add checker for accidental constants in boolean operators (#7273)

### Problem

It's possible to accidentally write something like `None or 'default_value'`, which is often an error indicating that the original value is dropped. One example is in 13b8505 for #7269 just now.

### Solution

- Add a checker for `and`/`or` expressions which ensures that the left-hand side is probably not a constant, and ensures that the right-hand side is probably not a constant for `and` expressions.

### Result

One possible class of error that I just made is impossible.

contrib/python/src/python/pants/contrib/python/checks/checker/BUILD

@@ -10,6 +10,7 @@ python_library(
     }
   ),
   dependencies=[
+    '3rdparty/python:future',
     '3rdparty/python:pycodestyle',
     '3rdparty/python:pyflakes',
     '3rdparty/python:six',

contrib/python/src/python/pants/contrib/python/checks/checker/checker.py

@@ -13,6 +13,7 @@
 
 from pants.contrib.python.checks.checker.class_factoring import ClassFactoring
 from pants.contrib.python.checks.checker.common import CheckSyntaxError, Nit, PythonFile
+from pants.contrib.python.checks.checker.constant_logic import ConstantLogic
 from pants.contrib.python.checks.checker.except_statements import ExceptStatements
 from pants.contrib.python.checks.checker.file_excluder import FileExcluder
 from pants.contrib.python.checks.checker.future_compatibility import FutureCompatibility
@@ -128,6 +129,7 @@ def plugins():
   """
   return (
     ClassFactoring,
+    ConstantLogic,
     ExceptStatements,
     FutureCompatibility,
     ImportOrder,

contrib/python/src/python/pants/contrib/python/checks/checker/constant_logic.py

@@ -0,0 +1,60 @@
+# coding=utf-8
+# Copyright 2019 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+import ast
+
+from future.utils import PY3
+
+from pants.contrib.python.checks.checker.common import CheckstylePlugin
+
+
+class ConstantLogic(CheckstylePlugin):
+  """Check for constants provided to boolean operators which result in a constant expression."""
+
+  @classmethod
+  def name(cls):
+    return 'constant-logic'
+
+  @classmethod
+  def iter_bool_ops(cls, tree):
+    for ast_node in ast.walk(tree):
+      if isinstance(ast_node, ast.BoolOp) and isinstance(ast_node.op, (ast.And, ast.Or)):
+        yield ast_node
+
+  @classmethod
+  def is_name_constant(cls, expr):
+    if PY3:
+      if isinstance(expr, ast.NameConstant):
+        return True
+    else:
+      if isinstance(expr, ast.Name) and expr.id in ['True', 'False', 'None']:
+        return True
+    return False
+
+  @classmethod
+  def is_probably_constant(cls, expr):
+    if isinstance(expr, (ast.Num, ast.Str)):
+      return True
+    if cls.is_name_constant(expr):
+      return True
+    return False
+
+  def nits(self):
+    for bool_op in self.iter_bool_ops(self.python_file.tree):
+      # We don't try to check anything in the middle of logical expressions with more than two
+      # values.
+      leftmost = bool_op.values[0]
+      rightmost = bool_op.values[-1]
+      if self.is_probably_constant(leftmost):
+        yield self.error('T804',
+                         'You are using a constant on the left-hand side of a logical operator. '
+                         'This is probably an error.',
+                         bool_op)
+      if isinstance(bool_op.op, ast.And) and self.is_probably_constant(rightmost):
+        yield self.error('T805',
+                         'You are using a constant on the right-hand side of an `and` operator. '
+                         'This is probably an error.',
+                         bool_op)

contrib/python/src/python/pants/contrib/python/checks/checker/except_statements.py

@@ -6,7 +6,7 @@
 
 import ast
 
-from six import PY3
+from future.utils import PY3
 
 from pants.contrib.python.checks.checker.common import CheckstylePlugin
 

contrib/python/src/python/pants/contrib/python/checks/checker/missing_contextmanager.py

@@ -6,7 +6,7 @@
 
 import ast
 
-from six import PY3
+from future.utils import PY3
 
 from pants.contrib.python.checks.checker.common import CheckstylePlugin
 

contrib/python/src/python/pants/contrib/python/checks/checker/print_statements.py

@@ -7,7 +7,7 @@
 import ast
 import re
 
-from six import PY3
+from future.utils import PY3
 
 from pants.contrib.python.checks.checker.common import CheckstylePlugin
 

contrib/python/tests/python/pants_test/contrib/python/checks/checker/test_constant_logic.py

@@ -0,0 +1,26 @@
+# coding=utf-8
+# Copyright 2019 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+from pants_test.contrib.python.checks.checker.plugin_test_base import CheckstylePluginTestBase
+
+from pants.contrib.python.checks.checker.constant_logic import ConstantLogic
+
+
+class ConstantLogicTest(CheckstylePluginTestBase):
+  plugin_type = ConstantLogic
+
+  def test_or(self):
+    self.assertNit('None or x', 'T804')
+    self.assertNit('True or x', 'T804')
+    self.assertNit('False or x', 'T804')
+    self.assertNit('1 or x', 'T804')
+    self.assertNit('"a" or x', 'T804')
+    self.assertNoNits('x or y')
+
+  def test_and(self):
+    self.assertNit('None and x', 'T804')
+    self.assertNit('x and False', 'T805')
+    self.assertNoNits('x and y')