Implement `flake8-no-pep420` (#1942)

Closes https://github.com/charliermarsh/ruff/issues/1844.

CONTRIBUTING.md

@@ -59,7 +59,7 @@ There are four phases to adding a new lint rule:
 1. Define the violation struct in `src/violations.rs` (e.g., `ModuleImportNotAtTopOfFile`).
 2. Map the violation struct to a rule code in `src/registry.rs` (e.g., `E402`).
 3. Define the logic for triggering the violation in `src/checkers/ast.rs` (for AST-based checks),
-   `src/checkers/tokens.rs` (for token-based checks), or `src/checkers/lines.rs` (for text-based checks).
+   `src/checkers/tokens.rs` (for token-based checks), `src/checkers/lines.rs` (for text-based checks) or `src/checkers/filesystem.rs` (for filesystem-based checks).
 4. Add a test fixture.
 5. Update the generated files (documentation and generated code).
 

README.md

@@ -137,6 +137,7 @@ developer of [Zulip](https://github.com/zulip/zulip):
    1. [Pylint (PLC, PLE, PLR, PLW)](#pylint-plc-ple-plr-plw)
    1. [flake8-pie (PIE)](#flake8-pie-pie)
    1. [flake8-commas (COM)](#flake8-commas-com)
+   1. [flake8-no-pep420 (INP)](#flake8-no-pep420-inp)
    1. [Ruff-specific rules (RUF)](#ruff-specific-rules-ruf)<!-- End auto-generated table of contents. -->
 1. [Editor Integrations](#editor-integrations)
 1. [FAQ](#faq)
@@ -1158,6 +1159,14 @@ For more, see [flake8-commas](https://pypi.org/project/flake8-commas/2.1.0/) on
 | COM818 | TrailingCommaOnBareTupleProhibited | Trailing comma on bare tuple prohibited |  |
 | COM819 | TrailingCommaProhibited | Trailing comma prohibited | 🛠 |
 
+### flake8-no-pep420 (INP)
+
+For more, see [flake8-no-pep420](https://pypi.org/project/flake8-boolean-trap/2.3.0/) on PyPI.
+
+| Code | Name | Message | Fix |
+| ---- | ---- | ------- | --- |
+| INP001 | ImplicitNamespacePackage | File `...` is part of an implicit namespace package. Add an `__init__.py`. |  |
+
 ### Ruff-specific rules (RUF)
 
 | Code | Name | Message | Fix |
@@ -1451,6 +1460,7 @@ natively, including:
 - [`flake8-errmsg`](https://pypi.org/project/flake8-errmsg/)
 - [`flake8-implicit-str-concat`](https://pypi.org/project/flake8-implicit-str-concat/)
 - [`flake8-import-conventions`](https://github.com/joaopalmeiro/flake8-import-conventions)
+- [`flake8-no-pep420`](https://pypi.org/project/flake8-no-pep420)
 - [`flake8-pie`](https://pypi.org/project/flake8-pie/) ([#1543](https://github.com/charliermarsh/ruff/issues/1543))
 - [`flake8-print`](https://pypi.org/project/flake8-print/)
 - [`flake8-quotes`](https://pypi.org/project/flake8-quotes/)
@@ -1518,6 +1528,7 @@ Today, Ruff can be used to replace Flake8 when used with any of the following pl
 - [`flake8-errmsg`](https://pypi.org/project/flake8-errmsg/)
 - [`flake8-implicit-str-concat`](https://pypi.org/project/flake8-implicit-str-concat/)
 - [`flake8-import-conventions`](https://github.com/joaopalmeiro/flake8-import-conventions)
+- [`flake8-no-pep420`](https://pypi.org/project/flake8-no-pep420)
 - [`flake8-pie`](https://pypi.org/project/flake8-pie/) ([#1543](https://github.com/charliermarsh/ruff/issues/1543))
 - [`flake8-print`](https://pypi.org/project/flake8-print/)
 - [`flake8-quotes`](https://pypi.org/project/flake8-quotes/)

licenses/LICENSE_adamchainz_flake8-no-pep420

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2020 Adam Johnson
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

resources/test/fixtures/flake8_no_pep420/test_fail_nonempty/example.py

@@ -0,0 +1 @@
+print('hi')

resources/test/fixtures/flake8_no_pep420/test_fail_shebang/example.py

@@ -0,0 +1,2 @@
+#!/bin/env/python
+print('hi')

resources/test/fixtures/flake8_no_pep420/test_ignored/example.py

@@ -0,0 +1 @@
+import os  # noqa: INP001

ruff.schema.json

@@ -1356,6 +1356,10 @@
         "ICN0",
         "ICN00",
         "ICN001",
+        "INP",
+        "INP0",
+        "INP00",
+        "INP001",
         "ISC",
         "ISC0",
         "ISC00",

