创建类的实例时如何限制可能的输入。 例如，仅创建类的整数实例

Question

In Python OOP is it possible to restrict the input when creating an object.

For eg if I only wanted int objects for a class. So if a string was entered it would return an error.

My initial thought was to write an if statement in the init method that would return an error msg if the conditions were not met.

The only problem with the above is that it seems that returning a statement in an init method is far from ideal.

Any guidance would be much appreciated

Answer 1

You can use type suggestions in the method header. This is considered better practices than forcing.

def __init__(name: str) -> str:
    return "Hello, " + name

If you did need to force you can add isinstance(variable, type) like this.

if not isinstance(name, str):
    raise TypeError

Answer 2

The basic way you would implement this is to add a check to your __init__ method that raises an error (usually a TypeError ) if an incorrect type is passed into the method. You would also want to hint to your readers about the parameters expected type using function annotations . In practice though, it is generally better to try and convert whatever was passed into the __init__ method into the type that you expect. Python is flexible, and using that flexibility to your advantage is beneficial for you in designing and building your application, as well as for anyone who potentially may want to utilize your code down the line.

An example:

class Test:
    def __init__(self, param: int):
        self.my_param = int(param)

In the above example, if you were to wrap your param with an exception, the entire application would crash if the exception was left unhandled. If you add the int() conversion, it will instead try to convert whatever is passed into an int , thereby saving your application from a crash as long as the passed item can be converted. There are cases where you would absolutely want to specify an exact type to be passed, but flexibility is usually more important.