Python: I changed b... and a changed?!PRO

b = a gives the same list a second name. a.copy() makes a new list.

# I changed b... and a changed ?!
# b = a does NOT copy the list: both names point at the SAME list.
a = [1, 2]
b = a
b.append(3)
print(a)          # [1, 2, 3] ?!
print(a is b)     # True: one list, two names

# Fix: make a real copy
a = [1, 2]
b = a.copy()
b.append(3)
print(a)          # [1, 2] ✓

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