Hello, can anyone explain this?

Question

a = [1,2,3]
b = a
a.append(4)
print(b) #[1,2,3,4]
print(a) #[1,2,3,4]

Till now they share the same memory location but when I write:

a = a + [5]
print(b) #[1,2,3,4]
print(a) #[1,2,3,4,5]

Now, they no more share the same memory location!!
Why does this happen?

Answer 1

Stravo1 okay earlier i thought that both the codes are different but they are not as you told me

here pay attention to the line a=a+[4] and a.append(4) let's see the difference when you do a.append(4) what Python does is that i add 4 to the same list which is pointed by a and b both names therefore in append case when you print a or b it shows the same output but in a=a+[4] what Python does is that as this line a=a+[4] is executed by the Python interpret the memory pointer of memory location 'a' creates a new memory location for the variable a in which the old list data and the newly added data is copied and the variable b is still pointing the old memory location but now variable a is pointing a new memory location see the difference here you did not commanded the Python to append the element in the same list but you commanded Python to add a element to list a so what Python does is that it creates a new list with old data and add new data in that new list

Answer 2

To Understand This You Must Know How The Assignment Operator (=) Works in The Memory Let me explain you About the assignment operator.
when you declare a variable in memory it must have a name such as in your case a list type data structures have name 'a' and this name 'a' points the data structures list location in memory but when you does b=a Python does not create another data structures of list type with name 'b' But what Python does is that it gives name 'b' to that old data structure 'a' so basically 'a' and 'b' both are pointing the same Memory location it is actually same as calling a single persona with two names.
in context with Python it can be said that a single Memory location is pointed by two names.

Answer 3

Khan Ayaan ...okay, I got your point but how does this explain the last part of my question? There too I have two names pointing to one location, isn't it??

Answer 4

no no there is not that case in the last part you have added one Element in a by a=a+[5] but you did not created a relationship between a and b as you had done that in the upper case by writing b=a and in the last case variable a and b both are pointing the different memory locations in the memory and the changes made in b will not affect a and same goes for a that is changes made in a will not affect b unless you create a relationship between then by assigning a to b by b=a or by assigning b to a by a=b.

Answer 5

Khan Ayaan
Okay...(actually the 2nd part of the code is a continuation of the 1st part where I've mentioned b=a) but still if I say:

a=[1,2,3]
b=a
a=a + [4]
print(a) #[1,2,3,4]
print (b) #[1,2,3]

How do you explain this? Here too 'b' is linked to 'a', and just like in the previous code [a.append(4)] I'm adding 4 to 'a' [instead of append() I'm using +], but unlike the previous code any change in 'a' this time won't affect 'b'? Why?

Answer 6

Khan Ayaan
Okay, perfect answer...
Thnx

Happy coding 😄😄

Answer 7

Stravo1 Welcome Enjoy Coding.