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Python (and Java) do not pass anything by reference. They pass by pointer-value. (If they passed by reference, you could change to which value a caller's variab

Python (and Java) do not pass anything by reference. They pass by pointer-value. (If they passed by reference, you could change to which value a caller's variable was bound, like you can in C++).

alexhutcheson 8y ago
> (If they passed by reference, you could change to which value a caller's variable was bound, like you can in C++) I might be misunderstanding you, but I don't think you can do this in C++. References can't be changed to point to a different object after initialization. If you have code like: void MyFunc(Foo& ref_param) { Foo new_foo; ref_param = new_foo; } The assignment above isn't "changing the value to which a caller's variable is bound". Instead, it's running the '=' operator on the Foo object to copy the state from new_foo to ref_param. To demonstrate this, you could run the following to see that the addresses are the same: Foo original_object; Foo& object_ref = original_object; MyFunc(object_ref); // object_ref still points to the same address assert(&original_object == &object_ref); Under the hood, C++ reference params are basically syntactic sugar for passing by pointer-value, so this behavior isn't surprising.
- colanderman 8y ago
  The proper analogy for a Python object is `std::shared_ptr<Foo>`: in Python, all "objects" are actually reference-counted pointers to objects (thus making them shareable). And then yes, in C++, you can change to which object the caller's "object" (pointer-to-object) points to, if you pass a reference to that. This is not possible in Python: all you can do is pass that shared-pointer value around, not the value of the object itself, nor a reference to the shared pointer. This is a well-known distinction that goes by several names, see e.g. https://en.m.wikipedia.org/wiki/Evaluation_strategy#Call_by_sharing https://en.m.wikipedia.org/wiki/Evaluation_strategy#Call_by_...
  - alexhutcheson 8y ago
    Ah, got it. Your basic point is that C++ params can use two layers of indirection (pointer-to-ref, ref-to-pointer, pointer-to-pointer, etc.), while doing so in Python is clunky. Makes sense.
lmm 8y ago
Those languages use references rather than pointers (that is, their referring-things do not support arithmetic), so any name for what they do that involves "pointer" is a poor one. The distinction between passing the value of a reference being used by the caller and passing a reference to the caller's stack is so rarely important or useful that there's no consensus terminology for it. The distinction that's actually relevant and useful is whether, when we write f(a), f receives the (thing we would call the) value of a or a reference to the value of a (and in particular whether f can change the value of a). In Python, f can change the value of a (that is to say, the thing Python programmers understand to be the value of a); therefore Python is pass by reference as the term is usually understood (and certainly any term for its call style that uses "value" is more misleading than calling it "pass by reference").
- jerf 8y ago
  "The distinction between passing the value of a reference being used by the caller and passing a reference to the caller's stack is so rarely important or useful that there's no consensus terminology for it." And the reason for that is essentially all modern languages make copies of something when passing argument parameters. The only question is what they are passing and exactly how hard the language layer works to make it look as if you are or are not passing something by reference. I have to reach to something as obscure as Forth to find a language that does not work that way, and truly does not copy parameters into a function. (I haven't dug into Factor enough to know if under the hood it still copies parameters.) It's a very unusual choice to not copy something for a function call. (This is one of the ways our CPUs have been optimized for the languages we run; they make this intuitively expensive operation otherwise cheaper than it would be on a naively designed CPU.)
  - com2kid 8y ago
    > And the reason for that is essentially all modern languages make copies of something when passing argument parameters. Then of course someone turns on whole program optimization for C++, half the code gets inlined into one terrifying large function, nothing is copied anywhere, and the code runs 2x faster. Then some poor SOB has to debug the assembly code for all of this and has nightmares for years after. Not like I'd know or anything. :)`
pdonis 8y ago
> They pass by pointer-value. No, Python doesn't even do that, because Python variables aren't names for storage locations to begin with. They're namespace bindings. Passing a variable to a Python function means transferring a namespace binding from the caller's namespace to the function's local namespace.
- arduinomancer 8y ago
  What is a namespace binding? Can you explain at a low level what that means?
  - pdonis 8y ago
    Compare a variable assignment in C and Python. In C: int i = 17; Means "set aside one int's worth of storage and fill it with the bit pattern for the number 17". In Python: i = 17 means "create an int object with the value 17 and bind it in the current namespace dictionary to the name i".
- colanderman 8y ago
  The semantics are identical.
  - pdonis 8y ago
    If you think this, I'm confused about what you mean by "semantics".

