[ruff] Validate arguments before offering a fix (RUF056) (#18631)

## Summary

Fixes #18628 by avoiding a fix
if there are "unknown" arguments, including any keyword arguments and
more than the expected 2 positional arguments.

I'm a bit on the fence here because it also seems reasonable to avoid a
diagnostic at all. Especially in the final test case I added (`not
my_dict.get(default=False)`), the hint suggesting to remove
`default=False` seems pretty misleading. At the same time, I guess the
diagnostic at least calls attention to the call site, which could help
to fix the missing argument bug too.

As I commented on the issue, I double-checked that keyword arguments are
invalid as far back as Python 3.8, even though the positional-only
marker was only added to the
[docs](https://docs.python.org/3.11/library/stdtypes.html#dict.get) in
3.12 (link is to 3.11, showing its absence).

## Test Plan

New tests derived from the bug report

## Stabilization

This was planned to be stabilized in 0.12, and the bug is less severe
than some others, but if there's nobody opposed, I will plan **not to
stabilize** this one for now.

Author: ntBre

crates/ruff_linter/resources/test/fixtures/ruff/RUF056.py

@@ -149,23 +149,39 @@ def inner():
 value = not my_dict.get("key", 0.0)  # [RUF056]
 value = not my_dict.get("key", "")  # [RUF056]
 
-# testing dict.get call using kwargs
-value = not my_dict.get(key="key", default=False)  # [RUF056]
-value = not my_dict.get(default=[], key="key")  # [RUF056]
-
 # testing invalid dict.get call with inline comment
 value = not my_dict.get("key", # comment1
                      [] # comment2
                      ) # [RUF056]
 
-# testing invalid dict.get call with kwargs and inline comment
-value = not my_dict.get(key="key", # comment1
-                        default=False # comment2
-                        )  # [RUF056]
-value = not my_dict.get(default=[], # comment1
-                        key="key" # comment2
-                        )  # [RUF056]
-
-# testing invalid dict.get calls
-value = not my_dict.get(key="key", other="something", default=False)
-value = not my_dict.get(default=False, other="something", key="test")
+# regression tests for https://github.com/astral-sh/ruff/issues/18628
+# we should avoid fixes when there are "unknown" arguments present, including
+# extra positional arguments, either of the positional-only arguments passed as
+# a keyword, or completely unknown keywords.
+
+# extra positional
+not my_dict.get("key", False, "?!")
+
+# `default` is positional-only, so these are invalid
+not my_dict.get("key", default=False)
+not my_dict.get(key="key", default=False)
+not my_dict.get(default=[], key="key")
+not my_dict.get(default=False)
+not my_dict.get(key="key", other="something", default=False)
+not my_dict.get(default=False, other="something", key="test")
+
+# comments don't really matter here because of the kwargs but include them for
+# completeness
+not my_dict.get(
+    key="key",  # comment1
+    default=False,  # comment2
+)  # comment 3
+not my_dict.get(
+    default=[],  # comment1
+    key="key",  # comment2
+)  # comment 3
+
+# the fix is arguably okay here because the same `takes no keyword arguments`
+# TypeError is raised at runtime before and after the fix, but we still bail
+# out for having an unrecognized number of arguments
+not my_dict.get("key", False, foo=...)

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

@@ -5,7 +5,7 @@ use ruff_text_size::Ranged;
 
 use crate::checkers::ast::Checker;
 use crate::fix::edits::{Parentheses, remove_argument};
-use crate::{AlwaysFixableViolation, Applicability, Fix};
+use crate::{Applicability, Fix, FixAvailability, Violation};
 
 /// ## What it does
 /// Checks for `dict.get(key, falsy_value)` calls in boolean test positions.
@@ -28,21 +28,34 @@ use crate::{AlwaysFixableViolation, Applicability, Fix};
 /// ```
 ///
 /// ## Fix safety
-/// This rule's fix is marked as safe, unless the `dict.get()` call contains comments between arguments.
+///
+/// This rule's fix is marked as safe, unless the `dict.get()` call contains comments between
+/// arguments that will be deleted.
+///
+/// ## Fix availability
+///
+/// This rule's fix is unavailable in cases where invalid arguments are provided to `dict.get`. As
+/// shown in the [documentation], `dict.get` takes two positional-only arguments, so invalid cases
+/// are identified by the presence of more than two arguments or any keyword arguments.
+///
+/// [documentation]: https://docs.python.org/3.13/library/stdtypes.html#dict.get
 #[derive(ViolationMetadata)]
 pub(crate) struct FalsyDictGetFallback;
 
