SIM907: Use Optional[Type] instead of Union[Type, None]

Closes #64

README.md

@@ -51,6 +51,7 @@ Python-specific rules:
 * `SIM124`: Reserved for [SIM904](#sim904) once it's stable
 * `SIM125`: Reserved for [SIM905](#sim905) once it's stable
 * `SIM126`: Reserved for [SIM906](#sim906) once it's stable
+* `SIM127`: Reserved for [SIM907](#sim907) once it's stable
 
 Simplifying Comparations:
 
@@ -598,3 +599,19 @@ os.path.join(a, os.path.join(b, c))
 # Good
 os.path.join(a, b, c)
 ```
+
+### SIM907
+
+This rule will be renamed to `SIM127` after its 6-month trial period is over.
+Please report any issues you encounter with this rule!
+
+```python
+# Bad
+def foo(a: Union[int, None]) -> Union[int, None]:
+    return a
+
+
+# Good
+def foo(a: Optional[int]) -> Optional[int]:
+    return a
+```

flake8_simplify/__init__.py

@@ -37,6 +37,7 @@
     get_sim401,
 )
 from flake8_simplify.rules.ast_ifexp import get_sim210, get_sim211, get_sim212
+from flake8_simplify.rules.ast_subscript import get_sim907
 from flake8_simplify.rules.ast_try import get_sim105, get_sim107
 from flake8_simplify.rules.ast_unary_op import (
     get_sim201,
@@ -109,6 +110,10 @@ def visit_For(self, node: ast.For) -> None:
         self.errors += get_sim113(node)
         self.generic_visit(node)
 
+    def visit_Subscript(self, node: ast.Subscript) -> None:
+        self.errors += get_sim907(node)
+        self.generic_visit(node)
+
     def visit_Try(self, node: ast.Try) -> None:
         self.errors += get_sim105(node)
         self.errors += get_sim107(node)

flake8_simplify/rules/ast_subscript.py

@@ -0,0 +1,59 @@
+# Core Library
+import ast
+from typing import List, Tuple
+
+# First party
+from flake8_simplify.constants import BOOL_CONST_TYPES
+from flake8_simplify.utils import to_source
+
+
+def get_sim907(node: ast.Subscript) -> List[Tuple[int, int, str]]:
+    """
+
+    Subscript(
+        value=Name(id='Union', ctx=Load()),
+        slice=Tuple(
+            elts=[
+                Name(id='int', ctx=Load()),
+                Name(id='str', ctx=Load()),
+                Constant(value=None, kind=None),
+            ],
+            ...
+        )
+    )
+    """
+    errors: List[Tuple[int, int, str]] = []
+
+    if not (isinstance(node.value, ast.Name) and node.value.id == "Union"):
+        return errors
+
+    if isinstance(node.slice, ast.Index) and isinstance(
+        node.slice.value, ast.Tuple  # type: ignore
+    ):
+        # Python 3.8
+        tuple = node.slice.value  # type: ignore
+    elif isinstance(node.slice, ast.Tuple):
+        # Python 3.9+
+        tuple = node.slice
+    else:
+        return errors
+
+    has_none = False
+    others = []
+    for elt in tuple.elts:  # type: ignore
+        if isinstance(elt, BOOL_CONST_TYPES) and elt.value is None:
+            has_none = True
+        else:
+            others.append(elt)
+
+    RULE = "SIM907 Use 'Optional[{type_}]' instead of '{original}'"
+    if len(others) == 1 and has_none:
+        type_ = to_source(others[0])
+        errors.append(
+            (
+                node.lineno,
+                node.col_offset,
+                RULE.format(type_=type_, original=to_source(node)),
+            )
+        )
+    return errors

tests/test_900_rules.py

@@ -85,3 +85,13 @@ def test_sim905():
 def test_sim906(s, msg):
     results = _results(s)
     assert results == {msg}
+
+
+def test_sim907():
+    results = _results(
+        """def foo(a: Union[int, None]) -> bool:
+  return a"""
+    )
+    assert results == {
+        "1:11 SIM907 Use 'Optional[int]' instead of 'Union[int, None]'"
+    }