feat: extend f-string rules to t-strings (#3613) by raza-khan0108 · Pull Request #3702 · wemake-services/wemake-python-styleguide

raza-khan0108 · 2026-07-05T10:20:17Z

Extend style rules for f-strings to t-strings (Python 3.14+)

Checklist

I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc)
I have created at least one test case for the changes I have made
I have updated the documentation for the changes I have made
I have added my changes to the CHANGELOG.md

Related issues

Closes Extend the rules for f-strings to t-strings #3613

Summary of Changes

Python 3.14 introduces Template Strings (t-strings) via PEP 750. Since t-strings share formatting and interpolation syntax with f-strings, this PR extends our existing formatted string style rules to cover t-strings as well.

1. AST Compatibility Shims (`compat/nodes.py`)

Added conditional shims for ast.TemplateStr and ast.Interpolation. On Python versions < 3.14, these fallback to safe stub classes inheriting from ast.expr so type checking and AST traversal work cleanly across all Python versions without raising AttributeError.

2. AST Visitor Extensions (`visitors/ast/`)

builtins.py: Added visit_TemplateStr handler and generalized _check_complex_formatted_string to inspect both ast.JoinedStr and nodes.TemplateStr, enforcing complexity restrictions (WPS237) on t-string interpolation expressions.
complexity/jones.py: Added TemplateStr and Interpolation to ignored node types so internal formatting nodes do not inflate inline Jones line complexity scores.
complexity/overuses.py: Updated _check_string_constant to ignore string constants nested inside nodes.TemplateStr.
operators.py: Added TemplateStr to string classes for math operator checking.

3. Tokenize Visitor Extensions (`visitors/tokenize/`)

primitives.py: Added visit_tstring_start handler and updated _multiline_fstring_pattern regex to match f and t string prefixes ((?:[ft]r?|r[ft])(['"])), enforcing triple-quote requirements for multiline formatted strings (WPS479).
comments.py: Added visit_tstring_start handler and updated _comment_in_fstring regex to forbid inline comments inside t-string expressions (WPS480).

4. Violation Documentation (`violations/`)

Updated violation descriptions and docstrings for WPS237 (TooComplexFormattedStringViolation), WPS479 (MultilineFormattedStringViolation), and WPS480 (CommentInFormattedStringViolation) to explicitly state they forbid complex expressions, multiline quotes, and inline comments in both f-strings and t-strings.

5. Automated Testing (`tests/`)

Added unit test cases covering simple and complex t-string usages across AST and tokenize visitors in test_formatted_string.py, test_mulitiline_formatted_string312.py, and test_comment_in_formatted_string312.py.
Guarded all new t-string tests with @pytest.mark.skipif(sys.version_info < (3, 14), reason='t-strings are only in Python 3.14+').

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Khabib73 · 2026-07-07T19:27:39Z

        """Used to define `TemplateStr` nodes in `python3.14+`."""

-        values: list[ast.expr]
+        values: list[ast.expr]  # noqa: WPS110


Please do not suppress the WPS rules, look at https://github.com/wemake-services/wemake-python-styleguide/blob/master/wemake_python_styleguide/constants.py

Khabib73 · 2026-07-07T19:32:12Z


 @final
-class WrongFormatStringVisitor(base.BaseNodeVisitor):
+class WrongFormatStringVisitor(base.BaseNodeVisitor):  # noqa: WPS214


…mplateStr

codecov · 2026-07-09T19:34:21Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (13912d3) to head (f959320).
⚠️ Report is 34 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master     #3702    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          369       371     +2     
  Lines        12425     12556   +131     
  Branches       858       870    +12     
==========================================
+ Hits         12425     12556   +131

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yangfan-yf-yf

The 3.10–3.13 jobs are failing only because the new Python 3.14-only paths reduce total coverage below the repository's required 100% threshold.

The job output identifies the skipped t-string test bodies in test_formatted_string.py (lines 280–285 and 302–307), plus the new visit_tstring_start methods in comments.py and primitives.py. Those methods cannot execute before Python 3.14, while the existing version-specific coverage convention already uses covdefaults pragmas such as # pragma: >=3.12 cover.

Please mark the t-string-only test functions and the two visit_tstring_start methods with the corresponding # pragma: >=3.14 cover condition (or use an equivalent existing version-gated test layout). That keeps the 3.14 execution covered while excluding paths that older interpreters cannot reach. The 3.14 job already passes; this should restore the 3.10–3.13 matrix without weakening the coverage requirement.

raza-khan0108 · 2026-07-26T14:13:04Z

Added # pragma: >=3.14 cover annotations to the Python 3.14-only visit_tstring_start visitor methods (comments.py, primitives.py) and all t-string test function definitions (test_formatted_string.py, test_comment_in_formatted_string312.py, and test_mulitiline_formatted_string312.py).

