What are mutable and immutable data types?

In Python, all objects are divided into mutable and immutable depending on whether their contents can be changed after creation.

Immutable:

• int, float, bool
• str
• tuple
• frozenset

When "modified," a new object is created:

x = 10
print(id(x))  # e.g.: 140234866357520
x += 1
print(id(x))  # Different id — this is a new object

Mutable:

• list
• dict
• set

The object is modified "in place":

lst = [1, 2, 3]
print(id(lst))  # e.g.: 140234866400064
lst.append(4)
print(id(lst))  # Same id — object was modified

Why this matters:

• Dictionary keys can only be immutable objects.
• Passing to functions: mutable objects can be changed inside a function, which may cause unexpected side effects.
• Default values: don't use mutable objects as default values in functions.

# Common mistake
def add_item(item, lst=[]):  # Same list across all calls!
    lst.append(item)
    return lst

# Correct approach
def add_item(item, lst=None):
    if lst is None:
        lst = []
    lst.append(item)
    return lst