Problem with the python object constructor

Question

es.

class item:
    def __init__(self, number:int):
        self.number = number
a = item("ciao")

When I instantiate the object I would like to make sure that the name parameter is of type string and otherwise an exception is raised. "name: str" doesn't actually check that the parameter is of the specified type

Answer 1

As per the documentation :

The Python runtime does not enforce function and variable type annotations. They can be used by third party tools such as type checkers, IDEs, linters, etc.

If you would like to enforce the type you can use isinstance() function to do so.

class item:
    def __init__(self, number: int):
        if not isinstance(number, int):
            raise TypeError('Value should be of type int')
        self.number = number

Answer 2

Annotations do not add more than information to the developer about the expected information. This is done through an informal contract because, as we know, simple annotations do not contain syntactic meaning but are accessible at runtime, so we can implement a generic decorator that is capable of checking types from the function annotations.

def ensure_annotations(function):
    signature = inspect.signature(function)
    parameters = signature.parameters

    @wraps(function)
    def wrapped(*args, **kwargs):
        bound = signature.bind(*args, **kwargs)
        for name, value in bound.arguments.items():
            annotation = parameters[name].annotation
            if annotation is inspect._empty:
                continue
            if not isinstance(value, annotation):
                raise TypeError(
                    "{}: {} doesn't actually check that the parameter"
                    "is of the specified type.".format(name, annotation)
                )
        function(*args, **kwargs)

    return wrapped


class item:
    @ensure_annotations
    def __init__(self, number: int):
        self.number = number

Answer 3

This might be what you want. You can use isinstance() to check if it is a string. I am not used to exeptions, so I don't know if that is written right, but the logic is.

class item:
    def __init__(self, name):
        if isinstance(name, str):
            self._name = name
        else:
            self._name = None
            raise Exception


a = item('ciai')