Fixes wemake-services#3405, forbids miltiline fstrings (wemake-services#3411)

Co-authored-by: sobolevn <mail@sobolevn.me>

Author: mkaraev

CHANGELOG.md

@@ -22,6 +22,7 @@ Semantic versioning in our case means:
 ### Features
 
 - Adds `WPS478`: forbids using non strict slice operations, #1011
+- Adds `WPS479`: forbids using multiline fstrings, #3405
 
 ### Bugfixes
 

tests/test_checker/test_noqa.py

@@ -243,6 +243,7 @@
         'WPS476': 1,
         'WPS477': 0,  # enabled only in python 3.13+
         'WPS478': 1,
+        'WPS479': 0,
         'WPS500': 1,
         'WPS501': 1,
         'WPS502': 0,  # disabled since 1.0.0

tests/test_visitors/test_tokenize/test_primitives/test_string_tokens/test_mulitiline_formatted_string312.py

@@ -0,0 +1,90 @@
+import pytest
+
+from wemake_python_styleguide.compat.constants import PY312
+from wemake_python_styleguide.violations.best_practices import (
+    MultilineFormattedStringViolation,
+)
+from wemake_python_styleguide.visitors.tokenize.primitives import (
+    MultilineFormattedStringTokenVisitor,
+)
+
+if not PY312:  # pragma: >=3.12 no cover
+    pytest.skip(
+        reason='unterminated string literal was added in 3.12',
+        allow_module_level=True,
+    )
+
+# Wrong:
+single_quote_formatted_string_wrong = """x=f'{ 1
+}'
+"""
+
+fr_prefix_formatted_string_wrong = """x=fr'{1
+}'"""
+
+double_quote_formatted_string_wrong = """x=f" {2} { 1
+}"
+"""
+
+# Correct:
+triple_quote_formatted_string_first_correct = """x=f'''{ 1
+}'''
+"""
+
+triple_quote_formatted_string_second_correct = '''x=f"""{ 1
+}"""
+'''
+single_line_formatted_string = (
+    """formatted_string_complex = f'1+1={1 + 1}'  # noqa: WPS237"""
+)
+
+fr_prefix_formatted_string = (
+    """formatted_string_complex = fr'1+1={1 + 1}'  # noqa: WPS237"""
+)
+
+
+@pytest.mark.parametrize(
+    'code',
+    [
+        triple_quote_formatted_string_first_correct,
+        triple_quote_formatted_string_second_correct,
+        single_line_formatted_string,
+        fr_prefix_formatted_string,
+    ],
+)
+def test_correctly_formatted_string(
+    parse_tokens,
+    assert_errors,
+    default_options,
+    code,
+):
+    """Ensures that correct formatting works."""
+    tokens = parse_tokens(code)
+    visitor = MultilineFormattedStringTokenVisitor(
+        default_options, file_tokens=tokens
+    )
+    visitor.run()
+    assert_errors(visitor, [])
+
+
+@pytest.mark.parametrize(
+    'code',
+    [
+        single_quote_formatted_string_wrong,
+        double_quote_formatted_string_wrong,
+        fr_prefix_formatted_string_wrong,
+    ],
+)
+def test_incorrectly_formatted_string(
+    parse_tokens,
+    assert_errors,
+    default_options,
+    code,
+):
+    """Ensures that correct formatting works."""
+    tokens = parse_tokens(code)
+    visitor = MultilineFormattedStringTokenVisitor(
+        default_options, file_tokens=tokens
+    )
+    visitor.run()
+    assert_errors(visitor, [MultilineFormattedStringViolation])

wemake_python_styleguide/presets/types/file_tokens.py

@@ -19,4 +19,5 @@
     primitives.WrongStringTokenVisitor,
     statements.MultilineStringVisitor,
     conditions.IfElseVisitor,
+    primitives.MultilineFormattedStringTokenVisitor,
 )

wemake_python_styleguide/violations/best_practices.py

@@ -95,6 +95,7 @@
    AwaitInLoopViolation
    SneakyTypeVarWithDefaultViolation
    NonStrictSliceOperationsViolation
+   MultilineFormattedStringViolation
 
 Best practices
 --------------
@@ -178,6 +179,7 @@
 .. autoclass:: AwaitInLoopViolation
 .. autoclass:: SneakyTypeVarWithDefaultViolation
 .. autoclass:: NonStrictSliceOperationsViolation
+.. autoclass:: MultilineFormattedStringViolation
 
 """
 
@@ -3074,3 +3076,33 @@ class NonStrictSliceOperationsViolation(ASTViolation):
 
     error_template = 'Found non strict slice operation'
     code = 478
+
+
+@final
+class MultilineFormattedStringViolation(TokenizeViolation):
+    """
+    Forbid using multi-line formatted string with single and double quotes.
+
+    Reasoning:
+        Multiline f-strings must use triple quotes for clarity.
+        Single f-strings may not span lines.
+
+    Solution:
+        Use triple quotes instead of single quotes.
+
+    Example::
+
+        # Correct:
+        x = f''' { 1
+        ...}'''
+
+        # Wrong:
+        x = f' { 1
+        ...}'
+
+    .. versionadded:: 1.2.0
+
+    """
+
+    error_template = 'Found multi-line formatted string'
+    code = 479

wemake_python_styleguide/visitors/tokenize/primitives.py

@@ -12,6 +12,7 @@
 from wemake_python_styleguide.violations import consistency
 from wemake_python_styleguide.violations.base import TokenizeViolation
 from wemake_python_styleguide.violations.best_practices import (
+    MultilineFormattedStringViolation,
     WrongUnicodeEscapeViolation,
 )
 from wemake_python_styleguide.visitors.base import BaseTokenVisitor
@@ -226,3 +227,33 @@ def visit_fstring_start(  # pragma: >3.12 cover
         # but, since we don't recommend `f`-string, this is a low-priority
         modifiers = token.string[:-1]
         self._checker.check_string_modifiers(token, modifiers)
+
+
+@final
+class MultilineFormattedStringTokenVisitor(
+    BaseTokenVisitor
+):  # pragma: >=3.12 cover
+    """Checks incorrect formatted string usages."""
+
+    _multiline_fstring_pattern: ClassVar[re.Pattern[str]] = re.compile(
+        r"""
+        .*                  # (1) anything before the f-string
+        fr?(['"])           # (2) `f` or `fr`prefix + a single or double quote
+        (?!\1\1)            # (3) not triple quote
+        .*                  # (4) any characters up to…
+        (\{.*\}.)*          # (5) any fully closed {…} expressions, if present
+        .*                  # (6) then more arbitrary chars
+        \{                  # (7) an opening brace of an f-expr
+        [^}]*\n             # (8) chars up to a newline (i.e. multiline)
+        """,
+        re.VERBOSE,
+    )
+
+    def visit_fstring_start(self, token: tokenize.TokenInfo) -> None:
+        """Performs check."""
+        self._check_fstring_is_multi_lined(token)
+
+    def _check_fstring_is_multi_lined(self, token: tokenize.TokenInfo) -> None:
+        """Finds if f-string is multi-line."""
+        if self._multiline_fstring_pattern.match(token.line):
+            self.add_violation(MultilineFormattedStringViolation(token))