what does it mean by 'passed by assignment'?

Question

As follow is my understanding of types & parameters passing in java and python: In java, there are primitive types and non-primitive types. Former are not object, latter are objects. In python, they are all objects.

In java, arguments are passed by value because: primitive types are copied and then passed, so they are passed by value for sure. non-primitive types are passed by reference but reference(pointer) is also value, so they are also passed by value. In python, the only difference is that 'primitive types'(for example, numbers) are not copied, but simply taken as objects.

Based on official doc, arguments are passed by assignment. What does it mean by 'passed by assignment'? Is objects in java work the same way as python? What result in the difference (passed by value in java and passed by argument in python)? And is there any wrong understanding above?

Answer 1

tl;dr: You're right that Python's semantics are essentially Java's semantics, without any primitive types.

---

"Passed by assignment" is actually making a different distinction than the one you're asking about.¹ The idea is that argument passing to functions (and other callables) works exactly the same way assignment works.

Consider:

def f(x):
    pass
a = 3
b = a
f(a)

b = a means that the target b, in this case a name in the global namespace, becomes a reference to whatever value a references.

f(a) means that the target x, in this case a name in the local namespace of the frame built to execute f, becomes a reference to whatever value a references.

The semantics are identical. Whenever a value gets assigned to a target (which isn't always a simple name—e.g., think lst[0] = a or spam.eggs = a), it follows the same set of assignment rules—whether it's an assignment statement, a function call, an as clause, or a loop iteration variable, there's just one set of rules.

---

But overall, your intuitive idea that Python is like Java but with only reference types is accurate: You always "pass a reference by value".

Arguing over whether that counts as "pass by reference" or "pass by value" is pointless. Trying to come up with a new unambiguous name for it that nobody will argue about is even more pointless. Liskov invented the term "call by object" three decades ago, and if that never caught on, anything someone comes up with today isn't likely to do any better.

You understand the actual semantics, and that's what matters.

---

And yes, this means there is no copying. In Java, only primitive values are copied, and Python doesn't have primitive values, so nothing is copied.

the only difference is that 'primitive types'(for example, numbers) are not copied, but simply taken as objects

It's much better to see this as "the only difference is that there are no 'primitive types' (not even simple numbers)", just as you said at the start.

---

It's also worth asking why Python has no primitive types—or why Java does.²

Making everything "boxed" can be very slow. Adding 2 + 3 in Python means dereferencing the 2 and 3 objects, getting the native values out of them, adding them together, and wrapping the result up in a new 5 object (or looking it up in a table because you already have an existing 5 object). That's a lot more work than just adding two ints.³

While a good JIT like Hotspot—or like PyPy for Python—can often automatically do those optimizations, sometimes "often" isn't good enough. That's why Java has native types: to let you manually optimize things in those cases.

Python, instead, relies on third-party libraries like Numpy, which let you pay the boxing costs just once for a whole array, instead of once per element. Which keeps the language simpler, but at the cost of needing Numpy.⁴

---

_{1. As far as I know, "passed by assignment" appears a couple times in the FAQs, but is not actually used in the reference docs or glossary. The reference docs already lean toward intuitive over rigorous, but the FAQ, like the tutorial, goes much further in that direction. So, asking what a term in the FAQ means, beyond the intuitive idea it's trying to get across, may not be a meaningful question in the first place.}

_{2. I'm going to ignore the issue of Java's lack of operator overloading here. There's no reason they couldn't include special language rules for a handful of core classes, even if they didn't let you do the same thing with your own classes—e.g., Go does exactly that for things like range, and people rarely complain.}

_{3. … or even than looping over two arrays of 30-bit digits, which is what Python actually does. The cost of working on unlimited-size "bigints" is tiny compared to the cost of boxing, so Python just always pays that extra, barely-noticeable cost. Python 2 did, like Java, have separate fixed and bigint types, but a couple decades of experience showed that it wasn't getting any performance benefits out of the extra complexity.}

_{4. The implementation of Numpy is of course far from simple. But using it is pretty simple, and a lot more people need to use Numpy than need to write Numpy, so that turns out to be a pretty decent tradeoff.}