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I have, it didn't really help. It does help if you have no tests, but a large project needs tests.

I guess you don't use an IDE. Use VSCode or IntelliJ with auto completion and error highlighting as you type. Type hints are a blessing.

PyCharm does this without the hints

How would that work across files and libraries?

Presumably with static analysis. I don't know exactly how this works, but it's surprisingly good.