[`ruff`] Ignore `str()` when not used for simple conversion (`RUF065`) (#21330)

## Summary

Fixed RUF065 (`logging-eager-conversion`) to only flag `str()` calls
when they perform a simple conversion that can be safely removed. The
rule now ignores `str()` calls with no arguments, multiple arguments,
starred arguments, or keyword unpacking, preventing false positives.

Fixes #21315

## Problem Analysis

The RUF065 rule was incorrectly flagging all `str()` calls in logging
statements, even when `str()` was performing actual conversion work
beyond simple type coercion. Specifically, the rule flagged:

- `str()` with no arguments - which returns an empty string
- `str(b"data", "utf-8")` with multiple arguments - which performs
encoding conversion
- `str(*args)` with starred arguments - which unpacks arguments
- `str(**kwargs)` with keyword unpacking - which passes keyword
arguments

These cases cannot be safely removed because `str()` is doing meaningful
work (encoding conversion, argument unpacking, etc.), not just redundant
type conversion.

The root cause was that the rule only checked if the function was
`str()` without validating the call signature. It didn't distinguish
between simple `str(value)` conversions (which can be removed) and more
complex `str()` calls that perform actual work.

## Approach

The fix adds validation to the `str()` detection logic in
`logging_eager_conversion.rs`:

1. **Check argument count**: Only flag `str()` calls with exactly one
positional argument (`str_call_args.args.len() == 1`)
2. **Check for starred arguments**: Ensure the single argument is not
starred (`!str_call_args.args[0].is_starred_expr()`)
3. **Check for keyword arguments**: Ensure there are no keyword
arguments (`str_call_args.keywords.is_empty()`)

This ensures the rule only flags cases like `str(value)` where `str()`
is truly redundant and can be removed, while ignoring cases where
`str()` performs actual conversion work.

The fix maintains backward compatibility - all existing valid test cases
continue to be flagged correctly, while the new edge cases are properly
ignored.

---------

Co-authored-by: Brent Westbrook <brentrwestbrook@gmail.com>

crates/ruff_linter/resources/test/fixtures/ruff/RUF065_1.py

@@ -0,0 +1,18 @@
+import logging
+
+# Test cases for str() that should NOT be flagged (issue #21315)
+# str() with no arguments - should not be flagged
+logging.warning("%s", str())
+
+# str() with multiple arguments - should not be flagged
+logging.warning("%s", str(b"\xe2\x9a\xa0", "utf-8"))
+
+# str() with starred arguments - should not be flagged
+logging.warning("%s", str(*(b"\xf0\x9f\x9a\xa7", "utf-8")))
+
+# str() with keyword unpacking - should not be flagged
+logging.warning("%s", str(**{"object": b"\xf0\x9f\x9a\xa8", "encoding": "utf-8"}))
+
+# str() with single keyword argument - should be flagged (equivalent to str("!"))
+logging.warning("%s", str(object="!"))
+

crates/ruff_linter/src/rules/ruff/mod.rs

@@ -112,7 +112,8 @@ mod tests {
     #[test_case(Rule::LegacyFormPytestRaises, Path::new("RUF061_warns.py"))]
     #[test_case(Rule::LegacyFormPytestRaises, Path::new("RUF061_deprecated_call.py"))]
     #[test_case(Rule::NonOctalPermissions, Path::new("RUF064.py"))]
-    #[test_case(Rule::LoggingEagerConversion, Path::new("RUF065.py"))]
+    #[test_case(Rule::LoggingEagerConversion, Path::new("RUF065_0.py"))]
+    #[test_case(Rule::LoggingEagerConversion, Path::new("RUF065_1.py"))]
     #[test_case(Rule::RedirectedNOQA, Path::new("RUF101_0.py"))]
     #[test_case(Rule::RedirectedNOQA, Path::new("RUF101_1.py"))]
     #[test_case(Rule::InvalidRuleCode, Path::new("RUF102.py"))]

