How is length 3? What is the working and process i.e how is code getting executed on a line by line basis?

Question

a=[1,2]
d=a
a.append(7)
print(len(d))

The output i.e length of d is 3, even though I have appended 7 to a, not d. How? What is the process of execution on a line by line basis.

Answer 1

d=a
Means you passed a as reference not as copy.
So changes made in a also affect d .
If you don't want it use
d=a.copy()