[flake8-bugbear] Implement mutable-contextvar-default (B039)

crates/ruff_linter/resources/test/fixtures/flake8_bugbear/B039.py

@@ -0,0 +1,36 @@
+from contextvars import ContextVar
+from types import MappingProxyType
+import re
+import collections
+import time
+
+# Okay
+ContextVar("cv")
+ContextVar("cv", default=())
+ContextVar("cv", default=(1, 2, 3))
+ContextVar("cv", default="foo")
+ContextVar("cv", default=tuple())
+ContextVar("cv", default=frozenset())
+ContextVar("cv", default=MappingProxyType({}))
+ContextVar("cv", default=re.compile("foo"))
+ContextVar("cv", default=float(1))
+
+# Bad
+ContextVar("cv", default=[])
+ContextVar("cv", default={})
+ContextVar("cv", default=list())
+ContextVar("cv", default=set())
+ContextVar("cv", default=dict())
+ContextVar("cv", default=[char for char in "foo"])
+ContextVar("cv", default={char for char in "foo"})
+ContextVar("cv", default={char: idx for idx, char in enumerate("foo")})
+ContextVar("cv", default=collections.deque())
+
+def bar() -> list[int]:
+    return [1, 2, 3]
+
+ContextVar("cv", default=bar())
+ContextVar("cv", default=time.time())
+
+def baz(): ...
+ContextVar("cv", default=baz())

crates/ruff_linter/resources/test/fixtures/flake8_bugbear/B039_extended.py

@@ -0,0 +1,7 @@
+from contextvars import ContextVar
+
+from fastapi import Query
+ContextVar("cv", default=Query(None))
+
+from something_else import Depends
+ContextVar("cv", default=Depends())

crates/ruff_linter/src/checkers/ast/analyze/expression.rs

@@ -580,6 +580,9 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
             if checker.enabled(Rule::NoExplicitStacklevel) {
                 flake8_bugbear::rules::no_explicit_stacklevel(checker, call);
             }
+            if checker.enabled(Rule::MutableContextvarDefault) {
+                flake8_bugbear::rules::mutable_contextvar_default(checker, call);
+            }
             if checker.enabled(Rule::UnnecessaryDictKwargs) {
                 flake8_pie::rules::unnecessary_dict_kwargs(checker, call);
             }

crates/ruff_linter/src/codes.rs

@@ -345,6 +345,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Flake8Bugbear, "033") => (RuleGroup::Stable, rules::flake8_bugbear::rules::DuplicateValue),
         (Flake8Bugbear, "034") => (RuleGroup::Stable, rules::flake8_bugbear::rules::ReSubPositionalArgs),
         (Flake8Bugbear, "035") => (RuleGroup::Stable, rules::flake8_bugbear::rules::StaticKeyDictComprehension),
+        (Flake8Bugbear, "039") => (RuleGroup::Preview, rules::flake8_bugbear::rules::MutableContextvarDefault),
         (Flake8Bugbear, "901") => (RuleGroup::Preview, rules::flake8_bugbear::rules::ReturnInGenerator),
         (Flake8Bugbear, "904") => (RuleGroup::Stable, rules::flake8_bugbear::rules::RaiseWithoutFromInsideExcept),
         (Flake8Bugbear, "905") => (RuleGroup::Stable, rules::flake8_bugbear::rules::ZipWithoutExplicitStrict),

