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x is 257 Was True on Monday and False on Tuesday
Why Python’s is and == are not the same thing, how one of them lies about numbers, and the single rule that keeps you safe
I had a check that compared two numbers. Something close to this, simplified:
if parsed_id is expected_id:
grant_access()It worked. Every test passed. Then one day, with slightly different data, the same comparison started returning False for two numbers that were obviously equal. Both were 257. I printed them. They were identical. And parsed_id is expected_id was False.
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So I changed the test data, and it went back to True. Smaller numbers worked. Bigger numbers did not. The comparison's correctness depended on how large the number was, which is not a sentence that should ever make sense.
The culprit was a single two-letter word I had used where I meant something else: is. In Python, is and == look interchangeable and are not. One asks "are these equal?" The other asks a completely different question, and the answer to that question happens to be unpredictable for numbers. Here is what is actually checks, why it lied to me, and the one rule that…