-impl AlwaysFixableViolation for FalsyDictGetFallback {
+impl Violation for FalsyDictGetFallback {
+    const FIX_AVAILABILITY: FixAvailability = FixAvailability::Sometimes;
+
     #[derive_message_formats]
     fn message(&self) -> String {
         "Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.".to_string()
     }
 
-    fn fix_title(&self) -> String {
-        "Remove falsy fallback from `dict.get()`".to_string()
+    fn fix_title(&self) -> Option<String> {
+        Some("Remove falsy fallback from `dict.get()`".to_string())
     }
 }
 
+/// RUF056
 pub(crate) fn falsy_dict_get_fallback(checker: &Checker, expr: &Expr) {
     let semantic = checker.semantic();
 
@@ -89,6 +102,16 @@ pub(crate) fn falsy_dict_get_fallback(checker: &Checker, expr: &Expr) {
 
     let mut diagnostic = checker.report_diagnostic(FalsyDictGetFallback, fallback_arg.range());
 
+    // All arguments to `dict.get` are positional-only.
+    if !call.arguments.keywords.is_empty() {
+        return;
+    }
+
+    // And there are only two of them, at most.
+    if call.arguments.args.len() > 2 {
+        return;
+    }
+
     let comment_ranges = checker.comment_ranges();
 
     // Determine applicability based on the presence of comments

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF056_RUF056.py.snap

@@ -323,7 +323,7 @@ RUF056.py:149:32: RUF056 [*] Avoid providing a falsy fallback to `dict.get()` in
     149 |+value = not my_dict.get("key")  # [RUF056]
 150 150 | value = not my_dict.get("key", "")  # [RUF056]
 151 151 | 
-152 152 | # testing dict.get call using kwargs
+152 152 | # testing invalid dict.get call with inline comment
 
 RUF056.py:150:32: RUF056 [*] Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
     |
@@ -332,7 +332,7 @@ RUF056.py:150:32: RUF056 [*] Avoid providing a falsy fallback to `dict.get()` in
 150 | value = not my_dict.get("key", "")  # [RUF056]
     |                                ^^ RUF056
 151 |
-152 | # testing dict.get call using kwargs
+152 | # testing invalid dict.get call with inline comment
     |
     = help: Remove falsy fallback from `dict.get()`
 
@@ -343,142 +343,131 @@ RUF056.py:150:32: RUF056 [*] Avoid providing a falsy fallback to `dict.get()` in
 150     |-value = not my_dict.get("key", "")  # [RUF056]
     150 |+value = not my_dict.get("key")  # [RUF056]
 151 151 | 
-152 152 | # testing dict.get call using kwargs
-153 153 | value = not my_dict.get(key="key", default=False)  # [RUF056]
+152 152 | # testing invalid dict.get call with inline comment
+153 153 | value = not my_dict.get("key", # comment1
 
-RUF056.py:153:36: RUF056 [*] Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
+RUF056.py:154:22: RUF056 [*] Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
     |
-152 | # testing dict.get call using kwargs
-153 | value = not my_dict.get(key="key", default=False)  # [RUF056]
-    |                                    ^^^^^^^^^^^^^ RUF056
-154 | value = not my_dict.get(default=[], key="key")  # [RUF056]
+152 | # testing invalid dict.get call with inline comment
+153 | value = not my_dict.get("key", # comment1
+154 |                      [] # comment2
+    |                      ^^ RUF056
+155 |                      ) # [RUF056]
     |
     = help: Remove falsy fallback from `dict.get()`
 