This excludes these 3.14-only execution paths on Python 3.10–3.13 while keeping 100% coverage requirement intact on 3.14+.

raza-khan0108 · 2026-07-26T15:07:02Z

Updated the testing suite and violation documentation:

Parameterized Template Strings:
- Refactored all string templates in test_formatted_string.py to use {0} prefix placeholders.
- Parameterized test functions with prefix ('f' and 't', with 't' skipped for Python < 3.14).
- Removed redundant standalone test_simple_t_string and test_complex_t_string functions since all simple and complex cases are now automatically tested for both f and t strings.
Violation Docstrings Note:
- Added notes to TooComplexFormattedStringViolation, MultilineFormattedStringViolation, and CommentInFormattedStringViolation indicating that t-strings are only checked on python3.14+.

sobolevn · 2026-07-27T03:39:45Z

+        "foo = t'test {a} # testing'",
+    ],
+)
+def test_correct_t_string_comments(  # pragma: >=3.14 cover


here we should do the same with PREFIXES parametrization :)

sobolevn · 2026-07-27T03:39:58Z

    assert_errors(visitor, [MultilineFormattedStringViolation])
+
+
+@pytest.mark.skipif(


and here :)

sobolevn · 2026-07-27T03:40:34Z

@@ -19,11 +20,15 @@ def is_doc_string(node: ast.AST) -> bool:


 def has_fstring_conversion(component: ast.AST) -> bool:


This needs to be renamed, it is not about fstring only anymore.

sobolevn · 2026-07-27T03:42:21Z

+        """Performs check for t-strings."""
+        self._check_fstring_is_multi_lined(token)
+
    def _check_fstring_is_multi_lined(self, token: tokenize.TokenInfo) -> None:


this should also be renamed, _check_fstring_is_multi_lined name is not correct anymore, it is not just about fstring now

Khabib73 · 2026-07-29T20:08:09Z

+        Don't write comments inside formatted strings.

    Example::



Please don't forget to update the versionchanged entries for all violations that were changed :)

feat: extend f-string rules to t-strings (wemake-services#3613)

3204c83

Copilot AI review requested due to automatic review settings July 5, 2026 10:20

Copilot AI reviewed Jul 5, 2026

pre-commit-ci Bot and others added 3 commits July 5, 2026 10:20

[pre-commit.ci] auto fixes from pre-commit.com hooks

2536bb9

fix: shorten docstring in strings.py to fix line-too-long ruff error

85e18ec

fix: resolve WPS110 and WPS214 violations in t-string support

36c71cf

Khabib73 reviewed Jul 7, 2026

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Comment thread wemake_python_styleguide/visitors/ast/builtins.py Outdated

raza-khan0108 and others added 4 commits July 8, 2026 01:53

refactor: use @decorators.alias to merge visit_JoinedStr and visit_Te…

44c3cb0

…mplateStr

refactor: extract AnyFormattedString type alias for f/t-string union

7435b82

[pre-commit.ci] auto fixes from pre-commit.com hooks

3f9555d

fix: restore missing visitor.run() in test_incorrectly_formatted_string

281fd88

yangfan-yf-yf suggested changes Jul 25, 2026

View reviewed changes

raza-khan0108 and others added 2 commits July 26, 2026 19:42

test: mark Python 3.14-only t-string paths with pragma: >=3.14 cover

f599c3f

[pre-commit.ci] auto fixes from pre-commit.com hooks

f959320

sobolevn reviewed Jul 26, 2026

View reviewed changes

Comment thread tests/test_visitors/test_ast/test_builtins/test_strings/test_formatted_string.py Outdated

Comment thread wemake_python_styleguide/violations/best_practices.py

test: parameterize formatted string templates for f and t strings

a2f5d9e

Merge branch 'master' into feat/issue-3613-tstrings

8b4b915

sobolevn reviewed Jul 27, 2026

View reviewed changes

Khabib73 reviewed Jul 29, 2026

View reviewed changes

		assert_errors(visitor, [MultilineFormattedStringViolation])


		@pytest.mark.skipif(

		@@ -19,11 +20,15 @@ def is_doc_string(node: ast.AST) -> bool:


		def has_fstring_conversion(component: ast.AST) -> bool:

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raza-khan0108 commented Jul 5, 2026

Extend style rules for f-strings to t-strings (Python 3.14+)

Checklist

Related issues

Summary of Changes

1. AST Compatibility Shims (compat/nodes.py)

2. AST Visitor Extensions (visitors/ast/)

3. Tokenize Visitor Extensions (visitors/tokenize/)

4. Violation Documentation (violations/)

5. Automated Testing (tests/)

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1. AST Compatibility Shims (`compat/nodes.py`)

2. AST Visitor Extensions (`visitors/ast/`)

3. Tokenize Visitor Extensions (`visitors/tokenize/`)

4. Violation Documentation (`violations/`)

5. Automated Testing (`tests/`)

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