What is the difference between shallow copy and deep copy, and what is the mutable default argument trap?

Python assigns by reference, so b = a gives two names for one object.

• Shallow copy — copy.copy(x), list(x), or x[:]. A new outer container holding the same inner objects. Mutating a nested list still affects both.
• Deep copy — copy.deepcopy(x). Recursively copies everything, and handles circular references.

The mutable default argument trap is the classic Python gotcha:

def add(item, bucket=[]):     # WRONG
    bucket.append(item)
    return bucket

add(1)   # [1]
add(2)   # [1, 2]  <- the same list!

The default is evaluated once, when the function is defined, not on each call. So every call without an explicit argument shares one list.

The fix:

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

Note: The same applies to dictionaries, sets, and any object constructed in the signature — including datetime.now(), which freezes at import time.