No single letter variables

This allows `_` to be used as a variable name, since it is conventionally
used for throwaway variables, but does not allow any other single letter
variables.

The `target` method of `ast.Assigns` will normally contain a list of nodes,
however for an unpacking assignment (such as `'a,b = c'`) it will contain
an `ast.Tuple` or an `ast.List` so we need to cover for all cases.

Author: klssmith

README.md

@@ -39,6 +39,7 @@ a linter for pandas usage, please see [pandas-vet](https://github.com/deppen8/pa
 | PDF020 | found private import across modules                                     |
 | PDF021 | found 'np.bool' or 'np.object' (use 'np.bool_' or 'np.object_' instead) |
 | PDF022 | found import from 'numpy.random'                                        |
+| PDF023 | found assignment to single-letter variable                              |
 ## contributing
 
 See `contributing.md` for how to get started.

pandas_dev_flaker/_plugins_tree/single_letter_variables.py

@@ -0,0 +1,24 @@
+import ast
+from typing import Iterator, Tuple
+
+from pandas_dev_flaker._data_tree import State, register
+
+MSG = "PDF023 found assignment to single-letter variable"
+
+
+@register(ast.Assign)
+def visit_Assign(
+    state: State,
+    node: ast.Assign,
+    parent: ast.AST,
+) -> Iterator[Tuple[int, int, str]]:
+
+    # Unpacking is represented by putting a Tuple or List within targets
+    if isinstance(node.targets[0], (ast.Tuple, ast.List)):
+        assignment_names = node.targets[0].elts
+    else:
+        assignment_names = node.targets
+
+    for item in assignment_names:
+        if isinstance(item, ast.Name) and item.id != "_" and len(item.id) == 1:
+            yield item.lineno, item.col_offset, MSG

tests/no_single_letter_variables_test.py renamed to tests/single_letter_variables_test.py

@@ -24,6 +24,14 @@ def results(s):
             "ab = 3",
             id="Multi-letter assignment",
         ),
+        pytest.param(
+            "_ = 3",
+            id="Underscore assignment",
+        ),
+        pytest.param(
+            "ab, cd, _ = (1, 2, 3)",
+            id="Unpacking including an underscore",
+        ),
     ),
 )
 def test_noop(source):
@@ -35,9 +43,19 @@ def test_noop(source):
     (
         pytest.param(
             "a = 3",
-            "1:0: PDF023 don't assign to single-letter variables",
+            "1:0: PDF023 found assignment to single-letter variable",
             id="Single letter variable",
         ),
+        pytest.param(
+            "bar = a = 1",
+            "1:6: PDF023 found assignment to single-letter variable",
+            id="Multiple assignment",
+        ),
+        pytest.param(
+            "a, bar = (3, 5)",
+            "1:0: PDF023 found assignment to single-letter variable",
+            id="Unpacking",
+        ),
     ),
 )
 def test_violation(source, expected):

tests/string_to_concatenate_test.py

@@ -21,7 +21,7 @@ def results(s):
     "source",
     (
         pytest.param(
-            "a = (\n" "    'foo'\n" "    'bar'\n" ")",
+            "var = (\n" "    'foo'\n" "    'bar'\n" ")",
             id="separate lines",
         ),
     ),
@@ -34,8 +34,8 @@ def test_noop(source):
     "source, expected",
     (
         pytest.param(
-            "a = 'foo''bar'",
-            "1:4: PDF007 line split in two unnecessarily by 'black' formatter",
+            "var = 'foo''bar'",
+            "1:6: PDF007 line split in two unnecessarily by 'black' formatter",
             id="consecutive strings",
         ),
     ),