src/checkers/filesystem.rs

@@ -0,0 +1,18 @@
+use std::path::Path;
+
+use crate::registry::{Diagnostic, RuleCode};
+use crate::rules::flake8_no_pep420::rules::implicit_namespace_package;
+use crate::settings::Settings;
+
+pub fn check_file_path(path: &Path, settings: &Settings) -> Vec<Diagnostic> {
+    let mut diagnostics: Vec<Diagnostic> = vec![];
+
+    // flake8-no-pep420
+    if settings.rules.enabled(&RuleCode::INP001) {
+        if let Some(diagnostic) = implicit_namespace_package(path) {
+            diagnostics.push(diagnostic);
+        }
+    }
+
+    diagnostics
+}

src/checkers/mod.rs

@@ -1,4 +1,5 @@
 pub mod ast;
+pub mod filesystem;
 pub mod imports;
 pub mod lines;
 pub mod noqa;

src/linter.rs

@@ -7,6 +7,7 @@ use rustpython_parser::lexer::LexResult;
 use crate::ast::types::Range;
 use crate::autofix::fix_file;
 use crate::checkers::ast::check_ast;
+use crate::checkers::filesystem::check_file_path;
 use crate::checkers::imports::check_imports;
 use crate::checkers::lines::check_lines;
 use crate::checkers::noqa::check_noqa;
@@ -62,6 +63,15 @@ pub fn check_path(
         diagnostics.extend(check_tokens(locator, &tokens, settings, autofix));
     }
 
+    // Run the filesystem-based rules.
+    if settings
+        .rules
+        .iter_enabled()
+        .any(|rule_code| matches!(rule_code.lint_source(), LintSource::Filesystem))
+    {
+        diagnostics.extend(check_file_path(path, settings));
+    }
+
     // Run the AST-based rules.
     let use_ast = settings
         .rules

src/registry.rs

@@ -416,6 +416,8 @@ ruff_macros::define_rule_mapping!(
     COM812 => violations::TrailingCommaMissing,
     COM818 => violations::TrailingCommaOnBareTupleProhibited,
     COM819 => violations::TrailingCommaProhibited,
+    // flake8-no-pep420
+    INP001 => violations::ImplicitNamespacePackage,
     // Ruff
     RUF001 => violations::AmbiguousUnicodeCharacterString,
     RUF002 => violations::AmbiguousUnicodeCharacterDocstring,
@@ -459,6 +461,7 @@ pub enum RuleOrigin {
     Pylint,
     Flake8Pie,
     Flake8Commas,
+    Flake8NoPep420,
     Ruff,
 }
 
@@ -525,6 +528,7 @@ impl RuleOrigin {
             RuleOrigin::Pyupgrade => Prefixes::Single(RuleCodePrefix::UP),
             RuleOrigin::Flake8Pie => Prefixes::Single(RuleCodePrefix::PIE),
             RuleOrigin::Flake8Commas => Prefixes::Single(RuleCodePrefix::COM),
+            RuleOrigin::Flake8NoPep420 => Prefixes::Single(RuleCodePrefix::INP),
             RuleOrigin::Ruff => Prefixes::Single(RuleCodePrefix::RUF),
         }
     }
@@ -537,6 +541,7 @@ pub enum LintSource {
     Tokens,
     Imports,
     NoQa,
+    Filesystem,
 }
 
 impl RuleCode {
@@ -567,6 +572,7 @@ impl RuleCode {
             | RuleCode::RUF003 => &LintSource::Tokens,
             RuleCode::E902 => &LintSource::Io,
             RuleCode::I001 | RuleCode::I002 => &LintSource::Imports,
+            RuleCode::INP001 => &LintSource::Filesystem,
             _ => &LintSource::Ast,
         }
     }

src/rules/flake8_no_pep420/mod.rs

@@ -0,0 +1,32 @@
+//! Rules from [flake8-no-pep420](https://pypi.org/project/flake8-boolean-trap/2.3.0/).
+pub(crate) mod rules;
+
+#[cfg(test)]
+mod tests {
+    use std::path::Path;
+
+    use anyhow::Result;
+    use test_case::test_case;
+
+    use crate::linter::test_path;
+    use crate::registry::RuleCode;
+    use crate::settings::Settings;
+
+    #[test_case(Path::new("test_pass"); "INP001_0")]
+    #[test_case(Path::new("test_fail_empty"); "INP001_1")]
+    #[test_case(Path::new("test_fail_nonempty"); "INP001_2")]
+    #[test_case(Path::new("test_fail_shebang"); "INP001_3")]
+    #[test_case(Path::new("test_ignored"); "INP001_4")]
+    fn test_flake8_no_pep420(path: &Path) -> Result<()> {
+        let snapshot = format!("{}", path.to_string_lossy());
+        let diagnostics = test_path(
+            Path::new("./resources/test/fixtures/flake8_no_pep420")
+                .join(path)
+                .join("example.py")
+                .as_path(),
+            &Settings::for_rule(RuleCode::INP001),
+        )?;
+        insta::assert_yaml_snapshot!(snapshot, diagnostics);
+        Ok(())
+    }
+}

