![]() It also seems like this will be enforced in future versions as described in What about existing uses of annotations: In Python 3.5 though, PEP 484 - Type Hints attaches a single meaning to this: -> is used to indicate the type that the function returns. PEP 3107 - Function Annotations described the specification, defining the grammar changes, the existence of func._annotations_ in which they are stored and, the fact that it's use case is still open. In more recent versions of Python >= 3.5, though, it has a defined meaning. ![]() '.format(var, value, pr)ĪssertionError: x=2 Test: _between: must be between 3 and 10Īs other answers have stated, the -> symbol is used as part of function annotations. Specifically, the -> marks the return function annotation.Įxamples: def kinetic_energy(m:'in KG', v:'in M/S')->'Joules': For example: `Callback = typing.Callable, Callback]` and `Foo = Union]`.These are function annotations covered in PEP 3107. Return super()._new_(cls, name, bases, attrs)Įdit: it looks like there is now work going into this and other cases where recursive types don't work. So, it's not metaclasses that trigger the error. If I use an example with a metaclass, that also typechecks. Obviously there's cycle handling in mypy, otherwise none of the examples would work. But other than the fact that it uses metaclasses, I'm not sure why it'd have an error. From looking at the code, they're using metaclass / _new_. I know they don't use `eval` and templating to create it anymore. I wonder why `typing.NamedTuple` is unique as far as it not working. Looks like just plain class level attribute declaration also works: Hey, thanks a lot for pointing this out! Yet another reason to use data classes vs. Having a JS tech easier to use than a Python tech is a good sign we can improve things. It's way faster, shows many more things that before, way less false positive, and has support for duck typing and dunder methods (named "protocol").īut it's sad to thing TypeScript is actually easier to use. I have to do:Īnd Guido explicitly rejected the proposal for those on github despite the fact most imports are due to those. you need to use Union and instead of |. instead of list, dict, set, iter, tuple, etc., which means an import in almost every file and a very unnatural workflow. you need to use List, Dict, Set, Iteratble, Tuple, etc. ![]() you must know to use the magic command line arguments to avoid mypy complaining about other libs and imports all the time What's more, while I found mypy useful myself in several instances, it's still cumbersome to use: While the first complaint is just a reflect of the lack of good tutorials on mypy, the second one is very valid: I too wish for automatic type detection for obvious cases. It serves as a sanity check that you aren't accidentally returning something you don't expect.Īnd of course, being familiar with the differences between iterables, iterators, sequences, containers, etc. But you should still annotate your public apis. Speaking of which, if you want type inference, check out pytype, it's like MyPy, but does do type inference on unannotated code. ![]() That's why you should annotate the public apis even when you can have them be type inferred. This is an intentional choice: function declarations are your apis, and explicit and clear APIs are useful for human readers of your code. But, in C++ or java, if you write a function that returns a Mycontainer>, you have to write that in the function. `auto foo = M圜ontainer>() ` or whatever is nice than having to double write the type declaration. Especially long ones that use generics/templates. `auto` and its variants in other languages are useful for eliding redundant type declarations. This is absolutely wrong! You cannot annotate a function as returning `auto` in C++. This is especially true when dealing with parametric container classes, There are many situations where writing the type information isn’t workable. > This complexity helped drive the introduction of the auto keyword to C++. As of python3.7, this stringification is done by automatically under a from future import, and will eventually become the default, so the original code will be valid. No, the workaround is to stringify "Node".ĭef get_subs(self) -> Sequence: #Or maybe 'Sequence' You can’t refer to Node within the Node class. mypy has no trouble understanding this, but it’s unfortunately not valid Python code.
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