crates/ruff_linter/src/rules/ruff/rules/logging_eager_conversion.rs

@@ -138,7 +138,12 @@ pub(crate) fn logging_eager_conversion(checker: &Checker, call: &ast::ExprCall)
         .zip(call.arguments.args.iter().skip(msg_pos + 1))
     {
         // Check if the argument is a call to eagerly format a value
-        if let Expr::Call(ast::ExprCall { func, .. }) = arg {
+        if let Expr::Call(ast::ExprCall {
+            func,
+            arguments: str_call_args,
+            ..
+        }) = arg
+        {
             let CFormatType::String(format_conversion) = spec.format_type else {
                 continue;
             };
@@ -146,8 +151,13 @@ pub(crate) fn logging_eager_conversion(checker: &Checker, call: &ast::ExprCall)
             // Check for various eager conversion patterns
             match format_conversion {
                 // %s with str() - remove str() call
+                // Only flag if str() has exactly one argument (positional or keyword) that is not unpacked
                 FormatConversion::Str
-                    if checker.semantic().match_builtin_expr(func.as_ref(), "str") =>
+                    if checker.semantic().match_builtin_expr(func.as_ref(), "str")
+                        && str_call_args.len() == 1
+                        && str_call_args
+                            .find_argument("object", 0)
+                            .is_some_and(|arg| !arg.is_variadic()) =>
                 {
                     checker.report_diagnostic(
                         LoggingEagerConversion {

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF065_RUF065_0.py.snap

@@ -2,7 +2,7 @@
 source: crates/ruff_linter/src/rules/ruff/mod.rs
 ---
 RUF065 Unnecessary `str()` conversion when formatting with `%s`
- --> RUF065.py:4:26
+ --> RUF065_0.py:4:26
   |
 3 | # %s + str()
 4 | logging.info("Hello %s", str("World!"))
@@ -11,7 +11,7 @@ RUF065 Unnecessary `str()` conversion when formatting with `%s`
   |
 
 RUF065 Unnecessary `str()` conversion when formatting with `%s`
- --> RUF065.py:5:39
+ --> RUF065_0.py:5:39
   |
 3 | # %s + str()
 4 | logging.info("Hello %s", str("World!"))
@@ -22,7 +22,7 @@ RUF065 Unnecessary `str()` conversion when formatting with `%s`
   |
 
 RUF065 Unnecessary `repr()` conversion when formatting with `%s`. Use `%r` instead of `%s`
- --> RUF065.py:8:26
+ --> RUF065_0.py:8:26
   |
 7 | # %s + repr()
 8 | logging.info("Hello %s", repr("World!"))
@@ -31,7 +31,7 @@ RUF065 Unnecessary `repr()` conversion when formatting with `%s`. Use `%r` inste
   |
 
 RUF065 Unnecessary `repr()` conversion when formatting with `%s`. Use `%r` instead of `%s`
-  --> RUF065.py:9:39
+  --> RUF065_0.py:9:39
    |
  7 | # %s + repr()
  8 | logging.info("Hello %s", repr("World!"))
@@ -42,7 +42,7 @@ RUF065 Unnecessary `repr()` conversion when formatting with `%s`. Use `%r` inste
    |
 
 RUF065 Unnecessary `str()` conversion when formatting with `%s`
-  --> RUF065.py:22:18
+  --> RUF065_0.py:22:18
    |
 21 | # %s + str()
 22 | info("Hello %s", str("World!"))
@@ -51,7 +51,7 @@ RUF065 Unnecessary `str()` conversion when formatting with `%s`
    |
 
 RUF065 Unnecessary `str()` conversion when formatting with `%s`
-  --> RUF065.py:23:31
+  --> RUF065_0.py:23:31
    |
 21 | # %s + str()
 22 | info("Hello %s", str("World!"))
@@ -62,7 +62,7 @@ RUF065 Unnecessary `str()` conversion when formatting with `%s`
    |
 
