ruff E714

• The rewrite fails when the 'is' expression is parenthesized (e.g., 'not (x is y)'), as removing 'not' from before the parentheses breaks the grouping and changes meaning.
• When 'not x is y' appears in a compound condition like 'not x is y and z', the rewrite 'x is not y and z' alters precedence, making it equivalent to '(x is not y) and z' instead of 'not (x is y and z)'.
• In 'not x is y or z', the rewrite becomes 'x is not y or z', which evaluates as '(x is not y) or z' rather than 'not (x is y or z)', changing logic due to operator precedence.
• If the identity comparison is part of a larger expression where 'not' applies to more than just the 'is' (e.g., 'not f() is g()'), the rewrite incorrectly scopes the negation only to the 'is' operation, ignoring potential side effects in function calls.
• The fix assumes 'is' is always an identity operator; if overloaded via __eq__ or __ne__ in a custom class (though rare and discouraged), the semantics of 'is not' vs 'not ... is' could diverge, though 'is' cannot be overloaded in Python.
• Comments placed between 'not' and the 'is' keyword (e.g., 'not # comment x is y') would be lost or misplaced during the rewrite, as the tool may not preserve interleaved whitespace/comments.
• In lambda expressions like 'lambda: not x is y', the rewrite to 'lambda: x is not y' is safe, but if the lambda is used in a context where the original form relied on low precedence of 'not', the change could alter behavior when composed with other operators.
• In comprehensions such as '[expr for expr in items if not expr is skip]', moving 'not' after 'is' is safe locally, but if the comprehension is nested or part of a larger expression, the altered precedence might interact unexpectedly with surrounding logic.
• The rewrite does not account for line continuation or backslash-joined lines; if 'not x is\n y' is split across lines, the fix may incorrectly place 'is not' or break the line continuation.
• If the code contains a syntax error or invalid token near the 'not is' pattern (e.g., due to a typo), the autofix might produce invalid syntax or crash during application.

--- orig.py	2026-10-17 16:49:33.431586635 -0400
+++ fixed.py	2026-10-17 16:49:33.599590063 -0400
@@ -1,6 +1,6 @@
 def f():
     x, y, z = [], [], True
-    r1 = not x is y and z
+    r1 = x is not y and z
     r2 = x is not y and z
     print('Original:', r1)
     print('Rewritten form evaluates to:', r2)

--- orig.py	2026-10-17 16:49:33.703592185 -0400
+++ fixed.py	2026-10-17 16:49:33.889595979 -0400
@@ -1,6 +1,6 @@
 from typing import List
 
 def foo(x: List[int]) -> bool:
-    return not x is None
+    return x is not None
 
 print(foo([1]))

True

--- orig.py	2026-10-17 16:49:34.002598285 -0400
+++ fixed.py	2026-10-17 16:49:34.178601872 -0400
@@ -2,7 +2,7 @@
     raise ValueError('bad')
 
 try:
-    if not get_x() is None:
+    if get_x() is not None:
         pass
 except ValueError as e:
     print('Caught:', type(e).__name__, str(e))

Caught: ValueError bad

--- orig.py	2026-10-17 16:49:34.425606906 -0400
+++ fixed.py	2026-10-17 16:49:34.589610248 -0400
@@ -3,5 +3,5 @@
     yield 2
 
 g = gen()
-print('First check:', not next(g) is None)
-print('Second check:', not next(g) is None)
+print('First check:', next(g) is not None)
+print('Second check:', next(g) is not None)

First check: True
Second check: True

--- orig.py	2026-10-17 16:49:34.829615139 -0400
+++ fixed.py	2026-10-17 16:49:35.016618951 -0400
@@ -1,5 +1,5 @@
 b = b'hello'
 s = 'hello'
-print('Bytes check:', not b is b'hello')
-print('Str check:', not s is 'hello')
-print('Cross-type:', not b is s)
+print('Bytes check:', b is not b'hello')
+print('Str check:', s is not 'hello')
+print('Cross-type:', b is not s)

Bytes check: False
Str check: False
Cross-type: True

--- orig.py	2026-10-17 16:49:35.133621332 -0400
+++ fixed.py	2026-10-17 16:49:35.298624692 -0400
@@ -5,5 +5,5 @@
 x = side()
 y = side()
 print('Step 1 done')
-result = not x is y
+result = x is not y
 print('Result:', result)

side effect
side effect
Step 1 done
Result: True

--- orig.py	2026-10-17 16:49:42.170764127 -0400
+++ fixed.py	2026-10-17 16:49:42.560772009 -0400
@@ -4,7 +4,7 @@
         return 42
     x = __get__()
 obj = C()
-if not obj.x is 42:
+if obj.x is not 42:
     print('fail')
 else:
     print('pass')

--- orig.py	2026-10-17 16:49:42.910779083 -0400
+++ fixed.py	2026-10-17 16:49:43.074782396 -0400
@@ -1,7 +1,7 @@
 x = 1
 y = 2
 z = 3
-if not (x is y) and z:
+if x is not y and z:
     print('true')
 else:
     print('false')

true
3

--- orig.py	2026-10-17 16:49:43.173784395 -0400
+++ fixed.py	2026-10-17 16:49:43.337787706 -0400
@@ -1,7 +1,6 @@
 x = 1
 y = 2
-if not x is\
- y:
+if x is not y:
     print('equal')
 else:
     print('not equal')

equal

--- orig.py	2026-10-17 16:49:43.432789624 -0400
+++ fixed.py	2026-10-17 16:49:43.597792955 -0400
@@ -1,6 +1,6 @@
 x = 'hello'
 y = 'hello'
-if not x is y:
+if x is not y:
     print('%s != %s' % (x, y))
 else:
     print(f'{x} is {y}')

hello is hello

--- orig.py	2026-10-17 16:49:43.696794953 -0400
+++ fixed.py	2026-10-17 16:49:43.858798224 -0400
@@ -1,7 +1,7 @@
 list = []
 list.append(1)
 x = list
-if not x is list:
+if x is not list:
     print('aliased')
 else:
     print('same')

same

--- orig.py	2026-10-17 16:49:43.964800364 -0400
+++ fixed.py	2026-10-17 16:49:44.132803754 -0400
@@ -8,7 +8,7 @@
 a = A(1)
 b = A(1)
 d = {a: 'x'}
-if not d.get(b) is None:
+if d.get(b) is not None:
     print('found:', d[b])
 else:
     print('missing')

found: x

--- orig.py	2026-10-17 16:49:44.237805871 -0400
+++ fixed.py	2026-10-17 16:49:44.408809320 -0400
@@ -3,7 +3,7 @@
     ref.append(1)
     return ref
 x = f()
-if not x is f():
+if x is not f():
     print('different call')
 else:
     print('same call')

same call
ref: [1, 1]