Gotchas

Two dimensional array initialization⚑

Imagine we want to initialize an array containing a few empty arrays inside, something like

arr = [[], [], []]

If we want to save time and/or keystrokes, we might be tempted to do it as follows

arr = [[]] * 3

>>> arr
[[], [], []]

Which looks like what we wanted right? Well it does, but the gotcha is that all of the inner arrays are actually the same object copied so if you modify one of them, "all of them" will be equally affected. For example,

>>> arr[0].append('test')
>>> arr
[['test'], ['test'], ['test']]

So what you should do instead is:

arr = [[] for _ in range(3)]