How does python append to list work?

Question

I came across an issue in python appending to a list. The code I implemented was:

a=[1,2]
b=[3,4]
a.append(b)
b.append(5)
print a
print b

My understanding of python append was that the expected output of this code would be:

Expected Output

a=[1,2,[3,4]]
b=[3,4,5]

But the actual output is something different. Actual Output

a=[1,2,[3,4,5]]
b=[3,4,5]

I just want to know why this happened.

Since I appended the list b to a, before appending 5 to b, list a should have [1,2,[3,4]]

Answer 1

Python names are references, and appending to a list appends a reference to the same object.

In other words, you did not append a copy of the b list. The a list and the name b share a reference to one and the same object:

>>> a = [1, 2]
>>> b = [3, 4]
>>> a.append(b)
>>> a[-1] is b  # is tests if two references point to the same object
True
>>> id(a[-1]), id(b)  # id produces a unique number per object
(4595716320, 4595716320)

If you expected to add a copy of the b list to a, do so explicitly:

a.append(b[:])

or

a.append(list(b))

See How to clone or copy a list?