Python is known as a dynamic, strong-typed language. Most developers love it but some feel mad without type checking or type-hinted auto-completion. In Python3.5, Type Hints is introduced to further delight developers who want those features.
Type Hints offers type checking on function parameters, return values and class attributes, as if it’s static-typed. If you pass something does not match the expected type, a warning will be given.
According to The Theory of Type Hints, here’s an example showing how the rules work out in practice:
Say there is an Employee class, and a subclass Manager:
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class Employee: ... class Manager(Employee): ... |
Let’s say variable e is declared with type Employee:
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e = Employee() # type: Employee |
Now it’s OK to assign a Manager instance to e:
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e = Manager() |
It’s not OK to assign an Employee instance to a variable declared with type Manager:
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m = Manager() # type: Manager m = Employee() # Fails static check |
Now, suppose we have a variable whose type is Any :
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a = some_func() # type: Any |
It’s OK to assign a to Employee e:
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e = a # OK |
Of course it’s also OK to assign Employee e to a:
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a = e # OK |