crates/ruff_linter/src/rules/flake8_bugbear/mod.rs

@@ -64,6 +64,7 @@ mod tests {
     #[test_case(Rule::UselessExpression, Path::new("B018.py"))]
     #[test_case(Rule::ReturnInGenerator, Path::new("B901.py"))]
     #[test_case(Rule::LoopIteratorMutation, Path::new("B909.py"))]
+    #[test_case(Rule::MutableContextvarDefault, Path::new("B039.py"))]
     fn rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!("{}_{}", rule_code.noqa_code(), path.to_string_lossy());
         let diagnostics = test_path(
@@ -124,4 +125,20 @@ mod tests {
         assert_messages!(snapshot, diagnostics);
         Ok(())
     }
+
+    #[test]
+    fn extend_mutable_contextvar_default() -> Result<()> {
+        let snapshot = "extend_mutable_contextvar_default".to_string();
+        let diagnostics = test_path(
+            Path::new("flake8_bugbear/B039_extended.py"),
+            &LinterSettings {
+                flake8_bugbear: super::settings::Settings {
+                    extend_immutable_calls: vec!["fastapi.Query".to_string()],
+                },
+                ..LinterSettings::for_rule(Rule::MutableContextvarDefault)
+            },
+        )?;
+        assert_messages!(snapshot, diagnostics);
+        Ok(())
+    }
 }

crates/ruff_linter/src/rules/flake8_bugbear/rules/mod.rs

@@ -15,6 +15,7 @@ pub(crate) use jump_statement_in_finally::*;
 pub(crate) use loop_iterator_mutation::*;
 pub(crate) use loop_variable_overrides_iterator::*;
 pub(crate) use mutable_argument_default::*;
+pub(crate) use mutable_contextvar_default::*;
 pub(crate) use no_explicit_stacklevel::*;
 pub(crate) use raise_literal::*;
 pub(crate) use raise_without_from_inside_except::*;
@@ -52,6 +53,7 @@ mod jump_statement_in_finally;
 mod loop_iterator_mutation;
 mod loop_variable_overrides_iterator;
 mod mutable_argument_default;
+mod mutable_contextvar_default;
 mod no_explicit_stacklevel;
 mod raise_literal;
 mod raise_without_from_inside_except;

crates/ruff_linter/src/rules/flake8_bugbear/rules/mutable_contextvar_default.rs

@@ -0,0 +1,108 @@
+use ruff_diagnostics::{Diagnostic, Violation};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast::name::QualifiedName;
+use ruff_python_ast::{self as ast, Expr};
+use ruff_python_semantic::analyze::typing::{is_immutable_func, is_mutable_expr, is_mutable_func};
+use ruff_python_semantic::Modules;
+use ruff_text_size::Ranged;
+
+use crate::checkers::ast::Checker;
+
+/// ## What it does
+/// Checks for uses of mutable objects as `ContextVar` defaults.
+///
+/// ## Why is this bad?
+///
+/// The `ContextVar` default is evaluated once, when the `ContextVar` is defined.
+///
+/// The same mutable object is then shared across all `.get()` method calls to
+/// the `ContextVar`. If the object is modified, those modifications will persist
+/// across calls, which can lead to unexpected behavior.
+///
+/// Instead, prefer to use immutable data structures; or, take `None` as a
+/// default, and initialize a new mutable object inside for each call using the
+/// `.set()` method.
+///
+/// Types outside the standard library can be marked as immutable with the
+/// [`lint.flake8-bugbear.extend-immutable-calls`] configuration option.
+///
+/// ## Example
+/// ```python
+/// from contextvars import ContextVar
+///
+///
+/// cv: ContextVar[list] = ContextVar("cv", default=[])
+/// ```
+///
+/// Use instead:
+/// ```python
+/// from contextvars import ContextVar
+///
+///
+/// cv: ContextVar[list | None] = ContextVar("cv", default=None)
+///
+/// ...
+///
+/// if cv.get() is None:
+///     cv.set([])
+/// ```
+///
+/// ## Options
+/// - `lint.flake8-bugbear.extend-immutable-calls`
+///
+/// ## References
+/// - [Python documentation: [`contextvars` — Context Variables](https://docs.python.org/3/library/contextvars.html)
+#[violation]
+pub struct MutableContextvarDefault;
+
+impl Violation for MutableContextvarDefault {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("Do not use mutable data structures for `ContextVar` defaults")
+    }
+
+    fn fix_title(&self) -> Option<String> {
+        Some("Replace with `None`; initialize with `.set()``".to_string())
+    }
+}
+
+/// B039
+pub(crate) fn mutable_contextvar_default(checker: &mut Checker, call: &ast::ExprCall) {
+    if !checker.semantic().seen_module(Modules::CONTEXTVARS) {
+        return;
+    }
+
+    let Some(default) = call
+        .arguments
+        .find_keyword("default")
+        .map(|keyword| &keyword.value)
+    else {
+        return;
+    };
+
+    let extend_immutable_calls: Vec<QualifiedName> = checker
+        .settings
+        .flake8_bugbear
+        .extend_immutable_calls
+        .iter()
+        .map(|target| QualifiedName::from_dotted_name(target))
+        .collect();
+
+    if (is_mutable_expr(default, checker.semantic())
+        || matches!(
+            default,
+            Expr::Call(ast::ExprCall { func, .. })
+                if !is_mutable_func(func, checker.semantic())
+                    && !is_immutable_func(func, checker.semantic(), &extend_immutable_calls)))
+        && checker
+            .semantic()
+            .resolve_qualified_name(&call.func)
+            .is_some_and(|qualified_name| {
+                matches!(qualified_name.segments(), ["contextvars", "ContextVar"])
+            })
+    {
+        checker
+            .diagnostics
+            .push(Diagnostic::new(MutableContextvarDefault, default.range()));
+    }
+}

