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Improved Error Messages

00:00 In the previous lesson, I talked about the improvements to f-strings. In this lesson, I’ll show you better arrows. Did you mean errors? I mentioned the addition of the PEG parser In Python 3.9.

00:14 PEG has allowed more descriptive error messages, including did-you-mean semantics. For example, if you misspell something, it tries to guess what you meant and tell you about it.

00:26 Python 3.12 has added more error improvements. The contents of the standard library have now been added to the source of guesses for the did-you-mean semantics, as well as the members of a class. Evidently, a common error during importation is to get the from module import name syntax in the wrong order. Rather than a plain SyntaxError, you now get a hint as to the correct way around. And the contents of a module themselves are now in the guess list.

00:59 So if you spell something in the imported module incorrectly, it will show in the did-you-mean error. Let’s go write some broken code and see the errors.

01:10 I’m in a Python 3.12 REPL here. Let me start by showing you something that worked before 3.12. Built-ins like print() were in the guess list for the did-you-mean code.

01:26 Compare pint with davis. davis isn’t close to anything, so it doesn’t get a did-you-mean to go with it, whereas pint is close to print and so it does. The ability to look for these near matches in the builtins module was added in Python 3.10. Let me show you the built-ins.

01:53 There’s a lot there,

01:58 and you can see that print is inside of it. In addition to looking in builtins, Python 3.12 now looks in the standard library. As I haven’t imported math yet but it is part of the standard library, I get the did-you-mean message.

02:18 If I try that with miles, I just get a plain-style error instead. It isn’t close enough to anything in the guess list to get a did-you-mean.

02:29 Like with builtins, ou can see everything in the standard library. This time, instead of using a module directly though, you use the .stdlib_module_names member of the sys module,

02:47 which of course it finds math inside. Just for giggles, let’s look at all of them.

02:57 That’s even worse than builtins. Similar guess list stuff has been added to the inside of classes, with the members of the class now working with, did-you-mean style errors.

03:28 Consider my Talker class. Inside of the .say() method, I use message, which doesn’t actually exist. I probably mean the class member self.message, and now the error gives me that hint.

03:51 Python’s 3.10 guess list included everything in the local scope but didn’t work with self inside of a class. This actually created a bit of corner case. When there was something in the local scope with a similar name to something in a class, it would did-you-mean on the global variable, possibly pointing you at the wrong thing.

04:11 Python 3.12 has fixed this by adding class context, and the class scope is checked first, fixing this potentially misleading error caused by the error message.

04:24 As I mentioned in the intro, a common mistake is to get the order of the from import syntax wrong. This is particularly easy for multilanguage coders, as other programming languages do this differently.

04:40 Python 3.12 explicitly looks for this kind of problem, and instead of giving a generic SyntaxError, it now gives you a bit of a hint. The did-you-mean feature has been added to the underlying import code as well.

04:59 If I try to import something from a module that isn’t there, it checks the module’s contents for near-named things and gives you a hint. Of cos I meant cosine. Sorry. I’ll leave now. Okay, maybe I won’t leave. I’ll just head to the next lesson, which is about a few small additions to three different standard library modules.

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