-ℹ Safe fix
+ℹ Unsafe fix
 150 150 | value = not my_dict.get("key", "")  # [RUF056]
 151 151 | 
-152 152 | # testing dict.get call using kwargs
-153     |-value = not my_dict.get(key="key", default=False)  # [RUF056]
-    153 |+value = not my_dict.get(key="key")  # [RUF056]
-154 154 | value = not my_dict.get(default=[], key="key")  # [RUF056]
-155 155 | 
-156 156 | # testing invalid dict.get call with inline comment
-
-RUF056.py:154:25: RUF056 [*] Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
-    |
-152 | # testing dict.get call using kwargs
-153 | value = not my_dict.get(key="key", default=False)  # [RUF056]
-154 | value = not my_dict.get(default=[], key="key")  # [RUF056]
-    |                         ^^^^^^^^^^ RUF056
-155 |
-156 | # testing invalid dict.get call with inline comment
+152 152 | # testing invalid dict.get call with inline comment
+153     |-value = not my_dict.get("key", # comment1
+154     |-                     [] # comment2
+    153 |+value = not my_dict.get("key" # comment2
+155 154 |                      ) # [RUF056]
+156 155 | 
+157 156 | # regression tests for https://github.com/astral-sh/ruff/issues/18628
+
+RUF056.py:163:24: RUF056 Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
+    |
+162 | # extra positional
+163 | not my_dict.get("key", False, "?!")
+    |                        ^^^^^ RUF056
+164 |
+165 | # `default` is positional-only, so these are invalid
     |
     = help: Remove falsy fallback from `dict.get()`
 
-ℹ Safe fix
-151 151 | 
-152 152 | # testing dict.get call using kwargs
-153 153 | value = not my_dict.get(key="key", default=False)  # [RUF056]
-154     |-value = not my_dict.get(default=[], key="key")  # [RUF056]
-    154 |+value = not my_dict.get(key="key")  # [RUF056]
-155 155 | 
-156 156 | # testing invalid dict.get call with inline comment
-157 157 | value = not my_dict.get("key", # comment1
-
-RUF056.py:158:22: RUF056 [*] Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
-    |
-156 | # testing invalid dict.get call with inline comment
-157 | value = not my_dict.get("key", # comment1
-158 |                      [] # comment2
-    |                      ^^ RUF056
-159 |                      ) # [RUF056]
+RUF056.py:166:24: RUF056 Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
+    |
+165 | # `default` is positional-only, so these are invalid
+166 | not my_dict.get("key", default=False)
+    |                        ^^^^^^^^^^^^^ RUF056
+167 | not my_dict.get(key="key", default=False)
+168 | not my_dict.get(default=[], key="key")
     |
     = help: Remove falsy fallback from `dict.get()`
 
-ℹ Unsafe fix
-154 154 | value = not my_dict.get(default=[], key="key")  # [RUF056]
-155 155 | 
-156 156 | # testing invalid dict.get call with inline comment
-157     |-value = not my_dict.get("key", # comment1
-158     |-                     [] # comment2
-    157 |+value = not my_dict.get("key" # comment2
-159 158 |                      ) # [RUF056]
-160 159 | 
-161 160 | # testing invalid dict.get call with kwargs and inline comment
-
-RUF056.py:163:25: RUF056 [*] Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
-    |
-161 | # testing invalid dict.get call with kwargs and inline comment
-162 | value = not my_dict.get(key="key", # comment1
-163 |                         default=False # comment2
-    |                         ^^^^^^^^^^^^^ RUF056
-164 |                         )  # [RUF056]
-165 | value = not my_dict.get(default=[], # comment1
+RUF056.py:167:28: RUF056 Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
+    |
+165 | # `default` is positional-only, so these are invalid
+166 | not my_dict.get("key", default=False)
+167 | not my_dict.get(key="key", default=False)
+    |                            ^^^^^^^^^^^^^ RUF056
+168 | not my_dict.get(default=[], key="key")
+169 | not my_dict.get(default=False)
     |
     = help: Remove falsy fallback from `dict.get()`
 
-ℹ Unsafe fix
-159 159 |                      ) # [RUF056]
-160 160 | 
-161 161 | # testing invalid dict.get call with kwargs and inline comment
-162     |-value = not my_dict.get(key="key", # comment1
-163     |-                        default=False # comment2
-    162 |+value = not my_dict.get(key="key" # comment2
-164 163 |                         )  # [RUF056]
-165 164 | value = not my_dict.get(default=[], # comment1
-166 165 |                         key="key" # comment2
-
-RUF056.py:165:25: RUF056 [*] Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
-    |
-163 |                         default=False # comment2
-164 |                         )  # [RUF056]
-165 | value = not my_dict.get(default=[], # comment1
-    |                         ^^^^^^^^^^ RUF056
-166 |                         key="key" # comment2
-167 |                         )  # [RUF056]
+RUF056.py:168:17: RUF056 Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
+    |
+166 | not my_dict.get("key", default=False)
+167 | not my_dict.get(key="key", default=False)
+168 | not my_dict.get(default=[], key="key")
+    |                 ^^^^^^^^^^ RUF056
+169 | not my_dict.get(default=False)
+170 | not my_dict.get(key="key", other="something", default=False)
     |
     = help: Remove falsy fallback from `dict.get()`
 