 RUF065 Unnecessary `repr()` conversion when formatting with `%s`. Use `%r` instead of `%s`
-  --> RUF065.py:26:18
+  --> RUF065_0.py:26:18
    |
 25 | # %s + repr()
 26 | info("Hello %s", repr("World!"))
@@ -71,7 +71,7 @@ RUF065 Unnecessary `repr()` conversion when formatting with `%s`. Use `%r` inste
    |
 
 RUF065 Unnecessary `repr()` conversion when formatting with `%s`. Use `%r` instead of `%s`
-  --> RUF065.py:27:31
+  --> RUF065_0.py:27:31
    |
 25 | # %s + repr()
 26 | info("Hello %s", repr("World!"))
@@ -82,7 +82,7 @@ RUF065 Unnecessary `repr()` conversion when formatting with `%s`. Use `%r` inste
    |
 
 RUF065 Unnecessary `repr()` conversion when formatting with `%s`. Use `%r` instead of `%s`
-  --> RUF065.py:44:32
+  --> RUF065_0.py:44:32
    |
 42 | logging.warning("Value: %r", repr(42))
 43 | logging.error("Error: %r", repr([1, 2, 3]))
@@ -92,7 +92,7 @@ RUF065 Unnecessary `repr()` conversion when formatting with `%s`. Use `%r` inste
    |
 
 RUF065 Unnecessary `repr()` conversion when formatting with `%s`. Use `%r` instead of `%s`
-  --> RUF065.py:45:30
+  --> RUF065_0.py:45:30
    |
 43 | logging.error("Error: %r", repr([1, 2, 3]))
 44 | logging.info("Debug info: %s", repr("test\nstring"))
@@ -103,7 +103,7 @@ RUF065 Unnecessary `repr()` conversion when formatting with `%s`. Use `%r` inste
    |
 
 RUF065 Unnecessary `ascii()` conversion when formatting with `%s`. Use `%a` instead of `%s`
-  --> RUF065.py:48:27
+  --> RUF065_0.py:48:27
    |
 47 | # %s + ascii()
 48 | logging.info("ASCII: %s", ascii("Hello\nWorld"))
@@ -112,7 +112,7 @@ RUF065 Unnecessary `ascii()` conversion when formatting with `%s`. Use `%a` inst
    |
 
 RUF065 Unnecessary `ascii()` conversion when formatting with `%s`. Use `%a` instead of `%s`
-  --> RUF065.py:49:30
+  --> RUF065_0.py:49:30
    |
 47 | # %s + ascii()
 48 | logging.info("ASCII: %s", ascii("Hello\nWorld"))
@@ -123,7 +123,7 @@ RUF065 Unnecessary `ascii()` conversion when formatting with `%s`. Use `%a` inst
    |
 
 RUF065 Unnecessary `oct()` conversion when formatting with `%s`. Use `%#o` instead of `%s`
-  --> RUF065.py:52:27
+  --> RUF065_0.py:52:27
    |
 51 | # %s + oct()
 52 | logging.info("Octal: %s", oct(42))
@@ -132,7 +132,7 @@ RUF065 Unnecessary `oct()` conversion when formatting with `%s`. Use `%#o` inste
    |
 
 RUF065 Unnecessary `oct()` conversion when formatting with `%s`. Use `%#o` instead of `%s`
-  --> RUF065.py:53:30
+  --> RUF065_0.py:53:30
    |
 51 | # %s + oct()
 52 | logging.info("Octal: %s", oct(42))
@@ -143,7 +143,7 @@ RUF065 Unnecessary `oct()` conversion when formatting with `%s`. Use `%#o` inste
    |
 
 RUF065 Unnecessary `hex()` conversion when formatting with `%s`. Use `%#x` instead of `%s`
-  --> RUF065.py:56:25
+  --> RUF065_0.py:56:25
    |
 55 | # %s + hex()
 56 | logging.info("Hex: %s", hex(42))
@@ -152,7 +152,7 @@ RUF065 Unnecessary `hex()` conversion when formatting with `%s`. Use `%#x` inste
    |
 
