Why Does My Python Code Throw a 'TypeError: Missing 1 Required Positional Argument: 'self''?

Why TypeError: Missing 1 Required Positional Argument: 'self' Occurs

In Python, when working with classes, one might encounter an exception while calling a class method. The specific error message "TypeError: Missing 1 required positional argument: 'self'" indicates that the constructor method was not properly called to create an instance of the class before invoking the method.

Understanding the Purpose of 'self'

'self' is a special variable that is automatically passed as the first argument to all class methods (including the constructor). It represents the instance of the class and provides access to its attributes and methods.

Constructor Invocation

A constructor (__init__) method must be called to initialize a class instance. To invoke it, one needs to instantiate the class by creating an object using the ClassName() syntax. This assigns the self argument to the instance and initializes its attributes and behaviors.

Example for Clarity

Consider the following code snippet:

class Pump:
    def __init__(self):
        print("init")

    def getPumps(self):
        pass

p = Pump.getPumps()  # Error: TypeError: getPumps() missing 1 required positional argument: 'self'
print(p)

In this code, the constructor (__init__) is not called before invoking getPumps(). Consequently, no instance of the Pump class is created, and the self argument is missing, resulting in the error.

Solution

To resolve the issue, one must first create an instance of the Pump class and then call the getPumps() method on the instance. The correct code would be:

p = Pump()
p.getPumps()
print(p)

By creating an instance first, the self argument is properly assigned, and the getPumps() method can be invoked successfully.

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