N811 & N814: eliminate false positives for single-letter names (#14584)

Co-authored-by: Micha Reiser <micha@reiser.io>
Co-authored-by: Alex Waygood <alex.waygood@gmail.com>

crates/ruff_linter/resources/test/fixtures/pep8_naming/N811.py

@@ -1,3 +1,9 @@
 import mod.CONST as const
 from mod import CONSTANT as constant
 from mod import ANOTHER_CONSTANT as another_constant
+import mod.CON as c
+from mod import C as c
+
+# These are all OK:
+import django.db.models.Q as Query1
+from django.db.models import Q as Query2

crates/ruff_linter/resources/test/fixtures/pep8_naming/N814.py

@@ -1,3 +1,7 @@
 import mod.Camel as CAMEL
 from mod import CamelCase as CAMELCASE
 from mod import AnotherCamelCase as ANOTHER_CAMELCASE
+
+# These are all OK:
+import mod.AppleFruit as A
+from mod import BananaFruit as B

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

@@ -30,6 +30,20 @@ use crate::rules::pep8_naming::settings::IgnoreNames;
 /// from example import MyClassName
 /// ```
 ///
+/// ## Note
+/// Identifiers consisting of a single uppercase character are ambiguous under
+/// the rules of [PEP 8], which specifies `CamelCase` for classes and
+/// `ALL_CAPS_SNAKE_CASE` for constants. Without a second character, it is not
+/// possible to reliably guess whether the identifier is intended to be part
+/// of a `CamelCase` string for a class or an `ALL_CAPS_SNAKE_CASE` string for
+/// a constant, since both conventions will produce the same output when given
+/// a single input character. Therefore, this lint rule does not apply to cases
+/// where the alias for the imported identifier consists of a single uppercase
+/// character.
+///
+/// A common example of a single uppercase character being used for a class
+/// name can be found in Django's `django.db.models.Q` class.
+///
 /// [PEP 8]: https://peps.python.org/pep-0008/
 #[derive(ViolationMetadata)]
 pub(crate) struct CamelcaseImportedAsConstant {
@@ -53,6 +67,9 @@ pub(crate) fn camelcase_imported_as_constant(
     stmt: &Stmt,
     ignore_names: &IgnoreNames,
 ) -> Option<Diagnostic> {
+    // Single-character names are ambiguous. It could be a class or a constant.
+    asname.chars().nth(1)?;
+
     if helpers::is_camelcase(name)
         && !str::is_cased_lowercase(asname)
         && str::is_cased_uppercase(asname)

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

@@ -4,7 +4,7 @@ use ruff_python_ast::{Alias, Stmt};
 use ruff_python_stdlib::str;
 use ruff_text_size::Ranged;
 
-use crate::rules::pep8_naming::settings::IgnoreNames;
+use crate::rules::pep8_naming::{helpers, settings::IgnoreNames};
 
 /// ## What it does
 /// Checks for constant imports that are aliased to non-constant-style
@@ -29,6 +29,19 @@ use crate::rules::pep8_naming::settings::IgnoreNames;
 /// from example import CONSTANT_VALUE
 /// ```
 ///
+/// ## Note
+/// Identifiers consisting of a single uppercase character are ambiguous under
+/// the rules of [PEP 8], which specifies `CamelCase` for classes and
+/// `ALL_CAPS_SNAKE_CASE` for constants. Without a second character, it is not
+/// possible to reliably guess whether the identifier is intended to be part
+/// of a `CamelCase` string for a class or an `ALL_CAPS_SNAKE_CASE` string for
+/// a constant, since both conventions will produce the same output when given
+/// a single input character. Therefore, this lint rule does not apply to cases
+/// where the imported identifier consists of a single uppercase character.
+///
+/// A common example of a single uppercase character being used for a class
+/// name can be found in Django's `django.db.models.Q` class.
+///
 /// [PEP 8]: https://peps.python.org/pep-0008/
 #[derive(ViolationMetadata)]
 pub(crate) struct ConstantImportedAsNonConstant {
@@ -52,7 +65,13 @@ pub(crate) fn constant_imported_as_non_constant(
     stmt: &Stmt,
     ignore_names: &IgnoreNames,
 ) -> Option<Diagnostic> {
-    if str::is_cased_uppercase(name) && !str::is_cased_uppercase(asname) {
+    if str::is_cased_uppercase(name)
+        && !(str::is_cased_uppercase(asname)
+            // Single-character names are ambiguous.
+            // It could be a class or a constant, so allow it to be imported
+            // as `SCREAMING_SNAKE_CASE` *or* `CamelCase`.
+            || (name.chars().nth(1).is_none() && helpers::is_camelcase(asname)))
+    {
         // Ignore any explicitly-allowed names.
         if ignore_names.matches(name) || ignore_names.matches(asname) {
             return None;

crates/ruff_linter/src/rules/pep8_naming/snapshots/ruff_linter__rules__pep8_naming__tests__N811_N811.py.snap

@@ -1,6 +1,5 @@
 ---
 source: crates/ruff_linter/src/rules/pep8_naming/mod.rs
-snapshot_kind: text
 ---
 N811.py:1:8: N811 Constant `CONST` imported as non-constant `const`
   |
@@ -16,6 +15,7 @@ N811.py:2:17: N811 Constant `CONSTANT` imported as non-constant `constant`
 2 | from mod import CONSTANT as constant
   |                 ^^^^^^^^^^^^^^^^^^^^ N811
 3 | from mod import ANOTHER_CONSTANT as another_constant
+4 | import mod.CON as c
   |
 
 N811.py:3:17: N811 Constant `ANOTHER_CONSTANT` imported as non-constant `another_constant`
@@ -24,4 +24,25 @@ N811.py:3:17: N811 Constant `ANOTHER_CONSTANT` imported as non-constant `another
 2 | from mod import CONSTANT as constant
 3 | from mod import ANOTHER_CONSTANT as another_constant
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ N811
+4 | import mod.CON as c
+5 | from mod import C as c
+  |
+
+N811.py:4:8: N811 Constant `CON` imported as non-constant `c`
+  |
+2 | from mod import CONSTANT as constant
+3 | from mod import ANOTHER_CONSTANT as another_constant
+4 | import mod.CON as c
+  |        ^^^^^^^^^^^^ N811
+5 | from mod import C as c
+  |
+
+N811.py:5:17: N811 Constant `C` imported as non-constant `c`
+  |
+3 | from mod import ANOTHER_CONSTANT as another_constant
+4 | import mod.CON as c
+5 | from mod import C as c
+  |                 ^^^^^^ N811
+6 | 
+7 | # These are all OK:
   |

crates/ruff_linter/src/rules/pep8_naming/snapshots/ruff_linter__rules__pep8_naming__tests__N814_N814.py.snap

@@ -1,6 +1,5 @@
 ---
 source: crates/ruff_linter/src/rules/pep8_naming/mod.rs
-snapshot_kind: text
 ---
 N814.py:1:8: N814 Camelcase `Camel` imported as constant `CAMEL`
   |
@@ -24,4 +23,6 @@ N814.py:3:17: N814 Camelcase `AnotherCamelCase` imported as constant `ANOTHER_CA
 2 | from mod import CamelCase as CAMELCASE
 3 | from mod import AnotherCamelCase as ANOTHER_CAMELCASE
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ N814
+4 | 
+5 | # These are all OK:
   |