Understanding Why Threading Uses self as an Argument in Python

A comprehensive guide to threading in Python, explaining why methods like `Function` need `self` implicitly and how this affects the number of arguments.
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Understanding Threading in Python: The Role of self as an Argument

When working with threading in Python, especially when using libraries like tkinter, you may encounter some unexpected behavior, particularly regarding how methods interact with thread arguments. One common question is: Why does using a class method as an argument result in an unexpected number of arguments? In this guide, we'll explore this question and clarify why your threading implementation behaves as it does.

The Problem at Hand

You’ve written a threading implementation within a tkinter GUI, trying to call a function that requires multiple arguments. However, instead of getting an error relating to too many or too few arguments, your code runs successfully, but with unexpected outputs.

Here's a summary of your experience:

You have a method Function that takes three arguments but provided only two when starting the thread.

Despite only providing two arguments, the program ran without errors, leading to confusion regarding the third argument’s role.

The Code Example

Here’s the relevant portion of your code that demonstrates the issue:

[[See Video to Reveal this Text or Code Snippet]]

Understanding the Behavior of Class Methods

Implicit Self Argument

In Python, class methods (like your Function) automatically receive the instance of the class (denoted as self) as the first argument when they are called through an instance. Thus, this means when you invoke self.Function, you are technically passing three arguments: self, x, and y.

Here’s the breakdown:

Declared Method: The method Function is defined to take three parameters.

Implicit Argument: When you call self.Function, Python automatically sends the instance (self) as the first argument.

Function Call: In the thread's target, you provided only two arguments: ('spam', 'eggs'). Thus, it effectively looks like this during the call:

[[See Video to Reveal this Text or Code Snippet]]

The self parameter is included, making the total argument count match the method signature.

Resulting Output

This is why the third argument doesn't raise an error; it gets the instance of the class that the method belongs to. When you run your program, the output shows the address of the instance before printing spam and eggs, which can be confusing.

[[See Video to Reveal this Text or Code Snippet]]

Solutions and Recommendations

Option 1: Use Static Method

If you don't want the implicit self to be passed to your function, you can define Function as a @ staticmethod. This way, the function will not implicitly require an instance:

[[See Video to Reveal this Text or Code Snippet]]

Option 2: Use a Separate Function

Instead of using a method, you could define a standalone function outside of the class that accepts the required number of arguments:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Understanding how threading interacts with class methods in Python is crucial for avoiding confusion. This behavior stems from the way Python automatically passes the self argument, which explains your unexpected results. By employing static methods or using standalone functions, you can achieve clearer and more predictable threading behavior.

Always remember to analyze methods and their expected arguments, especially when working in a multithreaded environment.

Happy coding!