Python (and Java) do not pass anything by reference. They pass by pointer-value. (If they passed by reference, you could change to which value a caller's variable was bound, like you can in C++).

alexhutcheson 8y ago

> (If they passed by reference, you could change to which value a caller's variable was bound, like you can in C++) I might be misunderstanding you, but I don't think you can do this in C++. References can't be changed to point to a different object after initialization. If you have code like: void MyFunc(Foo& ref_param) { Foo new_foo; ref_param = new_foo; } The assignment above isn't "changing the value to which a caller's variable is bound". Instead, it's running the '=' operator on the Foo object to copy the state from new_foo to ref_param. To demonstrate this, you could run the following to see that the addresses are the same: Foo original_object; Foo& object_ref = original_object; MyFunc(object_ref); // object_ref still points to the same address assert(&original_object == &object_ref); Under the hood, C++ reference params are basically syntactic sugar for passing by pointer-value, so this behavior isn't surprising.

colanderman 8y ago
The proper analogy for a Python object is `std::shared_ptr<Foo>`: in Python, all "objects" are actually reference-counted pointers to objects (thus making them shareable). And then yes, in C++, you can change to which object the caller's "object" (pointer-to-object) points to, if you pass a reference to that. This is not possible in Python: all you can do is pass that shared-pointer value around, not the value of the object itself, nor a reference to the shared pointer. This is a well-known distinction that goes by several names, see e.g. https://en.m.wikipedia.org/wiki/Evaluation_strategy#Call_by_sharing https://en.m.wikipedia.org/wiki/Evaluation_strategy#Call_by_...
- alexhutcheson 8y ago
  Ah, got it. Your basic point is that C++ params can use two layers of indirection (pointer-to-ref, ref-to-pointer, pointer-to-pointer, etc.), while doing so in Python is clunky. Makes sense.

lmm 8y ago

Those languages use references rather than pointers (that is, their referring-things do not support arithmetic), so any name for what they do that involves "pointer" is a poor one. The distinction between passing the value of a reference being used by the caller and passing a reference to the caller's stack is so rarely important or useful that there's no consensus terminology for it. The distinction that's actually relevant and useful is whether, when we write f(a), f receives the (thing we would call the) value of a or a reference to the value of a (and in particular whether f can change the value of a). In Python, f can change the value of a (that is to say, the thing Python programmers understand to be the value of a); therefore Python is pass by reference as the term is usually understood (and certainly any term for its call style that uses "value" is more misleading than calling it "pass by reference").

jerf 8y ago
"The distinction between passing the value of a reference being used by the caller and passing a reference to the caller's stack is so rarely important or useful that there's no consensus terminology for it." And the reason for that is essentially all modern languages make copies of something when passing argument parameters. The only question is what they are passing and exactly how hard the language layer works to make it look as if you are or are not passing something by reference. I have to reach to something as obscure as Forth to find a language that does not work that way, and truly does not copy parameters into a function. (I haven't dug into Factor enough to know if under the hood it still copies parameters.) It's a very unusual choice to not copy something for a function call. (This is one of the ways our CPUs have been optimized for the languages we run; they make this intuitively expensive operation otherwise cheaper than it would be on a naively designed CPU.)
- com2kid 8y ago
  > And the reason for that is essentially all modern languages make copies of something when passing argument parameters. Then of course someone turns on whole program optimization for C++, half the code gets inlined into one terrifying large function, nothing is copied anywhere, and the code runs 2x faster. Then some poor SOB has to debug the assembly code for all of this and has nightmares for years after. Not like I'd know or anything. :)`

pdonis 8y ago

> They pass by pointer-value. No, Python doesn't even do that, because Python variables aren't names for storage locations to begin with. They're namespace bindings. Passing a variable to a Python function means transferring a namespace binding from the caller's namespace to the function's local namespace.

arduinomancer 8y ago
What is a namespace binding? Can you explain at a low level what that means?
- pdonis 8y ago
  Compare a variable assignment in C and Python. In C: int i = 17; Means "set aside one int's worth of storage and fill it with the bit pattern for the number 17". In Python: i = 17 means "create an int object with the value 17 and bind it in the current namespace dictionary to the name i".
colanderman 8y ago
The semantics are identical.
- pdonis 8y ago
  If you think this, I'm confused about what you mean by "semantics".