Allow sys.path modifications between imports (#9047)

## Summary

It's common to interleave a `sys.path` modification between imports at
the top of a file. This is a frequent cause of `# noqa: E402` false
positives, as seen in the ecosystem checks. This PR modifies E402 to
omit such modifications when determining the "import boundary".

(We could consider linting against `sys.path` modifications, but that
should be a separate rule.)

Closes: https://github.com/astral-sh/ruff/issues/5557.

crates/ruff_linter/resources/test/fixtures/pycodestyle/E402.py

@@ -19,21 +19,26 @@ if x > 0:
 else:
     import e
 
-__some__magic = 1
+import sys
+sys.path.insert(0, "some/path")
 
 import f
 
+__some__magic = 1
 
-def foo() -> None:
-    import e
-
-
-if __name__ == "__main__":
 import g
 
-import h; import i
+
+def foo() -> None:
+    import h
 
 
 if __name__ == "__main__":
-    import j; \
-import k
+    import i
+
+import j; import k
+
+
+if __name__ == "__main__":
+    import l; \
+import m

crates/ruff_linter/src/checkers/ast/mod.rs

@@ -49,7 +49,7 @@ use ruff_python_ast::{helpers, str, visitor, PySourceType};
 use ruff_python_codegen::{Generator, Quote, Stylist};
 use ruff_python_index::Indexer;
 use ruff_python_parser::typing::{parse_type_annotation, AnnotationKind};
-use ruff_python_semantic::analyze::{typing, visibility};
+use ruff_python_semantic::analyze::{imports, typing, visibility};
 use ruff_python_semantic::{
     BindingFlags, BindingId, BindingKind, Exceptions, Export, FromImport, Globals, Import, Module,
     ModuleKind, NodeId, ScopeId, ScopeKind, SemanticModel, SemanticModelFlags, Snapshot,
@@ -303,9 +303,11 @@ where
             }
             _ => {
                 self.semantic.flags |= SemanticModelFlags::FUTURES_BOUNDARY;
-                if !self.semantic.seen_import_boundary()
-                    && !helpers::is_assignment_to_a_dunder(stmt)
-                    && !helpers::in_nested_block(self.semantic.current_statements())
+                if !(self.semantic.seen_import_boundary()
+                    || helpers::is_assignment_to_a_dunder(stmt)
+                    || helpers::in_nested_block(self.semantic.current_statements())
+                    || self.settings.preview.is_enabled()
+                        && imports::is_sys_path_modification(stmt, self.semantic()))
                 {
                     self.semantic.flags |= SemanticModelFlags::IMPORT_BOUNDARY;
                 }

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

@@ -65,6 +65,7 @@ mod tests {
     }
 
     #[test_case(Rule::IsLiteral, Path::new("constant_literals.py"))]
+    #[test_case(Rule::ModuleImportNotAtTopOfFile, Path::new("E402.py"))]
     #[test_case(Rule::TypeComparison, Path::new("E721.py"))]
     fn preview_rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!(

crates/ruff_linter/src/rules/pycodestyle/rules/imports.rs

@@ -41,6 +41,10 @@ impl Violation for MultipleImportsOnOneLine {
 /// According to [PEP 8], "imports are always put at the top of the file, just after any
 /// module comments and docstrings, and before module globals and constants."
 ///
+/// In [preview], this rule makes an exception for `sys.path` modifications,
+/// allowing for `sys.path.insert`, `sys.path.append`, and similar
+/// modifications between import statements.
+///
 /// ## Example
 /// ```python
 /// "One string"

crates/ruff_linter/src/rules/pycodestyle/snapshots/ruff_linter__rules__pycodestyle__tests__E402_E402.py.snap

@@ -1,27 +1,37 @@
 ---
 source: crates/ruff_linter/src/rules/pycodestyle/mod.rs
 ---
-E402.py:24:1: E402 Module level import not at top of file
+E402.py:25:1: E402 Module level import not at top of file
    |
-22 | __some__magic = 1
-23 | 
-24 | import f
+23 | sys.path.insert(0, "some/path")
+24 | 
+25 | import f
+   | ^^^^^^^^ E402
+26 | 
+27 | __some__magic = 1
+   |
+
+E402.py:29:1: E402 Module level import not at top of file
+   |
+27 | __some__magic = 1
+28 | 
+29 | import g
    | ^^^^^^^^ E402
    |
 
-E402.py:34:1: E402 Module level import not at top of file
+E402.py:39:1: E402 Module level import not at top of file
    |
-32 |     import g
-33 | 
-34 | import h; import i
+37 |     import i
+38 | 
+39 | import j; import k
    | ^^^^^^^^ E402
    |
 
-E402.py:34:11: E402 Module level import not at top of file
+E402.py:39:11: E402 Module level import not at top of file
    |
-32 |     import g
-33 | 
-34 | import h; import i
+37 |     import i
+38 | 
+39 | import j; import k
    |           ^^^^^^^^ E402
    |
 

crates/ruff_linter/src/rules/pycodestyle/snapshots/ruff_linter__rules__pycodestyle__tests__preview__E402_E402.py.snap

@@ -0,0 +1,28 @@
+---
+source: crates/ruff_linter/src/rules/pycodestyle/mod.rs
+---
+E402.py:29:1: E402 Module level import not at top of file
+   |
+27 | __some__magic = 1
+28 | 
+29 | import g
+   | ^^^^^^^^ E402
+   |
+
+E402.py:39:1: E402 Module level import not at top of file
+   |
+37 |     import i
+38 | 
+39 | import j; import k
+   | ^^^^^^^^ E402
+   |
+
+E402.py:39:11: E402 Module level import not at top of file
+   |
+37 |     import i
+38 | 
+39 | import j; import k
+   |           ^^^^^^^^ E402
+   |
+
+

crates/ruff_python_semantic/src/analyze/imports.rs

@@ -0,0 +1,37 @@
+use ruff_python_ast::{self as ast, Expr, Stmt};
+
+use crate::SemanticModel;
+
+/// Returns `true` if a [`Stmt`] is a `sys.path` modification, as in:
+/// ```python
+/// import sys
+///
+/// sys.path.append("../")
+/// ```
+pub fn is_sys_path_modification(stmt: &Stmt, semantic: &SemanticModel) -> bool {
+    let Stmt::Expr(ast::StmtExpr { value, range: _ }) = stmt else {
+        return false;
+    };
+    let Expr::Call(ast::ExprCall { func, .. }) = value.as_ref() else {
+        return false;
+    };
+    semantic
+        .resolve_call_path(func.as_ref())
+        .is_some_and(|call_path| {
+            matches!(
+                call_path.as_slice(),
+                [
+                    "sys",
+                    "path",
+                    "append"
+                        | "insert"
+                        | "extend"
+                        | "remove"
+                        | "pop"
+                        | "clear"
+                        | "reverse"
+                        | "sort"
+                ]
+            )
+        })
+}

crates/ruff_python_semantic/src/analyze/mod.rs

@@ -1,4 +1,5 @@
 pub mod function_type;
+pub mod imports;
 pub mod logging;
 pub mod type_inference;
 pub mod typing;