-ℹ Unsafe fix
-162 162 | value = not my_dict.get(key="key", # comment1
-163 163 |                         default=False # comment2
-164 164 |                         )  # [RUF056]
-165     |-value = not my_dict.get(default=[], # comment1
-    165 |+value = not my_dict.get(# comment1
-166 166 |                         key="key" # comment2
-167 167 |                         )  # [RUF056]
-168 168 | 
-
-RUF056.py:170:55: RUF056 [*] Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
-    |
-169 | # testing invalid dict.get calls
-170 | value = not my_dict.get(key="key", other="something", default=False)
-    |                                                       ^^^^^^^^^^^^^ RUF056
-171 | value = not my_dict.get(default=False, other="something", key="test")
+RUF056.py:169:17: RUF056 Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
+    |
+167 | not my_dict.get(key="key", default=False)
+168 | not my_dict.get(default=[], key="key")
+169 | not my_dict.get(default=False)
+    |                 ^^^^^^^^^^^^^ RUF056
+170 | not my_dict.get(key="key", other="something", default=False)
+171 | not my_dict.get(default=False, other="something", key="test")
     |
     = help: Remove falsy fallback from `dict.get()`
 
-ℹ Safe fix
-167 167 |                         )  # [RUF056]
-168 168 | 
-169 169 | # testing invalid dict.get calls
-170     |-value = not my_dict.get(key="key", other="something", default=False)
-    170 |+value = not my_dict.get(key="key", other="something")
-171 171 | value = not my_dict.get(default=False, other="something", key="test")
+RUF056.py:170:47: RUF056 Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
+    |
+168 | not my_dict.get(default=[], key="key")
+169 | not my_dict.get(default=False)
+170 | not my_dict.get(key="key", other="something", default=False)
+    |                                               ^^^^^^^^^^^^^ RUF056
+171 | not my_dict.get(default=False, other="something", key="test")
+    |
+    = help: Remove falsy fallback from `dict.get()`
 
-RUF056.py:171:25: RUF056 [*] Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
+RUF056.py:171:17: RUF056 Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
     |
-169 | # testing invalid dict.get calls
-170 | value = not my_dict.get(key="key", other="something", default=False)
-171 | value = not my_dict.get(default=False, other="something", key="test")
-    |                         ^^^^^^^^^^^^^ RUF056
+169 | not my_dict.get(default=False)
+170 | not my_dict.get(key="key", other="something", default=False)
+171 | not my_dict.get(default=False, other="something", key="test")
+    |                 ^^^^^^^^^^^^^ RUF056
+172 |
+173 | # comments don't really matter here because of the kwargs but include them for
     |
     = help: Remove falsy fallback from `dict.get()`
 
-ℹ Safe fix
-168 168 | 
-169 169 | # testing invalid dict.get calls
-170 170 | value = not my_dict.get(key="key", other="something", default=False)
-171     |-value = not my_dict.get(default=False, other="something", key="test")
-    171 |+value = not my_dict.get(other="something", key="test")
+RUF056.py:177:5: RUF056 Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
+    |
+175 | not my_dict.get(
+176 |     key="key",  # comment1
+177 |     default=False,  # comment2
+    |     ^^^^^^^^^^^^^ RUF056
+178 | )  # comment 3
+179 | not my_dict.get(
+    |
+    = help: Remove falsy fallback from `dict.get()`
+
+RUF056.py:180:5: RUF056 Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
+    |
+178 | )  # comment 3
+179 | not my_dict.get(
+180 |     default=[],  # comment1
+    |     ^^^^^^^^^^ RUF056
+181 |     key="key",  # comment2
+182 | )  # comment 3
+    |
+    = help: Remove falsy fallback from `dict.get()`
+
+RUF056.py:187:24: RUF056 Avoid providing a falsy fallback to `dict.get()` in boolean test positions. The default fallback `None` is already falsy.
+    |
+185 | # TypeError is raised at runtime before and after the fix, but we still bail
+186 | # out for having an unrecognized number of arguments
+187 | not my_dict.get("key", False, foo=...)
+    |                        ^^^^^ RUF056
+    |
+    = help: Remove falsy fallback from `dict.get()`