Why does mutability matter in Python?

Assignment never copies an object. It binds a name to a reference. For immutable values that distinction is invisible because nothing can change the object. For mutable values it is the source of many bugs, especially mutable default arguments and shared class attributes.

def add_item(item, bucket=[]):
    bucket.append(item)
    return bucket

print(add_item("a"))  # ['a']
print(add_item("b"))  # ['a', 'b']

def add_item_safe(item, bucket=None):
    if bucket is None:
        bucket = []
    bucket.append(item)
    return bucket

Mutable versus immutable

• Immutable: int, float, bool, str, bytes, tuple, frozenset, Decimal
• Mutable: list, dict, set, bytearray, most user-defined objects
• Immutable objects can be hashable and are safe to share across threads
• Mutable objects can be changed in place, which is efficient but requires ownership discipline

Identity and equality are separate. The is operator compares identity, and == compares value. Small integers and some strings are interned, so is may appear to work for them, but it is never the correct test for value equality. Use == for values and is only for None, True, False and sentinel objects.