What is the difference between shallow copy and deep copy?

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A shallow copy and a deep copy both produce a new container, but they differ in how they treat the objects referenced inside that container. Shallow copy — creates a new outer object whose elements are the same references as the original. Mutating a nested object through either copy is visible in both. Deep copy — recursively copies every nested object so the new structure shares nothing with the original. Mutating through one copy cannot affect the other. How to make each: import copy a = [[1, 2], [3, 4]] b = a.copy() # shallow (list.copy, slicing [:], list(a)) c = copy.copy(a) # shallow d = copy.deepcopy(a) # deep The difference in practice: a = [[1, 2], [3, 4]] b = copy.copy(a) d = copy.deepcopy(a) a[0].append(99) b[0] # [1, 2, 99] — same inner list, same mutation d[0] # [1, 2] — independent copy Show all three: assignment (same object), shallow copy (new outer, shared inner), deep copy (fully independent). The nested list mutation is the classic demonstration. Know the shorthand methods: list[:], list.copy(), dict.copy() for shallow copies.