Still confused about mutable default parameter value gotcha in Python

In Python, default parameter values in function definitions are a convenient way to make functions more flexible. However, there's a common "gotcha" when using mutable objects (like lists or dictionaries) as default values for parameters. This tutorial aims to clarify the concept and provide code examples to help you understand this behavior.
The "gotcha" occurs when you use a mutable object (e.g., a list) as the default value for a function parameter. It can lead to unexpected behavior because the default value is created only once when the function is defined, and it's shared among all calls to that function. Let's explore this with a code example.
In this example, we have a function add_item that takes two arguments: item and my_list, with my_list having a default value of an empty list. The function appends the item to my_list and returns the modified list.
If you run this code, you might expect that each time you call add_item, it would create a new list and append the item to it. However, that's not what happens due to the mutable default parameter value "gotcha." The default list is created only once when the function is defined and is shared among all calls to the function. This leads to the unexpected behavior in the example.
To avoid this gotcha, you should use an immutable object (like None) as the default value for parameters with mutable types and create a new mutable object within the function when necessary. Here's the revised code:
In this modified code, we use None as the default value for my_list and create a new list inside the function if my_list is None. This ensures that each call to the add_item function gets its independent list and prevents the unexpected behavior.
Understanding the mutable default parameter value "gotcha" in Python is crucial for writing reliable code. By using immutable default values and creating new mutable objects within the function when needed, you can avoid unexpected behavior and ensure your code works as intended.
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