JuniorTheoryCommon

What does the program print?

Python

a = [1, 2, 3]
b = a
c = a[:]
b.append(4)
print(len(a), len(c))

4 3

Expecting 3 3, as if b = a copied the list: it gives the same list a second name, so b.append(4) adds to a as well.
Expecting 4 4, as if a[:] were another name for a: a full slice builds a new list, so c keeps its 3 elements.