"strange" for loop behaviour [solved]

Question

Can someone explain what happens in the code below?

a = []
b = [a, a, a]
for x in b:
n = len(x)
x.append(n)
print(b[0]) # --> [0, 1, 2]

Why does x.append(n) also append n to a?
Something similar happens when I write x+=['test']
However, if I write x=['test'] or x='test' a does not change

Answer 1

gilex the logic goes like this:

a = []
b = [a, a, a] #Here, you put the list a in list b! It's a list in a list!
for x in b: #Here, the for loop iterates for each element in b, which is the list a three times! (since b contains list a three times) so, n is list a.
n = len(x) #The same as n = len(a)
x.append(n) #Since x is list a, it will append n!
print(b[0]) # b[0] or the first element of b is list a, so it prints list a, which is [0,1,2]!

Don't worry, this is only because you put a list inside a list! I didn't even know you can do that! 😅

Happy programming! :)

Answer 2

Thank you for your explanation and have a great day😀

Answer 3

gilex you're welcome and thanks! Have a great day as well 😊