src/rules/flake8_no_pep420/rules.rs

@@ -0,0 +1,18 @@
+use std::path::Path;
+
+use crate::ast::types::Range;
+use crate::registry::Diagnostic;
+use crate::violations;
+
+/// INP001
+pub fn implicit_namespace_package(path: &Path) -> Option<Diagnostic> {
+    if let Some(parent) = path.parent() {
+        if !parent.join("__init__.py").as_path().exists() {
+            return Some(Diagnostic::new(
+                violations::ImplicitNamespacePackage(path.to_string_lossy().to_string()),
+                Range::default(),
+            ));
+        }
+    }
+    None
+}

src/rules/flake8_no_pep420/snapshots/ruff__rules__flake8_no_pep420__tests__test_fail_empty.snap

@@ -0,0 +1,15 @@
+---
+source: src/rules/flake8_no_pep420/mod.rs
+expression: diagnostics
+---
+- kind:
+    ImplicitNamespacePackage: "./resources/test/fixtures/flake8_no_pep420/test_fail_empty/example.py"
+  location:
+    row: 1
+    column: 0
+  end_location:
+    row: 1
+    column: 0
+  fix: ~
+  parent: ~
+

src/rules/flake8_no_pep420/snapshots/ruff__rules__flake8_no_pep420__tests__test_fail_nonempty.snap

@@ -0,0 +1,15 @@
+---
+source: src/rules/flake8_no_pep420/mod.rs
+expression: diagnostics
+---
+- kind:
+    ImplicitNamespacePackage: "./resources/test/fixtures/flake8_no_pep420/test_fail_nonempty/example.py"
+  location:
+    row: 1
+    column: 0
+  end_location:
+    row: 1
+    column: 0
+  fix: ~
+  parent: ~
+

src/rules/flake8_no_pep420/snapshots/ruff__rules__flake8_no_pep420__tests__test_fail_shebang.snap

@@ -0,0 +1,15 @@
+---
+source: src/rules/flake8_no_pep420/mod.rs
+expression: diagnostics
+---
+- kind:
+    ImplicitNamespacePackage: "./resources/test/fixtures/flake8_no_pep420/test_fail_shebang/example.py"
+  location:
+    row: 1
+    column: 0
+  end_location:
+    row: 1
+    column: 0
+  fix: ~
+  parent: ~
+

src/rules/flake8_no_pep420/snapshots/ruff__rules__flake8_no_pep420__tests__test_ignored.snap

@@ -0,0 +1,6 @@
+---
+source: src/rules/flake8_no_pep420/mod.rs
+expression: diagnostics
+---
+[]
+

src/rules/flake8_no_pep420/snapshots/ruff__rules__flake8_no_pep420__tests__test_pass.snap

@@ -0,0 +1,6 @@
+---
+source: src/rules/flake8_no_pep420/mod.rs
+expression: diagnostics
+---
+[]
+

src/rules/mod.rs

@@ -13,6 +13,7 @@ pub mod flake8_debugger;
 pub mod flake8_errmsg;
 pub mod flake8_implicit_str_concat;
 pub mod flake8_import_conventions;
+pub mod flake8_no_pep420;
 pub mod flake8_pie;
 pub mod flake8_print;
 pub mod flake8_pytest_style;

src/violations.rs

@@ -6088,6 +6088,22 @@ impl AlwaysAutofixableViolation for TrailingCommaProhibited {
     }
 }
 
+// flake8-no-pep420
+
+define_violation!(
+    pub struct ImplicitNamespacePackage(pub String);
+);
+impl Violation for ImplicitNamespacePackage {
+    fn message(&self) -> String {
+        let ImplicitNamespacePackage(filename) = self;
+        format!("File `{filename}` is part of an implicit namespace package. Add an `__init__.py`.")
+    }
+
+    fn placeholder() -> Self {
+        ImplicitNamespacePackage("...".to_string())
+    }
+}
+
 // Ruff
 
 define_violation!(