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__B039_B039.py.snap

@@ -0,0 +1,127 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+B039.py:19:26: B039 Do not use mutable data structures for `ContextVar` defaults
+   |
+18 | # Bad
+19 | ContextVar("cv", default=[])
+   |                          ^^ B039
+20 | ContextVar("cv", default={})
+21 | ContextVar("cv", default=list())
+   |
+   = help: Replace with `None`; initialize with `.set()``
+
+B039.py:20:26: B039 Do not use mutable data structures for `ContextVar` defaults
+   |
+18 | # Bad
+19 | ContextVar("cv", default=[])
+20 | ContextVar("cv", default={})
+   |                          ^^ B039
+21 | ContextVar("cv", default=list())
+22 | ContextVar("cv", default=set())
+   |
+   = help: Replace with `None`; initialize with `.set()``
+
+B039.py:21:26: B039 Do not use mutable data structures for `ContextVar` defaults
+   |
+19 | ContextVar("cv", default=[])
+20 | ContextVar("cv", default={})
+21 | ContextVar("cv", default=list())
+   |                          ^^^^^^ B039
+22 | ContextVar("cv", default=set())
+23 | ContextVar("cv", default=dict())
+   |
+   = help: Replace with `None`; initialize with `.set()``
+
+B039.py:22:26: B039 Do not use mutable data structures for `ContextVar` defaults
+   |
+20 | ContextVar("cv", default={})
+21 | ContextVar("cv", default=list())
+22 | ContextVar("cv", default=set())
+   |                          ^^^^^ B039
+23 | ContextVar("cv", default=dict())
+24 | ContextVar("cv", default=[char for char in "foo"])
+   |
+   = help: Replace with `None`; initialize with `.set()``
+
+B039.py:23:26: B039 Do not use mutable data structures for `ContextVar` defaults
+   |
+21 | ContextVar("cv", default=list())
+22 | ContextVar("cv", default=set())
+23 | ContextVar("cv", default=dict())
+   |                          ^^^^^^ B039
+24 | ContextVar("cv", default=[char for char in "foo"])
+25 | ContextVar("cv", default={char for char in "foo"})
+   |
+   = help: Replace with `None`; initialize with `.set()``
+
+B039.py:24:26: B039 Do not use mutable data structures for `ContextVar` defaults
+   |
+22 | ContextVar("cv", default=set())
+23 | ContextVar("cv", default=dict())
+24 | ContextVar("cv", default=[char for char in "foo"])
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^ B039
+25 | ContextVar("cv", default={char for char in "foo"})
+26 | ContextVar("cv", default={char: idx for idx, char in enumerate("foo")})
+   |
+   = help: Replace with `None`; initialize with `.set()``
+
+B039.py:25:26: B039 Do not use mutable data structures for `ContextVar` defaults
+   |
+23 | ContextVar("cv", default=dict())
+24 | ContextVar("cv", default=[char for char in "foo"])
+25 | ContextVar("cv", default={char for char in "foo"})
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^ B039
+26 | ContextVar("cv", default={char: idx for idx, char in enumerate("foo")})
+27 | ContextVar("cv", default=collections.deque())
+   |
+   = help: Replace with `None`; initialize with `.set()``
