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Comment by rbanffy | Hacker News Reader

You may want to explicitly test for a type depending on the interface contract. If you build software for consumption by others, you may want to ensure that, whatever the inputs, it’ll always return something you said would be returned. While the consumer would be insane not to verify what you sent, it’s polite not to send a complex number when someone is expecting a count.

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Certainly, but you're typically much more in control of what you send than what you receive.

You may also want to do sanity checks but, in those cases, I tend to deliberately abuse the inputs. Send a string, a byte string, some invalid UTF-8, an emoji, an invoice, and ensure that the thing can take more abuse than what the manual says.

If, however, the manual says “integer” then it should complain loudly the user is breaking the contract.

> If, however, the manual says “integer” then it should complain loudly the user is breaking the contract.

Yep, that's why I prefer mypy/typeguard and type annotations for those sorts of things over checking in the body. It's frequently much, much easier to read, and self-documenting, since most IDEs have support for that when calling the function nowadays.

The post is not about what you're sending, it is about making sure what you're receiving is what you're getting. The motivation begins with: "Many of our programs accept input from the user. Often we need to validate this input before continuing processing and, in the case of errors, inform the user of any problems."

The post I was replying to is about what you send, though.

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