 RUF065 Unnecessary `hex()` conversion when formatting with `%s`. Use `%#x` instead of `%s`
-  --> RUF065.py:57:28
+  --> RUF065_0.py:57:28
    |
 55 | # %s + hex()
 56 | logging.info("Hex: %s", hex(42))
@@ -161,7 +161,7 @@ RUF065 Unnecessary `hex()` conversion when formatting with `%s`. Use `%#x` inste
    |
 
 RUF065 Unnecessary `ascii()` conversion when formatting with `%s`. Use `%a` instead of `%s`
-  --> RUF065.py:63:19
+  --> RUF065_0.py:63:19
    |
 61 | from logging import info, log
 62 |
@@ -171,7 +171,7 @@ RUF065 Unnecessary `ascii()` conversion when formatting with `%s`. Use `%a` inst
    |
 
 RUF065 Unnecessary `ascii()` conversion when formatting with `%s`. Use `%a` instead of `%s`
-  --> RUF065.py:64:32
+  --> RUF065_0.py:64:32
    |
 63 | info("ASCII: %s", ascii("Hello\nWorld"))
 64 | log(logging.INFO, "ASCII: %s", ascii("test"))
@@ -181,7 +181,7 @@ RUF065 Unnecessary `ascii()` conversion when formatting with `%s`. Use `%a` inst
    |
 
 RUF065 Unnecessary `oct()` conversion when formatting with `%s`. Use `%#o` instead of `%s`
-  --> RUF065.py:66:19
+  --> RUF065_0.py:66:19
    |
 64 | log(logging.INFO, "ASCII: %s", ascii("test"))
 65 |
@@ -191,7 +191,7 @@ RUF065 Unnecessary `oct()` conversion when formatting with `%s`. Use `%#o` inste
    |
 
 RUF065 Unnecessary `oct()` conversion when formatting with `%s`. Use `%#o` instead of `%s`
-  --> RUF065.py:67:32
+  --> RUF065_0.py:67:32
    |
 66 | info("Octal: %s", oct(42))
 67 | log(logging.INFO, "Octal: %s", oct(255))
@@ -201,7 +201,7 @@ RUF065 Unnecessary `oct()` conversion when formatting with `%s`. Use `%#o` inste
    |
 
 RUF065 Unnecessary `hex()` conversion when formatting with `%s`. Use `%#x` instead of `%s`
-  --> RUF065.py:69:17
+  --> RUF065_0.py:69:17
    |
 67 | log(logging.INFO, "Octal: %s", oct(255))
 68 |
@@ -211,7 +211,7 @@ RUF065 Unnecessary `hex()` conversion when formatting with `%s`. Use `%#x` inste
    |
 
 RUF065 Unnecessary `hex()` conversion when formatting with `%s`. Use `%#x` instead of `%s`
-  --> RUF065.py:70:30
+  --> RUF065_0.py:70:30
    |
 69 | info("Hex: %s", hex(42))
 70 | log(logging.INFO, "Hex: %s", hex(255))

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF065_RUF065_1.py.snap

@@ -0,0 +1,10 @@
+---
+source: crates/ruff_linter/src/rules/ruff/mod.rs
+---
+RUF065 Unnecessary `str()` conversion when formatting with `%s`
+  --> RUF065_1.py:17:23
+   |
+16 | # str() with single keyword argument - should be flagged (equivalent to str("!"))
+17 | logging.warning("%s", str(object="!"))
+   |                       ^^^^^^^^^^^^^^^
+   |

crates/ruff_python_ast/src/nodes.rs

@@ -3269,6 +3269,13 @@ impl<'a> ArgOrKeyword<'a> {
             ArgOrKeyword::Keyword(keyword) => &keyword.value,
         }
     }
+
+    pub const fn is_variadic(self) -> bool {
+        match self {
+            ArgOrKeyword::Arg(expr) => expr.is_starred_expr(),
+            ArgOrKeyword::Keyword(keyword) => keyword.arg.is_none(),
+        }
+    }
 }
 
 impl<'a> From<&'a Expr> for ArgOrKeyword<'a> {