+
+B039.py:26:26: B039 Do not use mutable data structures for `ContextVar` defaults
+   |
+24 | ContextVar("cv", default=[char for char in "foo"])
+25 | ContextVar("cv", default={char for char in "foo"})
+26 | ContextVar("cv", default={char: idx for idx, char in enumerate("foo")})
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ B039
+27 | ContextVar("cv", default=collections.deque())
+   |
+   = help: Replace with `None`; initialize with `.set()``
+
+B039.py:27:26: B039 Do not use mutable data structures for `ContextVar` defaults
+   |
+25 | ContextVar("cv", default={char for char in "foo"})
+26 | ContextVar("cv", default={char: idx for idx, char in enumerate("foo")})
+27 | ContextVar("cv", default=collections.deque())
+   |                          ^^^^^^^^^^^^^^^^^^^ B039
+28 | 
+29 | def bar() -> list[int]:
+   |
+   = help: Replace with `None`; initialize with `.set()``
+
+B039.py:32:26: B039 Do not use mutable data structures for `ContextVar` defaults
+   |
+30 |     return [1, 2, 3]
+31 | 
+32 | ContextVar("cv", default=bar())
+   |                          ^^^^^ B039
+33 | ContextVar("cv", default=time.time())
+   |
+   = help: Replace with `None`; initialize with `.set()``
+
+B039.py:33:26: B039 Do not use mutable data structures for `ContextVar` defaults
+   |
+32 | ContextVar("cv", default=bar())
+33 | ContextVar("cv", default=time.time())
+   |                          ^^^^^^^^^^^ B039
+34 | 
+35 | def baz(): ...
+   |
+   = help: Replace with `None`; initialize with `.set()``
+
+B039.py:36:26: B039 Do not use mutable data structures for `ContextVar` defaults
+   |
+35 | def baz(): ...
+36 | ContextVar("cv", default=baz())
+   |                          ^^^^^ B039
+   |
+   = help: Replace with `None`; initialize with `.set()``

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__extend_mutable_contextvar_default.snap

@@ -0,0 +1,10 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+B039_extended.py:7:26: B039 Do not use mutable data structures for `ContextVar` defaults
+  |
+6 | from something_else import Depends
+7 | ContextVar("cv", default=Depends())
+  |                          ^^^^^^^^^ B039
+  |
+  = help: Replace with `None`; initialize with `.set()``

crates/ruff_python_semantic/src/model.rs

@@ -1233,6 +1233,7 @@ impl<'a> SemanticModel<'a> {
             "_typeshed" => self.seen.insert(Modules::TYPESHED),
             "builtins" => self.seen.insert(Modules::BUILTINS),
             "collections" => self.seen.insert(Modules::COLLECTIONS),
+            "contextvars" => self.seen.insert(Modules::CONTEXTVARS),
             "dataclasses" => self.seen.insert(Modules::DATACLASSES),
             "datetime" => self.seen.insert(Modules::DATETIME),
             "django" => self.seen.insert(Modules::DJANGO),
@@ -1820,6 +1821,7 @@ bitflags! {
         const TYPESHED = 1 << 16;
         const DATACLASSES = 1 << 17;
         const BUILTINS = 1 << 18;
+        const CONTEXTVARS = 1 << 19;
     }
 }