🔒 Embarrassing Question

[regal nexus]

Genuinely embarrassed to ask this question but I feel like my brain is about to explode over the simple ahh topic:

What is passing by value and passing by reference.

How does passing a parameter differ between the two. I know its such an easy thing, I think it may just be the fact im on 2hours of sleep icl ahah

[regal sail]

@regal nexus

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[timid cypress]

What's your understanding of the two? This sounds like a homework question, so I'm hesitant to just give an answer here.

[restive needle]

give this a read, it might help: https://nedbatchelder.com/text/names.html

[regal nexus]

It was originally from a past paper so revision not homework

[regal nexus]

thank you

[regal nexus]

I know the answer for the question, but understanding the question is different

I feel like ive just memorised an answer from the textbook from ages ago, without actually understanding whats happenign

[timid cypress]

A thing that confuses matters here is "pass by reference" has different meanings depending on the language. Python is essentially pass by value (meaning things are copied on passing), but the thing being passed is a pointer to the object, not the object itself.

No object is ever copied on assignment/passing, but a function also can't reassign variables outside of itself by reassigning its parameters.

[regal nexus]

Yeah, I've read about this from what I've seen, how Python is create different from e.g. C when talking about by values and by pararmeters

For context

This was the question on past paper

question ii)

[timid cypress]

Python basically works the same as Java and Javascript when it comes to their reference types.

Oh, this doesn't have anything to do with Python.

[regal nexus]

Yeah exam boards never refer to a specfic language, but I thought that this server may help

its mroe pseudocode

My intial thought process was that Python would have similar or the same kind of logic for this question if ygm

[trail crow]

I have a question concerning this if it wont derail the OPs topic to much.
as Ive noticed in the same script file if i send a dict to a func ad dont return it, itll get updated, vrs sending it to a func in a mod file i would have to return it.

[timid cypress]

"Mod file"?

[trail crow]

a file imported with the same function...

[timid cypress]

A function being in a different module changes nothing. No matter where a function comes from, copies are never made. If a imported function mutates its argument, that change will be seen externally.

[stone kelp]

!e

with open("mod.py", "w") as f:
  f.write("def update(d): d['key'] = 'value'")

import mod
stuff = {}
mod.update(stuff)
print(stuff) 

[regal sail]

:white_check_mark: Your 3.12 eval job has completed with return code 0.

{'key': 'value'}

[timid cypress]

TIL the bot has access to a filesystem.

[stone kelp]

it even sends pngs
so you can use matplotlib and save it to a png and it will send the graph

[timid cypress]

Oh, cool.

[timid cypress]

Back to the main question, "pass by value" just means a copy of the value is made on passing. "Pass by reference" though is a bit harder to define since it can vary a bit between languages. If they previously taught you C++, for example, they may expect a different answer than if they had taught you Java.

[glossy pivot]

doesn't that just mean pass by reference?

[stone kelp]

"that" being what?

[glossy pivot]

the thing being passed is a pointer to the object, we're passing a reference

[regal nexus]

I figured it out through linking it to a previous topic (I think I have anyway). Im kinda just making the connection that it works similiarly to how indirect and direct memory addressing works in LMC/Assembly for the CPU

[stone kelp]

yeah that makes sense in python, but thats where the "depending on the language" can come in, e.g.

void f(int &x) {
  x += 1;
}

void g() {
  int x = 0;
  f(x);
  // x = 1
}

[regal nexus]

Like this:
pass by reference ≈ indirect addressing
and pass by value ≈ direct addressing

Thats how Im kinda understanding it atm

[stone kelp]

pass by value would be mov'ing the value into the respective register / putting on the stack for the argument (so there would be no way for the function to mutate it in a way visible to the caller), pass by reference would be mov'ing its' address

[glossy pivot]

i don't think i've seen this before, is this making x inside f refer to g's stack frame? it's like passing a pointer, but looks like a local variable on the surface

odd feature, i'm only used to passing pointers in c/zig and writing into the locations of those pointers

[timid cypress]

I might be giving C++'s definition too much weight because it was my first language. In most circumstances, "pass by reference" works as it does in Python and Java and such. I think C++ is the odd one out here where a reference allows reassigning the passed variable.

[pine dew]

what language even is this?

[timid cypress]

The assignment says it's pseudocode.

[molten moth]

My take:
In compiled languages, variables basically are space in memory. So pass by value means passing the value only and storing it in a new space.
Vs pass by reference is actually passing that same "space".

In python, variables are just names. They're akin to pointer to just any object but without being able to do pointer arithmetic. Reassigning stuff changes the stored address.
That's how the arrows in Ned's blog basically work, any var= changes where the arrow points.

Python's passing is sometimes referred to as "pass by assignment" because it follows exact rules of python assigning values to a variable (and without people being confused by different meanings of those "pass by" in other langs).
Meaning mutable stuff can be mutated... But arg1=something inside a function will be visible only in that function (because we change what we point to for that name only, not modifying the outside name).

[regal sail]

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