3.17 getattr vs setattr

This lesson shows the usage of setattr and getattr

The getattr and setattr are builtin functions which are used to get and set an attribute to a python object. Following examples show how to use these functions to set and get attributes of a class.

setattr

class Human:

    def __init__(self, name):
        self.name = name

    def grow(self):
        setattr(self, 'empathy', 10)
        return


human = Human("Ali")

When we created the instance of Human class, it did not have empathy attribute.

# uncomment following line
# human.empathy  # -> AttributeError: 'Human' object has no attribute 'empathy'

After executing the grow method of Human class, the empathy attribute is set to it using setattr function.

human.grow()

print(human.empathy)

10

The first argument to setattr is the object to which we want to set the attribute. The second argument is the name of attribute and the third argument is the value of the attribute.

If we want to set/change the value of empathy attribute of human to 14, we can do this using setattr as below.

setattr(human, "empathy", 14)

print(human.empathy)

14

It will be obvious from above examples that the function setattr can be used both inside the class and outside the class definition.

human.empathy = 100

print(human.empathy)

100

From above example we can infer that doing human.empathy = 10 is similar to setattr(human, ‘empathy’, 100). This can be translated as, setting the attribute of human with the name empathy to 100.

getattr

getattr is opposite of setattr. It is used to fetch the attribute value of an object.

print(getattr(human, 'empathy'))

100

In other words, doing human.empathy is similar to running getattr(human, ‘empathy’) The second argument to both setattr and getattr is string (str) type.

class Human:

    def __init__(self, name):
        self.name = name

    def grow(self):
        setattr(self, 'empathy', 10)
        return

    def info(self):
        empathy = getattr(self, 'empathy', None)
        return empathy


human = Human("Ali")

But what if the object does not have the attribute that we are trying to fetch?

# uncomment following line
# human.empathy  # -> AttributeError: 'Human' object has no attribute 'empathy'

Running the above code will give us AttributeError because, the human does not yet have empathy attribute.

human.info()

But why above cell did not throw AttributeError, even though we are getting, empathy attribute in it? This is because the 3rd argument in getattr function in info method is None. The 3rd argument is the default value of the attribute which we are trying to fetch. This means, when the object human did not have empathy attribute and we tried to get it using getattr, the default value None was returned.

We can verify this by printing the output of human.info().

print(human.info())

None

However, if we run the grow method first, this will result in setting the empathy attribute to human. Consequently, we can see a different output when we run human.info after that.

human.grow()

print(human.info())

10