The difference between '=' and 'append' in list in this example?

Question

I know list is a kind of changeable collection of data object,but why the output is different. I thought they should be the same.

a = [1]
b = a
b = [1,2]
print(a)

output:

[1]

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

output:

[1,2]

Answer 1

In the first example you are overwriting:

a=[1]
b=a     # b=[1] and b=a
b=[1,2] # b=[1,2] but not a

in the second example you apply a function built into lists:

a=[1]
b=a         # b=[1] and b=a
b.append(1) # applies append to b which is a so a.append is done

Answer 2

a = [1]
b = a
b = [1,2]
print(a)

When you do this, the value for b is reassigned therefore losing the connection with a .

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

But here, appending to b means the same list in the memory is appended a value. Since a and b still refer to the same memory, printing a results the same as b since they two are just two different aliases for the list.