. code-block:: python 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: .. code-block:: python 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#expression-evaluation`: 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: .. code-block:: python 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: .. code-block:: python 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``. Examples ======== All examples in this section of the PEP have fully tested reference implementations available in the public `pep750-examples `_ git repository. 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: .. code-block:: python 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: .. code-block:: python 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) .. note:: Example code See `fstring.py`__ and `test_fstring.py`__. __ https://github.com/davepeck/pep750-examples/blob/main/pep/fstring.py __ https://github.com/davepeck/pep750-examples/blob/main/pep/test_fstring.py 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. Approach 1: Custom Log Messages ''''''''''''''''''''''''''''''' The :ref:`Python Logging Cookbook ` has a short section on `how to implement structured logging `_. The logging cookbook suggests creating a new "message" class, ``StructuredMessage``, that is constructed with a simple text message and a separate dictionary of values: .. code-block:: python message = StructuredMessage("user action", { "action": "traded", "amount": 42, "item": "shrubs" }) logging.info(message) # Outputs: # user action >>> {"action": "traded", "amount": 42, "item": "shrubs"} The ``StructuredMessage.__str__()`` method formats both the human-readable message *and* the values, combining them into a final string. (See the `logging cookbook `_ for its full example.) We can implement an improved version of ``StructuredMessage`` using template strings: .. code-block:: python 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"} Template strings give us a more elegant way to define the custom message class. With template strings it is no longer necessary for developers to make sure that their format string and values dictionary are kept in sync; a single template string literal is all that is needed. The ``TemplateMessage`` implementation can automatically extract structured keys and values from the ``Interpolation.expr`` and ``Interpolation.value`` attributes, respectively. Approach 2: Custom Formatters ''''''''''''''''''''''''''''' Custom messages are a reasonable approach to structured logging but can be a little awkward. To use them, developers must wrap every log message they write in a custom class. This can be easy to forget. An alternative approach is to define custom ``logging.Formatter`` classes. This approach is more flexible and allows for more control over the final output. In particular, it's possible to take a single template string and output it in multiple formats (human-readable and JSON) to separate log streams. We define two simple formatters, a ``MessageFormatter`` for human-readable output and a ``ValuesFormatter`` for JSON output: .. code-block:: python 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)) We can then use these formatters when configuring our logger: .. code-block:: python 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"} This approach has a couple advantages over the custom message approach to structured logging: - Developers can log a t-string directly without wrapping it in a custom class. - Human-readable and structured output can be sent to separate log streams. This is useful for log aggregation systems that process structured data independently from human-readable data. .. note:: Example code See `logging.py`__ and `test_logging.py`__. __ https://github.com/davepeck/pep750-examples/blob/main/pep/logging.py __ https://github.com/davepeck/pep750-examples/blob/main/pep/test_logging.py 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. Backwards Compatibility ======================= Like f-strings, use of template strings will be a syntactic backwards incompatibility with previous versions. Security Implications ===================== The security implications of working with template strings, with respect to interpolations, are as follows: 1. Scope lookup is the same as f-strings (lexical scope). This model has been shown to work well in practice. 2. Code that processes ``Template`` instances can ensure that any interpolations are processed in a safe fashion, including respecting the context in which they appear. How To Teach This ================= Template strings have several audiences: - Developers using template strings and processing functions - Authors of template processing code - Framework authors who build interesting machinery with template strings We hope that teaching developers will be straightforward. At a glance, template strings look just like f-strings. Their syntax is familiar and the scoping rules remain the same. The first thing developers must learn is that template string literals don't evaluate to strings; instead, they evaluate to a new type, ``Template``. This is a simple type intended to be used by template processing code. It's not until developers call a processing function that they get the result they want: typically, a string, although processing code can of course return any arbitrary type. Because developers will learn that t-strings are nearly always used in tandem with processing functions, they don't necessarily need to understand the details of the ``Template`` type. As with descriptors and decorators, we expect many more developers will use t-strings than write t-string processing functions. Over time, a small number of more advanced developers *will* wish to author their own template processing code. Writing processing code often requires thinking in terms of formal grammars. Developers will need to learn how to parse the ``args`` attribute of a ``Template`` instance and how to process interpolations in a context-sensitive fashion. More sophisticated grammars will likely require parsing to intermediate representations like an AST. Great template processing code will handle format specifiers and conversions when appropriate. Writing production-grade template processing code -- for instance, to support HTML templates -- can be a large undertaking. We expect that template strings will provide framework authors with a powerful new tool in their toolbox. While the functionality of template strings overlaps with existing tools like template engines, t-strings move that logic into the language itself. Bringing the full power and generality of Python to bear on string processing tasks opens new possibilities for framework authors. Common Patterns Seen in Processing Templates ============================================ Structural Pattern Matching --------------------------- Iterating over the ``Template.args`` with structural pattern matching is the expected best practice for many template function implementations: .. code-block:: python 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: .. code-block:: python 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: .. code-block:: python 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: .. code-block:: python @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: .. code-block:: python attributes = {"id": "main"} attribute_value = "shrubbery" content = "hello" template = t"
{content}
" element = html(template) assert str(element) == '
hello
' 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: .. code-block:: python name = "World" content = html(t"

Hello {name}

") template = t"
{content}
" element = html(template) assert str(element) == '

Hello World

' 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: .. code-block:: python 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: .. code-block:: python 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: .. code-block:: python 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. .. note:: Example code See `aformat.py`__ and `test_aformat.py`__. __ https://github.com/davepeck/pep750-examples/blob/main/pep/aformat.py __ https://github.com/davepeck/pep750-examples/blob/main/pep/test_aformat.py 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: .. code-block:: python 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. Reference Implementation ======================== At the time of this PEP's announcement, a fully-working implementation is `available `_. There is also a public repository of `examples and tests `_ built around the reference implementation. If you're interested in playing with template strings, this repository is a great place to start. Rejected Ideas ============== This PEP has been through several significant revisions. In addition, quite a few interesting ideas were considered both in revisions of :pep:`501` and in the `Discourse discussion `_. We attempt to document the most significant ideas that were considered and rejected. 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: .. code-block:: python 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: .. code-block:: python 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: .. code-block:: python 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: .. code-block:: python 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 :func:`python: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. Acknowledgements ================ Thanks to Ryan Morshead for contributions during development of the ideas leading to template strings. Special mention also to Dropbox's `pyxl `_ for tackling similar ideas years ago. Finally, thanks to Joachim Viide for his pioneering work on the `tagged library `_. Tagged was not just the precursor to template strings, but the place where the whole effort started via a GitHub issue comment! Copyright ========= This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.