使用'is'关键字检查null是否可行？代码评审场景咨询

Great question! Let's break this down clearly—using the is keyword to check for null (or None in Python, the most common context where this question pops up) is not just 可行, it's actually the recommended approach in many cases.

Why It Works #

Most languages that have a null-like value (such as Python's None, C#'s null) treat this value as a singleton: there's only one instance of it in the runtime. Since the is keyword checks for identity (whether two variables point to the exact same object in memory), it will always correctly identify if a variable holds that singleton null value.

Example in Python:

some_property = None
if some_property is None:
    print("This check works flawlessly!")

Why It's Better Than == #

The equality operator (==) can be overridden by custom classes, which might lead to unexpected results. For instance, if a class defines an __eq__ method that incorrectly returns True when compared to null/None, using == would give you a wrong answer—but is will still work because it skips custom equality logic and checks for exact identity.

Counterexample:

class CustomClass:
    def __eq__(self, other):
        # Buggy equality check that matches None
        return other is None

obj = CustomClass()
print(obj == None)  # Prints True (incorrect!)
print(obj is None)  # Prints False (correct!)

A Quick Caveat #

This logic applies specifically to the language's built-in null singleton. For example, in JavaScript, the strict equality check (=== null) is the analogous safe approach (since == null can match undefined too). But in all languages where null is a single, unique instance, is (or its equivalent strict identity check) is the most reliable choice.

内容的提问来源于stack exchange，提问作者meJustAndrew