Why the result is 3 not 2?

Question

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

Answer 1

Variable names are like labels in python. "b=a" adds another label to the list a, so you can now use both "a" and "b" to address the same list. Whatever happens to b, happens to a and vice versa.

If you want to copy the list instead of just adding another label, use b = a[:] or b = list(a).

Answer 2

In this case b and a are two variables that reference the same list (point to the same memory location).
Therefore if the value of a changes, same thing happens to b.
To copy the list a to a separate and independent variable, you would have to slice it for example:
b = a[:]

Answer 3

A list is what is known as a ‘mutable’ object. This essentially means that, when you have a statement b=a, changing a also changes b.

So in your code, b=a means that b is set as the same as a, and when b is changed, so is a. So when a gets 3 appended to it, it affects b too. Since the length of a (and hence b too) is now 3, that is what is returned.

Answer 4

b is a reference to a and not a copy of it.

Answer 5

if you want the result is 2 you should applying copy method.