Newbie query regarding lists

[jazmad]

I am just working through some intros to Python. Basically I read about a concept and then have a play around to make sure I understand it. Right now, I'm on lists.

I wrote the following program to test a few commands:

test_list = [1,2,3,4]
print (test_list[0])
print (test_list[2])
print (test_list[-1])
sub_test_list = test_list[1:3]
print (sub_test_list)
naturals = [0,1,2,3,4,5,6,7,8,9,10,11,12]
evens = naturals[2::2]
print(evens)
odds = naturals[1::2]
print(odds)
primes = odds
primes[4:6]=[11,13]
print(odds) # OP originally had this bolded
print(primes)
naturals.append(13)
print(naturals)

The bit that confused me is the command 'print(odds)' towards the bottom in bold. I was still expecting odds to be [1,3,5,7,9,11] as the change was applied to the list 'primes'. However, both 'odds' and 'primes' were printed as [1,3,5,7,11,13].

Please could someone explain to me why this is the case?

Thanks

Stephen

[micseydel]

On line 12, the assignment reuses the same object. It's not a copy. So when one is modified, so is the other. You can make a "shallow" copy by replacing line 12 with

primes = odds[:]

This works because with [:] the variable "primes" no longer refers to the same object.

[jazmad]

Thank you very much

[jazmad]

Can I follow up with one additional question? I appreciate how to fix my initial query, but am not sure why the assignment in the original code reuses the object whereas in the code below it appears not to.

x=8
print('x=',x)
z=x
print('x=',x)
print('z=',z)
z=z+2
print('x=',x)
print('z=',z)

i.e. when z is modified x remain unmodified.

I assume it is that

variable2 = variable1

assigns variable2 independent to variable1 going forward (just of identical value at this stage) whereas

datatype2 = datatype1

assigns datatype2 and datatype1 to be identical going forward such that changes to one are automatically applied to the other.

However, I'd like to be 100% sure before moving on.

[micseydel]

Great question. Basically, lists are mutable, meaning can be edited. Numbers cannot change, though the variable that points to a number can be changed to point to a new number, which is what happens in your code. For this issue you've run into with lists, you won't run into with immutable types like numbers, tuples and frozensets.

[jazmad]

Thanks again. Want to spend time getting the basics right before moving on (and only installed Python on Thursday!)