An exercise to help build the right mental model for Python data.
- Solution: https://memory-graph.com/#codeurl=https%3A%2F%2Fraw.githubusercontent.com%2Fbterwijn%2Fmemory_graph_videos%2Frefs%2Fheads%2Fmain%2Fexercises%2Fexercise9.py&play=
- Explanation: https://github.com/bterwijn/memory_graph?tab=readme-ov-file#python-data-model
The “Solution” link visualizes execution and reveals what’s actually happening using 𝗺𝗲𝗺𝗼𝗿𝘆_𝗴𝗿𝗮𝗽𝗵: https://github.com/bterwijn/memory_graph
Let’s look at the suspects.
b.append(), you add to the existing list. This mutates the current list. You can not be the culprit.
b = b + [], you join up lists, making a new list in the process, that then gets stored in variable b, without changing the original list that’s still stored in variable a. You are not the culprit either.
No, the culprit must be someone that ambiguously looks both like a mutation and an instantiation.
Isn’t that right, b += []? Because the culprit… IS YOU!
Hot take: if
a += bis not the same asa = a + b, you done fucked upIt’s definitely not the same. Similarly for a class you can define the
__add__dunder method fora + band separately the__iadd__dunder method fora += b. The first creates a new object, the latter changes/mutates the existing objecta. For immutable types it is the same though.For immutable types it is the same though.
The most twisted thing I learned is that all
ints below a fixed limit share the sameid()result, so>>> x = 1 >>> id(x) 135993914337648 >>> y = 1 >>> id(y) 135993914337648But then suddenly:
>>> x = 1000000 >>> id(x) 135993893250992 >>> y = 1000000 >>> id(y) 135993893251056Using
id()as a key indict()may get you into trouble.Oh absolutely, I understand that the language allows implementations to violate my proposed equivalence — I’m saying that’s a bad implementation (some might say a bad language, for allowing bad implementations, but I don’t necessarily agree)
I expect A if “b” is a clone, or E if it’s a reference. But I also wouldnt combine array operations like this.
The answer being C feels like a bug.
Eh, I get it. The equal operator creates a reference but the plus operator isn’t destructive so it creates a new list and overwrites the variable b with a new list, when assigned.
Of course, this would all be avoided if creating copies was the norm; which is why I stick with functional languages.
Copying a list with a million elements every time you make a small change is not fun. Sure, you can optimize a bit behind the scenes, but that still gives a lot of overhead.
That’s what you get because you’re afraid of pointers 😁! /j
What the fuck
Tell me again how python is easy to learn for beginner programmers.
Other languages that have similar behavior include Java and JavaScript, and yes you have to be careful with list / array operations in those languages as well, lest you operate on the wrong list inadvertently. Happened to me. It will happen to you.
You don’t have to compile. You don’t need semicolons.
Python was my first programming language, and those two things alone honestly are really nice. Doesn’t mean there aren’t a million other issues and difficulties, though, lol.
