gistfile1.txt

```
x = [1, 2]

a = []
a.append(x)

x.append(3)

print(a)
# [[1, 2, 3]]
```
`append(x)` stores a reference to the same list object. When `x` changes later, the object inside a changes too.

You can see it with `id()`:
```
x = [1, 2]
a = []

a.append(x)

print(id(x))
print(id(a[0]))
# same number
```