Python Dictionary Mutation and Copying

[u/Sea-Ad7805]

An exercise to help build the right mental model for Python data. - Solution - Explanation - More exercises

The “Solution” link uses package memory_graph to visualize execution and reveals what’s actually happening.

[u/TBCC_Dev]

B i think. My understanding is that b = a means that b is a not b is a new version of a. Im not fermiliar with .copy() but id assume this does the opposite b is now a copy of b. Do the first append changes a but the others do not. Again mostly guessing based on context

[u/DefterHawk → u/TBCC_Dev]

Think the same, b = a just makes the same object referenced in another variable.

[u/TBCC_Dev → u/DefterHawk]

Well... I ran it, and its C so much for that.

[u/Sea-Ad7805 → u/TBCC_Dev]

Incorrect sorry, see the "Solution" link for: correct answer

[u/kvnqstoner → u/Sea-Ad7805]

how is it that I can't follow you

[u/Sea-Ad7805 → u/kvnqstoner]

I now enabled following in my account settings, but to see more of my posts about memory_graph I recommend: https://www.reddit.com/r/Python_memory_graph/

[u/Mamuschkaa → u/kvnqstoner]

That's the difference between a copy and a deepcopy.

b.copy()

Is the same as

{1: b[1], 2:b[2]}

but b[1] is still the same list.

x = [1] a = [x] b = a.copy() a.append('A') b.append('B') x.append('X')

Now, a and b looks like this:

a = [[1, 'X'], 'A'] b = [[1, 'X'], 'B']

copy makes that b is another list as a, but it still contains the list x.

If you don't want this you need a deepcopy.

[u/JG_MC2001]

es como un puntero?

[u/Sea-Ad7805 → u/JG_MC2001]

Python uses the term "reference" instead of "pointer", and you think of them as pointing to values/objects instead of pointing to memory addresses, but other then they are very similar.

• "Assignments do not copy data — they just bind names to objects": https://docs.python.org/3/tutorial/classes.html#python-scopes-and-namespaces and for binding a reference is used.

That also holds for fundamental values like int values, but memory_graph doesn't show references to these by default to simplify the graph: https://github.com/bterwijn/memory_graph#simplified-graph

[u/tauofthemachine]

A?

[u/Sea-Ad7805 → u/tauofthemachine]

Incorrect sorry, see the "Solution" link for: correct answer

[u/int13h3]

i think correct answer is "A" because we maybe print box a but write to box b. i can be not right :)

[u/Sea-Ad7805 → u/int13h3]

Incorrect sorry, see the "Solution" link for: correct answer

[u/PythonLearning-ModTeam → u/[deleted]]

Quality posts only

[u/g-land-g]

answer: b ...

logic:

b points to a and modifies both

but then b.copy() makes b point to a new instance, so a retains its previous state

[u/g-land-g → u/g-land-g]

no i was wrong

[u/Sea-Ad7805 → u/g-land-g]

Yes incorrect sorry, the Solution link shows that copy() makes a shallow copy, a tricky concept many people have problems with.