A quick explanation of a python bug to watch out for

weirdpythonbehavior.py

# When working with nested lists created with []*x, watch out for this bug:
a = [[0, 0, 0], [0, 0, 0], [0, 0, 0]]
a[0][1] += 1
print(a)
# gives [[0, 1, 0], [0, 0, 0], [0, 0, 0]]
# totally straightforward! but …
a = [[0]*3]*3
a[0][1] += 1
print(a)
# gives [[0, 1, 0], [0, 1, 0], [0, 1, 0]]
#even though
[[0, 0, 0], [0, 0, 0], [0, 0, 0]] == [[0]*3]*3
# is True

# Probably due to the []*x syntax making a deep copy as opposed to a shallow one.

# If you want a list that behaves like the first a, use:
[[0 for x in range(3)] for x in range(3)]

Nice explanation. []*x copies the references of what is inside the list. It's fine for immutable values, but as demonstrated, the nested list's reference gets copied to create the final list. Similar "results" can be found when using an object constructor instead of a list.