Seth Larson noticed that people don’t act on deprecation warnings. The response.getheader method in urllib has been deprecated since 2023 because the response.headers dictionary is what should be used instead. When the method was eventually removed, lots of code broke.

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Deprecation warnings try to solve the fat step function associated with backwards-incompatible api changes, by allowing people to schedule the maintenance burden, rather than having it imposed on them suddenly all at once. The problem is the economic cost of waiting is not tangible. You can ignore the deprecation warning right up until the api change happens, and then it becomes very expensive to delay it further.

People aren’t great at planning for sudden changes.

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What if we intentionally made deprecated functions return the wrong result … sometimes? Every time it intentionally returns the wrong result, it logs the deprecation warning.1¹ Users that are very sensitive to the correctness of the results might want to swap the wrong result for an artificial delay instead.

Initially, it should never return the wrong result. But after it’s been deprecated for a few months, it should start to return the wrong result once every million invocations, say. That would probably not trigger anyone’s midnight pager, but it would make it clear that relying on the deprecated functionality is a bug lurking in the code.

Then after a few more months, turn it up to once every ten thousand invocations. It’s probably going to start to hurt a little to delay the maintenance. After a year, make it return the wrong thing once every thousand invocations. At this point, users can only delay maintenance if it’s an unimportant auxiliary usage. And finally, as we bump up into the deadline, it should return the wrong thing every other invocation. Now it’s practically useless, just like when it is removed.

This makes the deprecated parts of the api increasingly buggy until they’re removed, and makes the economic tradeoff of when to schedule the maintenance more immediate to users.