Is python no longer a modern language? Objects are certainly not copied when passed to a function.
$ python3
Python 3.12.3 (main, Jul 31 2024, 17:43:48) [GCC 13.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> def x():
... v = 1
... y(v)
... print(v)
...
>>> def y(val):
... val += 1
...
>>> x()
1
Everything in a modern language is passed by copy. Exactly what is copied varies and can easily be pointers/references. But there were languages once upon a time that didn't work that way. It's a dead distinction now, though, unless you go dig one of them up.
If you want a specific one to look at, look at Forth. Note how when you call a function ("invoke a word", closest equivalent concept), the function/word doesn't get a copy of anything. It directly gets the actual value. There is no new copy, no new memory location, it gets the actual same memory as the caller was using, and not as a "pointer"... directly. Nothing works like that any more.
The classic example of "pass by copy-reference is less expressive" is you can't have pass a reference to number and have the caller modify it. You have to explicitly box it. I understand you understand this, but it's worth considering when thinking about whether the distinction means absolutely nothing at all.
This is really not true. Depending on how your language implements pass-by-reference, you can pass a reference to an int without boxing in one of two ways: either pass a pointer to the stack location where the int is stored (more common today), or simply arrange the stack in such a way that the local int in the caller is at the location of the corresponding parameter in the callee (or in a register).
The second option basically means that the calling convention for reference parameters is different from the calling convention for non-reference parameters, which makes it complicated. It also doesn't work if you're passing a heap variable by reference, you need extra logic to implement that. But, for local variables, it's extremely efficient, no need to do an extra copy or store a pointer at all.
Python passes primitive types by value, out rather "as if by value", because it copies them on write.
if you modify your experiment to pass around a dict or list and modify that in the 'y', you'll see y is happily modified.
so Python passes by reference, however it either blocks updates (tuple) or copies on write (int, str, float) or updates in place (dict, list, class)
No, you won't.
x = {'a' : 1}
foo(x)
print(x)
def foo(z):
z = {'b' : 2}
I would sooner believe the example is showing you shadowing the z argument to foo, than foo being able to modify the in-parameter sometimes even if it's pass by value.
The important point is that it's not "a reference to x" that gets passed, it's a copy of x's value. x's value, like the value of all Python variables, is a reference to some object. The same thing applies to setting variables in Python in general:
x = {1:2} # x is a new variable that references some dict
y = x # y is a new variable that references the same dict
y[1] = 7 # the dict referenced by x and y was modified
x = None # x no longer references the dict
print(y) # y still references the dict, so this will print {1:7}
y = None # now neither x nor y reference that dict; since y was the last reference to it, the dict's memory will be freedHonestly, "x's value, like the value of all Python variables, is a reference to some object" makes me think it's more accurate to call Python pass-by-reference only.
What those values represent and how they can be used is a completely different topic. Take the following code:
x = "/dirs/sub/file.txt"
with open(x, "w") as file:
file.write("abc")
foo(x)
with open(x, "r") as file:
print(file.read_all()) #prints "def"
def foo(z):
with open(z, "w") as file:
file.write("def")
def foo(x):
x['a'] = 1
y = {'b': 2}
foo(y)
print(y)
so we disagree on terminology?
in my CS upbringing, sharing the memory location of a thing as parameter was tagged "call by reference". the hallmark was: you can in theory modify the referenced thing, and you just need to copy the address.
call by value, in contrast, would create an independent clone, such that the called function has no chance to modify the outside value.
now python does fancy things, as we both agree. the result of which is that primitives (int, flot, str) behave as if they were passed by value, while dict and list and its derivatives show call by reference semantics.
I get how that _technically_ sounds like call by value. and indeed there is no assignment dunder. you can't capture reassignment of a name.
but other than that a class parameter _behaves_ like call by reference.
Because it is passing a pointer by value under the hood.
This is the part that messes everyone up. Passing pointers by value is not what passing by reference used to mean.
And it matters, precisely because that is extremely realistic Python code that absolutely will mess you up if you don't understand exactly what is going on. You were passed a reference by value. If you go under the hood, you will find it is quite literally being copied and a ref count is being incremented. It's a new reference to the same stuff as the passed-in reference. But if you assign directly to the variable holding that reference, that variable will then be holding the new reference. This is base level, "I'd use it on an interview to see if you really know Python", level stuff.
Everything in a modern language involves passing things by value. Sometimes the language will gloss over it for you, but it's still a gloss. There were languages where things fundamentally, at the deepest level, were not passed by value. They're gone. Passing references by copy is not the same thing, and that Python code is precisely why it's not the same thing.
Passing a pointer-by-value similarly seems to be avoiding the point; if you tell me Python is always pass-by-value I'll expect an object I pass to a function to be a copy & not a reference, thus not be able to be mutated, and that's not the case.
That would be a misunderstanding. It would only make sense if you think Python variables are Python objects. They are not: Python variables are pointers to objects. The fact that assignment to a variable never modifies the object pointed to by that variable is a consequence of that, and doesn't apply just to passing that variable to a function.
however if you do z['b'] = 2 in foo, then you'll see the global dict bound to x has been modified, as you have stated.
well, that's _exactly_ pass by reference.
Pass-by-reference doesn't exist in Python. Here's what it looks like in C#, which does suport it:
auto x = Dictionary<string, int >() ;
x.Add("a", 1);
foo(ref x);
System.Println(x); //prints {b: 2}
void foo(ref Dictionary<string, int> z) {
auto k = new Dictionary<string, int>();
k.Add("b", 2);
z = k;
}
https://stackoverflow.com/questions/373419/whats-the-differe...