Python setattr() Function Explained

In this tutorial, you’ll learn about the Python setattr() function, including what it does, when to use it, and when it’s redundant to use it. The function is used to set an attribute to a value of an object, whether the attribute exists or not.

By the end of this tutorial, you’ll have learned:

• What the Python setattr() function does and how to use it
• When the function will fail and how to prevent this error
• Why the function can be redundant for setting an attribute
• How the function can be used to add set multiple object attributes at once

Understanding the Python setattr Function

The Python setattr function is used to set the value of an attribute of an object. The function can be used to:

1. Set the attribute of an existing attribute, or
2. Create a new attribute and set it to a provided value

Let’s take a look at what this function looks like:

# Understanding the Python setattr() Function
setattr(
    object, # The object to set the attribute on
    name,   # The name of the attribute
    value   # The value of the attribute
)

We can see that the function takes three parameters: the object itself, the name of the attribute, and the value to be set.

Using setattr to Set an Attribute that Exists

Let’s take a look at how we can use the function to set an attribute’s value:

# Setting the attribute of an object
class Employee:
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

Nik = Employee("Nik", 75000)

print('Before changing attribute...')
print('Nik.salary:', Nik.salary)

setattr(Nik, "salary", 80000)

print('After changing attribute...')
print('Nik.salary:', Nik.salary)

# Returns:
# Before changing attribute...
# Nik.salary: 75000
# After changing attribute...
# Nik.salary: 80000

Let’s break down what the code above does:

1. We first create a class, Employee, which has two attributes, name and salary
2. We then instantiate a new Employee object and set two initial attributes
3. When then use the setattr() function to modify the salary attribute

Using setattr to Set an Attribute that Doesn’t Exist

In the example above, we used the setattr() function to modify the value of an attribute that already exists. However, we can also use it to set an attribute that doesn’t yet exist. Let’s add an attribute using the setattr() function to the same class:

# Setting the attribute of an object when the attribute doesn't exist
class Employee:
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

Nik = Employee("Nik", 75000)

setattr(Nik, "location", "Toronto")
print('Nik.location:', Nik.location)

# Returns:
# Nik.location: Toronto

We can see that the function works in setting an attribute’s value, even if that attribute didn’t previously exist.

Why Python setattr May Be Redundant

The Python setattr() function works in setting attributes in Python objects. However, it may also be redundant since there are much simpler and, arguably, more Pythonic ways of accomplishing the same thing. We can use dot notation to do this.

Let’s recreate the example above using dot notation:

# Using other methods to set attributes
class Employee:
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

Nik = Employee("Nik", 75000)
Nik.salary = 80000
Nik.location = 'Toronto'

print("Nik.salary:", Nik.salary)
print("Nik.location:", Nik.location)

We can see that both of these methods worked to set attributes easily. There are some perks to using the setattr() function, however – read on to learn more.

How to Set Multiple Object Attributes in Python with setattr

One of the perks of using the setattr() function is to be able to set multiple attributes at once. Imagine that you’re provided with a dictionary of attributes belonging to an object. You can loop over these and set them for the different attributes of an object.

Let’s see what this looks like:

# Setting multiple attributes with Python setattr()
class Employee:
    def __init__(self, name):
        self.name = name

Nik = Employee("Nik")
attributes = {
    'Age': 33,
    'Location': 'Toronto',
    'Salary': 75000
}

for attribute, value in attributes.items():
    setattr(Nik, attribute, value)

print(vars(Nik))

# Returns: {'name': 'Nik', 'Age': 33, 'Location': 'Toronto', 'Salary': 75000}

In the example above, we loop over each key and value in the dictionary. We then use the setattr() function to add these attributes and values. Finally, we use the vars() function to print out a dictionary that contains the object’s attributes.

When setattr Will Fail and How to Resolve Errors

The Python setattr() function will fail if it’s used on objects that don’t yet exist. This means that an object needs to first be instantiated in order to prevent any errors. Because of this, the function can’t be used to instantiate an object, but rather only work with ones that already exist.

# setattr() Will Fail When an Object Doesn't Exist
class Employee:
    def __init__(self, name):
        self.name = name

setattr(Nik, "age", 30)

# Returns: NameError: name 'Nik' is not defined

In order to resolve this, we first need to create the object:

# Resolving the NameError Raised by setattr()
class Employee:
    def __init__(self, name):
        self.name = name

Nik = Employee("Nik")
setattr(Nik, "age", 30)

Conclusion

In this tutorial, you learned how to use the Python setattr() function to set the attribute of an object to a value. You first learned how the function is defined and how to use it for both attributes that already exist and for ones that don’t. You then learned why the function can be redundant since you can simply use dot notation.

Then, you learned how to use the function to set multiple attributes using a for loop as well as how to resolve errors that may occur when using the function.

Additional Resources

To learn more about related topics, check out the tutorials below:

• Python Object-Oriented Programming (OOP) for Data Science
• Python: Print an Object’s Attributes
• Python isinstance() Function Explained with Examples
• Official Documentation: Python setattr()