[pep8-naming] Allow Django model loads in `non-lowercase-variable-i…

…n-function` (`N806`) (#8917)

## Summary

Allows assignments of the form, e.g., `Attachment =
apps.get_model("zerver", "Attachment")`, for better compatibility with
Django.

Closes #7675.

## Test Plan

`cargo test`

crates/ruff_linter/resources/test/fixtures/pep8_naming/N806.py

@@ -1,6 +1,6 @@
 import collections
 from collections import namedtuple
-from typing import TypeAlias, TypeVar, NewType, NamedTuple, TypedDict
+from typing import Type, TypeAlias, TypeVar, NewType, NamedTuple, TypedDict
 
 GLOBAL: str = "foo"
 
@@ -40,3 +40,15 @@ def loop_assign():
     global CURRENT_PORT
     for CURRENT_PORT in range(5):
         pass
+
+
+def model_assign() -> None:
+    Bad = apps.get_model("zerver", "Stream")  # N806
+    Attachment = apps.get_model("zerver", "Attachment")  # OK
+    Recipient = apps.get_model("zerver", model_name="Recipient")  # OK
+    Address: Type = apps.get_model("zerver", "Address")  # OK
+
+    from django.utils.module_loading import import_string
+
+    Bad = import_string("django.core.exceptions.ValidationError")  # N806
+    ValidationError = import_string("django.core.exceptions.ValidationError")  # OK

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

@@ -205,19 +205,9 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
                 ExprContext::Store => {
                     if checker.enabled(Rule::NonLowercaseVariableInFunction) {
                         if checker.semantic.current_scope().kind.is_function() {
-                            // Ignore globals.
-                            if !checker
-                                .semantic
-                                .current_scope()
-                                .get(id)
-                                .is_some_and(|binding_id| {
-                                    checker.semantic.binding(binding_id).is_global()
-                                })
-                            {
-                                pep8_naming::rules::non_lowercase_variable_in_function(
-                                    checker, expr, id,
-                                );
-                            }
+                            pep8_naming::rules::non_lowercase_variable_in_function(
+                                checker, expr, id,
+                            );
                         }
                     }
                     if checker.enabled(Rule::MixedCaseVariableInClassScope) {

crates/ruff_linter/src/rules/pep8_naming/helpers.rs

@@ -1,4 +1,5 @@
 use itertools::Itertools;
+use ruff_python_ast::call_path::collect_call_path;
 use ruff_python_ast::{self as ast, Arguments, Expr, Stmt};
 
 use ruff_python_semantic::SemanticModel;
@@ -72,6 +73,7 @@ pub(super) fn is_type_alias_assignment(stmt: &Stmt, semantic: &SemanticModel) ->
     }
 }
 
+/// Returns `true` if the statement is an assignment to a `TypedDict`.
 pub(super) fn is_typed_dict_class(arguments: Option<&Arguments>, semantic: &SemanticModel) -> bool {
     arguments.is_some_and(|arguments| {
         arguments
@@ -81,6 +83,67 @@ pub(super) fn is_typed_dict_class(arguments: Option<&Arguments>, semantic: &Sema
     })
 }
 
+/// Returns `true` if a statement appears to be a dynamic import of a Django model.
+///
+/// For example, in Django, it's common to use `get_model` to access a model dynamically, as in:
+/// ```python
+/// def migrate_existing_attachment_data(
+///     apps: StateApps, schema_editor: BaseDatabaseSchemaEditor
+/// ) -> None:
+///     Attachment = apps.get_model("zerver", "Attachment")
+/// ```
+pub(super) fn is_django_model_import(name: &str, stmt: &Stmt, semantic: &SemanticModel) -> bool {
+    fn match_model_import(name: &str, expr: &Expr, semantic: &SemanticModel) -> bool {
+        let Expr::Call(ast::ExprCall {
+            func, arguments, ..
+        }) = expr
+        else {
+            return false;
+        };
+
+        // Match against, e.g., `apps.get_model("zerver", "Attachment")`.
+        if let Some(call_path) = collect_call_path(func.as_ref()) {
+            if matches!(call_path.as_slice(), [.., "get_model"]) {
+                if let Some(argument) =
+                    arguments.find_argument("model_name", arguments.args.len() - 1)
+                {
+                    if let Some(string_literal) = argument.as_string_literal_expr() {
+                        return string_literal.value.to_str() == name;
+                    }
+                }
+            }
+        }
+
+        // Match against, e.g., `import_string("zerver.models.Attachment")`.
+        if let Some(call_path) = semantic.resolve_call_path(func.as_ref()) {
+            if matches!(
+                call_path.as_slice(),
+                ["django", "utils", "module_loading", "import_string"]
+            ) {
+                if let Some(argument) = arguments.find_argument("dotted_path", 0) {
+                    if let Some(string_literal) = argument.as_string_literal_expr() {
+                        if let Some((.., model)) = string_literal.value.to_str().rsplit_once('.') {
+                            return model == name;
+                        }
+                    }
+                }
+            }
+        }
+
+        false
+    }
+
+    match stmt {
+        Stmt::AnnAssign(ast::StmtAnnAssign {
+            value: Some(value), ..
+        }) => match_model_import(name, value.as_ref(), semantic),
+        Stmt::Assign(ast::StmtAssign { value, .. }) => {
+            match_model_import(name, value.as_ref(), semantic)
+        }
+        _ => false,
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::{is_acronym, is_camelcase, is_mixed_case};

crates/ruff_linter/src/rules/pep8_naming/rules/non_lowercase_variable_in_function.rs

@@ -53,6 +53,15 @@ impl Violation for NonLowercaseVariableInFunction {
 
 /// N806
 pub(crate) fn non_lowercase_variable_in_function(checker: &mut Checker, expr: &Expr, name: &str) {
+    // Ignore globals.
+    if checker
+        .semantic()
+        .lookup_symbol(name)
+        .is_some_and(|id| checker.semantic().binding(id).is_global())
+    {
+        return;
+    }
+
     if checker
         .settings
         .pep8_naming
@@ -72,6 +81,7 @@ pub(crate) fn non_lowercase_variable_in_function(checker: &mut Checker, expr: &E
         || helpers::is_typed_dict_assignment(parent, checker.semantic())
         || helpers::is_type_var_assignment(parent, checker.semantic())
         || helpers::is_type_alias_assignment(parent, checker.semantic())
+        || helpers::is_django_model_import(name, parent, checker.semantic())
     {
         return;
     }

crates/ruff_linter/src/rules/pep8_naming/snapshots/ruff_linter__rules__pep8_naming__tests__N806_N806.py.snap

@@ -20,4 +20,22 @@ N806.py:13:5: N806 Variable `CONSTANT` in function should be lowercase
 14 |     _ = 0
    |
 
+N806.py:46:5: N806 Variable `Bad` in function should be lowercase
+   |
+45 | def model_assign() -> None:
+46 |     Bad = apps.get_model("zerver", "Stream")  # N806
+   |     ^^^ N806
+47 |     Attachment = apps.get_model("zerver", "Attachment")  # OK
+48 |     Recipient = apps.get_model("zerver", model_name="Recipient")  # OK
+   |
+
+N806.py:53:5: N806 Variable `Bad` in function should be lowercase
+   |
+51 |     from django.utils.module_loading import import_string
+52 | 
+53 |     Bad = import_string("django.core.exceptions.ValidationError")  # N806
+   |     ^^^ N806
+54 |     ValidationError = import_string("django.core.exceptions.ValidationError")  # OK
+   |
+