Can you please explain the output of this code?

Question

a = []
b = [a, a, a]
for x in b:
n = len(x)
x.append(n)
print(b[0])

Answer is [0, 1, 2]

Answer 1

All three elemets of list “b” actually are the only one element - list “a”, there are just three references to list “a”.

When “for loop” takes first element - list “a” - this element is empty, so len(x)=0. It appends the integer 0 into the list “a”.

Then “for loop” takes second element of list “b” - the same list “a”, which already has one element inside it - the integer 0. That’s why at this point len(x)=1. We append the integer 1 into list “a”, so list “a” has two elements inside, so a==[0,1].

As you guess at the third iteration len(x)==2, it appends integer 2 into list “a”.

At the end, when you see print(b[0]), b[0] is list “a”, which now contains 0,1 and 2 inside.

Hope it is clear=)

Answer 2

For x in b means a
At first len(x=a)=0 , so n=0 and 0 append to a, so now a=[0]
For Second len(x=a) = 1, so n=1 and 1 append to a , so a=[0,1]
For Third one len(a)=2, so n=2 and 2 append to a , so now a=[0,1,2]
Now we know b[0]=b[1]=b[2]=a
And a =[0,1,2]

Answer 3

Yes I made a mistake and I edited it Mikhail

Answer 4

Continuing Jente's explanation:
0. b = [[], [], []]
1. b = [[0], [0], [0]]
2. b = [[0, 1], [0, 1], [0, 1]]
3. b = [[0, 1, 2], [0, 1, 2], [0, 1, 2]]

Answer 5

All three elemets of list “b” actually are the only one element - list “a”, there are just three references to list “a”.

When “for loop” takes first element - list “a” - this element is empty, so len(x)=0. It appends the integer 0 into the list “a”.

Then “for loop” takes second element of list “b” - the same list “a”, which already has one element inside it - the integer 0. That’s why at this point len(x)=1. We append the integer 1 into list “a”, so list “a” has two elements inside, so a==[0,1].

As you guess at the third iteration len(x)==2, it appends integer 2 into list “a”.

At the end, when you see print(b[0]), b[0] is list “a”, which now contains 0,1 and 2 inside.
Continuing Jente's explanation:
0. b = [[], [], []]
1. b = [[0], [0], [0]]
2. b = [[0, 1], [0, 1], [0, 1]]
3. b = [[0, 1, 2], [0, 1, 2], [0, 1, 2]]
For x in b means a
At first len(x=a)=0 , so n=0 and 0 append to a, so now a=[0]
For Second len(x=a) = 1, so n=1 and 1 append to a , so a=[0,1]
For Third one len(a)=2, so n=2 and 2 append to a

Answer 6

Mikhail

Answer 7

I tried to rewrite and analyze it:

a = []
b = [a]*3
for x in b:
n = len(x)
x.append(n)
print(b[0])

Answer is [0, 1, 2]

So, if we change to b = [a]*4,
the answer would be:
[0, 1, 2, 3]

to b = [a]*5
the answer would be
[0, 1, 2, 3, 4]
and so on.

Also print(b) give us all the list ([0, 1, 2, ... n-1] n times)

Answer 8

It's simple

List b has sublist a three times...

b = [ [] [] [] ]. Initially

In the for loop a gets appended with length of a i.e. 0

Now b = [ [0] [0] [0] ]

Answer 9

first

Answer 10

6969

Answer 11

I have no idea ;)