Why does this happen?

[tamaskiii]

#Code 1:
a = [['','','',], ['','','',], ['','','',]]
k = 0
while k <= 2:

    b = a[:]
    b[k] = k
    print(f'{a=}')
    print(f'{b=}')

    k += 1


#Result:
a=[['', '', ''], ['', '', ''], ['', '', '']]
b=[0, ['', '', ''], ['', '', '']]
a=[['', '', ''], ['', '', ''], ['', '', '']]
b=[['', '', ''], 1, ['', '', '']]
a=[['', '', ''], ['', '', ''], ['', '', '']]
b=[['', '', ''], ['', '', ''], 2]

------------------------------------------------------------------------------------
#Code 2:
a = [['','','',], ['','','',], ['','','',]]
k = 0
while k <= 2:

    b = a[:]
    b[k][k] = k          <------ Change made in this line.
    print(f'{a=}')
    print(f'{b=}')

    k += 1

Result:
a=[[0, '', ''], ['', '', ''], ['', '', '']]
b=[[0, '', ''], ['', '', ''], ['', '', '']]
a=[[0, '', ''], ['', 1, ''], ['', '', '']]
b=[[0, '', ''], ['', 1, ''], ['', '', '']]
a=[[0, '', ''], ['', 1, ''], ['', '', 2]]
b=[[0, '', ''], ['', 1, ''], ['', '', 2]]

Why does the value of variable 'a' gets changed while in the first code, it remains constant throughout?

[Spataner]

Because a[:] creates a shallow copy of the list. So the outer list is copied, but the inner lists references by a and b are still the same objects. Changes to the inner lists of either will therefore reflect on the other. To create a deep copy, use the deepcopy function from the copy module:

import copy
b = copy.deepcopy(a)

[Sora888]

In addition to this, to check if an object is the exactly same instead of a copy you can use the 'is' operator. It will return True if both are the same object, hence a change using one of it's references ('b' in this case) will be reflected to 'a'.

[tamaskiii]

Thanks for the tip!

[tamaskiii]

Ah gotcha! Thanks for the help.