What is the pythonic way to check the vaildity of constructor arguments?

Question

For example take a simple class representing a person. The only class attribute is a string representing the persons name. I want to make sure that nobody tries to pass the constructor some other type of object like an int, or list...etc. This was my first attempt below I thought that this would return an obj of type None if the argument was not a str but it still seems to return a Person obj. I come from a "more" strongly typed language background and I am a little confused as how to handle my self in python. What does pythonic style say about this situation and type safety more generally? Should I raise an exception? Or find a way to return None? Or something else entirely.

class Person: 

    name = None

    def __init__(self, name):
        if not isinstance(name, str):
            return None
        self.name = name

Answer 1

You'd want to raise an error within the __init__ method:

if not isinstance(name,basestring):
    raise TypeError("I don't think that is a name ...")

*Note that basestring also includes unicode for python2.x, but isn't available in python3.x.

Careful though, there is nothing here to prevent a user from re-setting the person's name to a list after the person has been constructed.

jack = Person("Jack")
jack.name = ["cheese","steak"]  #???

If you want to have this safety built in, you'll need to start learning about property.

Answer 2

That's about it - short of using some form of decorator, but generally, type checking isn't very Pythonic, although valid in some cases, your code should be:

def __init__(self, name):
    if not isinstance(name, basestring):
        raise TypeError('name must be str/unicode')
    # ...

Note that basestring is the parent of str and unicode in Python 2.x, so this would allow both - either use str (as you are now in 2.x) to not allow unicode (or vice versa - for any particular reason). In 3.x, basestring doesn't exist, you've just got str.

Answer 3

I would not return None here but rather raise and exception to signify the incorrect object type.