CPython中IS_OP指令为何要对oparg执行异或操作？

Great question—this is a clever bit of code that handles both is and is not with a single instruction, and the ^ oparg part is key to that reuse. Let's break it down step by step:

First, let's clarify what left == right does here. In CPython, all Python objects are referenced via pointers (PyObject*). So when you compare two PyObject* variables with == in C, you're checking if they point to the exact same memory address—which is exactly what Python's is operator is supposed to do (verify object identity). That part alone would give us the result for the is case.

But here's the catch: the IS_OP instruction doesn't just handle is—it also handles is not. That's where oparg comes in:

• When the instruction is generated for an is check, oparg is set to 0.
• When it's generated for an is not check, oparg is set to 1.

The XOR (^) operation flips the boolean result based on oparg:

• For is: res = (left == right) ^ 0 → this leaves the result unchanged (since XOR with 0 does nothing). So res is 1 if the objects are the same, 0 otherwise—exactly what we need for is.
• For is not: res = (left == right) ^ 1 → this flips the result. So if the objects are the same, we get 0; if they're different, we get 1—which matches what is not should return.

Then the code converts that integer res to the appropriate Python boolean (Py_True or Py_False) to return as the final result of the operation.

To make it concrete:

• If you run a is b, the C code checks if a and b point to the same object, returns True if they do.
• If you run a is not b, it checks the same pointer equality, then flips the result to return True only if they don't point to the same object.

This is a neat way to reuse the same instruction logic for both positive and negative forms of the operator, keeping the interpreter code concise and avoid redundant logic.

内容的提问来源于stack exchange，提问作者user459872