Python points both lists in vec = v to the same spot of memory.
To copy a list use vec=v[:]
This might all seem counter-intuitive. Why not make copying the list the default behavior? Consider the situation
def foo():
my_list = some_function()
# Do stuff with my_list
Wouldn't you want my_list to contain the exact same list that was created in some_function and not have to spend time creating a copy of it. For large lists copying the data can take some time. Because of this reason, Python does not copy a list upon assignment.
Misc Notes:
If you're familiar with languages that use pointers. Internally, in the resulting assembly language, vec and v are just pointers that reference the address in memory where the list starts.
Other languages have been able to overcome the obstacles I mentioned through the use of copy on write which allows objects to share memory until they are modified. Unfortunately, Python never implemented this.
For other ways of copying a list, or to do a deep copy, see How to clone or copy a list?
