reference - How does Python referencing work?

Question

I am confused with Python referencing. Consider the following example:

My task : To edit each element in the list

d = { 'm': [1,2,3] }
m = d['m']
m = m[1:]   # m changes its reference to the new sliced list, edits m but not d (I wanted to change d)

Similarly:

d = { 'm': [1,2,3] }
m = d['m']
m = m[0]    # As per python referencing, m should be pointing to d['m'] and should have edited d

In python everything goes by reference, then when is a new object created? Do we always need copy and deepcopy from copy module to make object copies?

Please clarify.

Answer 1

In Python a variable is not a box that holds things, it is a name that points to an object. In your code:

• d = { 'm': [1,2,3] } --> binds the name d to a dictionary
• m = d['m'] --> binds the name m to a list
• m = m[1:] --> binds the name m to another list

Your third line is not changing m itself, but what m is pointing to.

To edit the elements in the list what you can do is:

m = d['m']
for i, item in enumerate(m):
    result = do_something_with(item)
    m[i] = result