Is it good or bad to pass mutable data types as function arguments?

Passing mutable data types to a function can be both useful and dangerous. If the function modifies the passed object, it can lead to side effects that are hard to track, especially in large projects.

Pros:

• Saves memory and time, as no copies of data are created.
• Allows the function to directly modify the object's state.

Cons:

• Debugging becomes difficult due to unexpected data changes outside the function.
• Violates the principles of locality and purity of functions.

Recommendation: If a function is supposed to modify an object, this should be explicitly documented. Otherwise, it is better to pass copies or use immutable types to avoid side effects.

Example in Python:

def add_item(lst, item):
    lst.append(item)  # modifies the original list

my_list = [1, 2]
add_item(my_list, 3)
print(my_list)  # [1, 2, 3] — the list has changed outside the function