SIM909: Avoid reflexive assignments

Closes #114

README.md

@@ -87,6 +87,7 @@ General Code Style:
 * [`SIM112`](https://github.com/MartinThoma/flake8-simplify/issues/19): Use CAPITAL environment variables ([example](#SIM112))
 * `SIM122`: Reserved for SIM902 once it's stable
 * `SIM123`: Reserved for SIM903 once it's stable
+* `SIM124`: Reserved for SIM909 once it's stable
 
 **Experimental rules:**
 
@@ -631,3 +632,24 @@ if "some_key" in some_dict:
 # Good
 name = some_dict.get("some_key", "some_default")
 ```
+
+### SIM909
+
+Thank you Ryan Delaney for the idea!
+
+```python
+# Bad
+foo = foo
+
+# Good: Nothing. Reflexive assignments have no purpose.
+```
+
+or
+
+```python
+# Bad
+foo = foo = 42
+
+# Good
+foo = 42
+```

flake8_simplify/__init__.py

@@ -5,7 +5,7 @@
 from typing import Any, Generator, List, Tuple, Type
 
 # First party
-from flake8_simplify.rules.ast_assign import get_sim904
+from flake8_simplify.rules.ast_assign import get_sim904, get_sim909
 from flake8_simplify.rules.ast_bool_op import (
     get_sim101,
     get_sim109,
@@ -68,6 +68,7 @@ def __init__(self) -> None:
 
     def visit_Assign(self, node: ast.Assign) -> Any:
         self.errors += get_sim904(node)
+        self.errors += get_sim909(node)
         self.generic_visit(node)
 
     def visit_Call(self, node: ast.Call) -> Any:

flake8_simplify/rules/ast_assign.py

@@ -67,3 +67,47 @@ def get_sim904(node: ast.Assign) -> List[Tuple[int, int, str]]:
         )
     )
     return errors
+
+
+def get_sim909(node: ast.Assign) -> List[Tuple[int, int, str]]:
+    """
+    Avoid reflexive assignments
+
+    Example
+    -------
+    Code:
+        # Bad
+        foo = foo
+
+        # Good: Just remove them
+    Bad AST:
+        [
+            Assign(
+                targets=[Name(id='foo', ctx=Store())],
+                value=Name(id='foo', ctx=Load()),
+                type_comment=None,
+            ),
+        ]
+    """
+    RULE = "SIM909 Remove reflexive assignment '{code}'"
+    errors: List[Tuple[int, int, str]] = []
+
+    names = []
+    if isinstance(node.value, (ast.Name, ast.Subscript, ast.Tuple)):
+        names.append(to_source(node.value))
+    for target in node.targets:
+        names.append(to_source(target))
+
+    if len(names) == len(set(names)):
+        return errors
+
+    code = to_source(node)
+
+    errors.append(
+        (
+            node.lineno,
+            node.col_offset,
+            RULE.format(code=code),
+        )
+    )
+    return errors

flake8_simplify/utils.py

@@ -69,7 +69,7 @@ def __init__(self, orig: ast.For) -> None:
 
 
 def to_source(
-    node: Union[None, ast.expr, ast.Expr, ast.withitem, ast.slice]
+    node: Union[None, ast.expr, ast.Expr, ast.withitem, ast.slice, ast.Assign]
 ) -> str:
     if node is None:
         return "None"

tests/test_900_rules.py

@@ -193,3 +193,47 @@ def test_sim908():
         "2:0 SIM908 Use 'some_dict.get(\"some_key\")' instead of "
         '\'if "some_key" in some_dict: some_dict["some_key"]\''
     }
+
+
+@pytest.mark.parametrize(
+    ("s", "msg"),
+    (
+        # Credits to Ryan Delaney for the following two example
+        # https://github.com/MartinThoma/flake8-simplify/issues/114
+        (
+            "foo = foo",
+            "1:0 SIM909 Remove reflexive assignment 'foo = foo'",
+        ),
+        (
+            "foo = foo = 42",
+            "1:0 SIM909 Remove reflexive assignment 'foo = foo = 42'",
+        ),
+        (
+            "foo = bar = foo = 42",
+            "1:0 SIM909 Remove reflexive assignment 'foo = bar = foo = 42'",
+        ),
+        (
+            "n, m = n, m",
+            "1:0 SIM909 Remove reflexive assignment 'n, m = n, m'",
+        ),
+        (
+            "a['foo'] = a['foo']",
+            "1:0 SIM909 Remove reflexive assignment 'a['foo'] = a['foo']'",
+        ),
+    ),
+    ids=["simple", "double", "multiple", "tuple", "dict"],
+)
+def test_sim909(s, msg):
+    results = _results(s)
+    assert results == {msg}
+
+
+@pytest.mark.parametrize(
+    ("s"),
+    ("n, m = m, n", "foo = 'foo'"),
+    ids=["tuple-switch", "variable"],
+)
+def test_sim909_false_positives(s):
+    results = _results(s)
+    for result in results:
+        assert "SIM909" not in result