TypeError: missing 1 required positional argument: 'self'
Tags: python, abotwrotethis
Published: 2023-04-12 01:28:03

If you're a Python developer, there's a good chance you've come across the TypeError: Missing 1 Required Positional Argument: 'self' error at some point in your coding journey. This error occurs when you attempt to call a method on an instance of a class without passing in a reference to that instance as the first argument. In this post, we'll cover what this error means, how to fix it, and some common scenarios where it might occur.

## Understanding the Error

Let's say you have a class named `SomeClass` with a method named `some_method`. You create an instance of this class named `some_instance`, and then attempt to call the `some_method` method on that instance:

```
class SomeClass:
    def some_method(self, arg1):
        # do something

some_instance = SomeClass()
some_instance.some_method("test")
```

If you run this code, you will get the following error:

```
TypeError: some_method() missing 1 required positional argument: 'arg1'
```

The error message indicates that you are missing a required argument for the `some_method` method. But why is this happening? The answer lies in the first argument of the method definition: `self`.

In Python, classes have a special parameter named `self` that is used to refer to the instance of the class. When you define a method in a class, the first parameter should always be `self`, even if you do not need to use it inside the method. This is because Python automatically passes the instance of the class as the first argument when you call a method on an instance.

## Fixing the Error

To fix the TypeError: Missing 1 Required Positional Argument: 'self' error, you need to make sure you are passing in a reference to the instance of the class as the first argument when calling a method on that instance.

Using the previous example, you would need to modify the code to include the `self` parameter in the method definition, like this:

```
class SomeClass:
    def some_method(self, arg1):
        # do something

some_instance = SomeClass()
some_instance.some_method(some_instance, "test")
```

Notice that we are passing in `some_instance` as the first argument to the `some_method` method. This tells Python to pass in the instance of the class as the first argument, so that the `self` parameter inside the method points to the correct instance.

## Common Scenarios

Now that you understand what the TypeError: Missing 1 Required Positional Argument: 'self' error means and how to fix it, let's look at some common scenarios where you might encounter this error in your code.

### Missing 'self' Parameter

The most common cause of the TypeError: Missing 1 Required Positional Argument: 'self' error is simply forgetting to include the `self` parameter in the method definition. If you define a method without `self` as the first parameter, you will get this error when you try to call that method on an instance of the class.

```
class SomeClass:
    def some_method(arg1):  # missing 'self' parameter
        # do something

some_instance = SomeClass()
some_instance.some_method("test")
```

To fix this error, simply add the `self` parameter to the method definition.

### Incorrect First Parameter

Another common scenario is passing in the wrong object as the first argument when calling a method on an instance. For example, you might accidentally pass in a different instance of the same class, or a completely unrelated object.

```
class SomeClass:
    def some_method(self, arg1):
        # do something

some_instance1 = SomeClass()
some_instance2 = SomeClass()
some_instance1.some_method(some_instance2, "test")  # incorrect first parameter
```

In this case, Python will interpret `some_instance2` as the `self` parameter inside the `some_method` method, which will cause the TypeError: Missing 1 Required Positional Argument: 'self' error.

To fix this error, make sure you are passing in the correct instance of the class as the first argument when calling a method on that instance.

### Incorrect Method Name

Finally, it's possible to get the TypeError: Missing 1 Required Positional Argument: 'self' error if you accidentally call a non-existent method on an instance of a class. This can happen if you misspell the method name, or if you call a method that doesn't exist in the class definition.

```
class SomeClass:
    def some_method(self, arg1):
        # do something

some_instance = SomeClass()
some_instance.nonexistent_method("test")  # incorrect method name
```

In this case, Python will not be able to find the `nonexistent_method` method in the `SomeClass` class definition, which will cause the TypeError: Missing 1 Required Positional Argument: 'self' error.

To fix this error, make sure you are calling the correct method name on an instance of the class.

## Conclusion

The TypeError: Missing 1 Required Positional Argument: 'self' error is a common error that Python developers may encounter when working with classes and methods. To fix this error, you need to make sure you are including the `self` parameter in the method definition, and passing in a reference to the instance of the class as the first argument when calling a method on that instance. By understanding the causes and solutions to this error, you can write better Python code and avoid potential errors in your programs.