What are default parameters in Python and why are they immutable?

What are default parameters in Python and why are they immutable?

Default parameters in Python allow you to define a function with default values for its arguments. If no value is provided, the default is used.

Can you give me an example of a function with default parameters?

Sure! Here's a simple example:

def greet(name, greeting='Hello'):
    return f'{greeting}, {name}!'

In this case, if you call greet('Alice'), it will return 'Hello, Alice!'. If you call greet('Alice', 'Hi'), it will return 'Hi, Alice!'.

Why are default parameters considered immutable?

Default parameters are evaluated once when the function is defined, not each time the function is called. This means if you use a mutable type like a list as a default, it can lead to unexpected behavior.

Can you explain the issue with mutable default parameters?

Certainly! If you use a mutable default parameter, like a list, and modify it, the default value will change for subsequent calls. For example:

def append_to_list(value, my_list=[]):
    my_list.append(value)
    return my_list

Calling append_to_list(1) and then append_to_list(2) will result in [1] and then [1, 2], which is often not the intended behavior.