The NameError: name ‘self’ is not defined is a common error that Python programmers encounter while working with classes. This error occurs when your code tries to access the self variable in a class method, but the self has not been initialized properly.
While the exact causes may differ, the root of the problem is that Python cannot find the self variable in the current scope. As self plays a critical role in Python’s object-oriented programming, failing to define it correctly will lead to NameError exceptions being raised.
This can happen even to experienced Pythonistas, as the specifics of self-initialization and usage are easy to get wrong.
In this detailed guide, we will demystify the meaning of self in Python classes, outline the typical situations where you may get a NameError related to self, and provide actionable solutions and tips on how to define and use self properly. Read on to learn how to troubleshoot and fix the infamous “name ‘self’ is not defined” error in Python.
What is ‘self’ in Python?
In Python, self is a reference to the instance of the class. It is used to access variables and methods belonging to the class. Within a class method, self refers to the instance calling that method.
For example:
class MyClass:
def __init__(self):
self.x = 10
def method(self):
print(self.x)
Here, self allows the __init__ method to assign the value 10 to a member variable x. And self.x allows the method to access the member variable x.
So self gives access to the attributes and methods of a class from within the class itself. All methods of a class have self as their first parameter (by convention).
Common causes of NameError: name ‘self’ is not defined
There are a few common scenarios where you may encounter this error:
1. Forgetting to add self parameter
This happens when you define a method inside a class but forget to add self as the first parameter:
class MyClass:
def my_method(): #no self parameter
print(self.x)
This will throw NameError because self is not defined within the scope of my_method().
The solution is to add self as the first parameter:
class MyClass:
def my_method(self):
print(self.x)
2. Calling method without instantiating class
You try to call a method directly on the class without creating an instance object first:
class MyClass:
def my_method(self):
print(self.x)
MyClass.my_method() #Error!
This will not work because self is undefined. The class needs to be instantiated first:
obj = MyClass()
obj.my_method() #works!
Now self will refer to obj when my_method() is called.
3. Using self outside a class
You try to use self outside the context of a class:
#outside a class
self.x = 10 #Error!
self only makes sense inside class methods to refer to instance attributes and methods. Outside a class, it has no meaning.
4. Forgetting to call parent init method
When inheriting from a parent class, you forget to call the parent’s __init__() method which initializes self:
class Parent:
def __init__(self):
self.x = 5
class Child(Parent):
def __init__(self):
print(self.x) #Error!
child = Child()
This will fail with a NameError because self.x was never initialized. To fix it, call super().__init__():
class Child(Parent):
def __init__(self):
super().__init__()
print(self.x) #works!
Solutions to NameError: name ‘self’ is not defined
Here are some ways to troubleshoot and fix this error:
- Check that all methods have self as the first parameter
- Make sure to instantiate classes before calling methods on them
- Do not try to use self outside class methods
- Call super().__init__() in child class __init__ methods
- Use a linter/IDE to detect missing self parameters
- Double check spelling to make sure you didn’t misspell self
Some other tips:
- The error will tell you exactly where self is missing – check that line/method
- Try printing self to see if it exists in a method
- Understand that self refers to an instance, while class attributes use the class name
Using these steps, you should be able to identify and fix any NameError related to self in Python. Proper use of self is key to working with classes in Python.
Conclusion
The NameError: name ‘self’ is not defined occurs when trying to use self without initializing it first, usually within a class method. The most common causes are forgetting the self parameter, calling methods directly on the class, using self outside a class, and forgetting to call super().__init__() in child classes.
By passing self correctly, instantiating classes, and understanding the meaning of self, you can avoid this error. Fixing it is just a matter of following Python’s class and variable scope rules properly when using self.
