r/PythonLearning • u/Sea-Ad7805 • 9d ago
Python Data Model Exercise
An exercise to help build the right mental model for Python data. - Solution - Explanation - More exercises
The โSolutionโ link visualizes execution and reveals whatโs actually happening using ๐บ๐ฒ๐บ๐ผ๐ฟ๐_๐ด๐ฟ๐ฎ๐ฝ๐ต.
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u/arivictor 7d ago
https://parsnip.dev/editor/qDQDFUs4QrHX
C
At the start b = a does not create a copy, b simply references a. However, as soon as you do b = b + [[3]] you create a new list in b, a is no longer linked to b.
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u/Sea-Ad7805 6d ago
Nice one, do check the "Solution" link for a visualization of what you explain here.
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u/Dapper_Bad_8728 9d ago
I think it's D.
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u/Sea-Ad7805 9d ago
Incorrect sorry, see the "Solution" link for the correct answer.
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u/Dapper_Bad_8728 8d ago
Oh yeah it prints the "a" list not the "b" one. I though it was printing the "b"
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u/ghost59 8d ago
It's A.
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u/Sea-Ad7805 8d ago
Incorrect sorry, see the "Solution" link for the correct answer.
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u/ghost59 8d ago
That's weird. Why is it printing C when a didn't touch again.
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u/Sea-Ad7805 8d ago
See the "Explanation" link for an explanation of Python's data model.
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u/ghost59 8d ago
Oh! Wait. I'm stupid. You didn't make a copy of the values with b. You copied the original and when you change b you mutate a as well. It's because the list is something that can be mutated.
If I wanted a not to mutate. It would need to be B = a.copy()
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u/Sea-Ad7805 8d ago
Be careful,
b = a.copy()makes a shallow copy. Are you sure you want that? https://github.com/bterwijn/memory_graph?tab=readme-ov-file#copying-values-of-mutable-type2
u/ghost59 8d ago
No. I want a deep copy. Because you're right. Looks like I'll be hitting training grounds on boot.dev tonight.
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u/Sea-Ad7805 8d ago edited 8d ago
I tried to make the exercise difficult so don't feel bad. Keep the memory_graph tool around to visualize tricky Python situations, also with boot.dev exercises.
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u/Samar_Upreti 8d ago
Option C correct Concept of indexing (No index [2] exists) eliminate option D As we know list are mutable in python and we use .append
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u/OopsAllCharisma 8d ago
What's the line b = b + [3] for?
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u/Sea-Ad7805 8d ago
Line
b = b + [[3]]adds[3]to theblist, and this exercises is about how this relates toa. First think about it, then check the Solution.
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u/FoolsSeldom 8d ago edited 8d ago
C.
Line 4 re-assigns b to a new list object so severs the link to the original list object referenced by a and b up to that point.
EDIT: More detail for learners regarding variables and lists in Python (on back of a challenge from u/HardyDaytn):
Variables in Python do not contain values, but simply reference where in Python a Python object (int, float, str, list, etc) is located in memory. Generally, you don't need to concern yourself with the memory matters.
A Python list object similarly does not contain any variables or values, but only a collection of zero or more references to other Python objects.
Two variables can reference the same object and if the object is mutable (able to be changed), such as a list, then changing the object can be done via either variable.
Thus,
alpha = 10
beta = 'eleven'
charlie = 12.1
delta = ['mary', 'had', 'flower']
stuff = [alpha, beta, charlie, delta]
This does the following:
- Assigns a memory reference to the pre-defined
intbinary object representation of decimal 10 to the variable namealpha - Assigns a memory reference to the new
strobject (consisting of binary unicode numbers for each character in the string) to the variable namebeta - Assigns the memory reference to the new
floatbinary object representation of the decimal floating point number 12.1 to the variable namecharlie - Assigns the memory reference to the new
listobject, containing 3 entries to newstrobjects, to the variable namedelta, where each entry in thelistis itself a memory reference to a new Python object - Assigns the memory reference to the new
listobject, containing 4 entries, to the variable namestuff, where each entry in thelistis itself a memory reference to a Python object as below,- the first entry in the
listholds the same memory reference asalphaholds - the second entry in the
listholds the same memory reference asbetaholds - the third entry in the
listholds the same memory reference ascharlieholds - the fourth entry in the
listholds the same memory reference asdeltaholds, i.e. they both refer to the samelistobject
- the first entry in the
It is worth noting that if you change the contents of the list referenced by delta, that change will also show up in the list referenced by stuff. This is shown in the example below.
Complete code:
alpha = 10
beta = 'eleven'
charlie = 12.1
delta = ['mary', 'had', 'flower']
stuff = [alpha, beta, charlie, delta]
print(stuff)
delta[1] = 'lost'
print(stuff)
Outputs,
[10, 'eleven', 12.1, ['mary', 'had', 'flower']]
[10, 'eleven', 12.1, ['mary', 'lost', 'flower']]
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u/Sea-Ad7805 8d ago edited 8d ago
Nice one, do check the "Solution" link for visualization of the correct answer.
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u/FoolsSeldom 8d ago
No thanks, could visualise it virtually (mentally) with ease. Good exercise for learners though.
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u/Sea-Ad7805 8d ago
Thanks, but are you sure? It's a really powerful visualizer that can help to understand and debug tricky Python situations: https://memory-graph.com/#codeurl=https://raw.githubusercontent.com/bterwijn/memory_graph_videos/refs/heads/main/exercises/exercise18.py&play
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u/FoolsSeldom 8d ago
Yes. I probably should not have answered the challenge. It was something of a fly-by.
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u/HardyDaytn 8d ago
Not a very beginner friendly explanation though, as line 5 is still changing list a.
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u/FoolsSeldom 8d ago
The OP has provided linked comprehensive explanations including detailed diagrams.
However, if any beginner wants to ask for clarification, I'd be more than happy to expand.
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u/HardyDaytn 8d ago
The OP has provided linked comprehensive explanations including detailed diagrams.
Right, so instead of trying to be helpful, you only wanted to toot your own horn here.
A healthy dose of "look ma', I know this one too!".
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u/Sea-Ad7805 8d ago edited 8d ago
It's even worse, it's a "look ma', I've built this tool" situation. The Python data model can seem difficult at first, but I hope the visualization can help beginners understand it more easily. Beginners will have to face it at some point if they want to get to advanced.
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u/FoolsSeldom 8d ago
So, I've edited the original comment to add additional information for learners if they don't want to follow the links provided by the OP. (This is using repurposed content from classes I've run.)
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u/HardyDaytn 8d ago
My proverbial hat's off to you for adding detailed info. It's an annoying and difficult concept to learn and understand compared to other more intuitive things.
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u/Embarrassed-Crow9283 8d ago
C. The first two lists in b were still the same lists as the lists in a, but the third list was added and the third element would only be in b and not a. You appended the list in position 0 with 11 and in position 1 with 22. This gives result C when printing out a.