الدرس 28 · Under the hood

Names, objects and references

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In Python a variable is not a box that holds a value. It is a name in a namespace that refers to an object on the heap, and an object can have many names. Step through a seven-line program, refs.py, and watch the arrows: two names share one list, a change through one name shows through the other, == and is give different answers, and CPython frees the list the moment its last reference goes away.

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01After a = [1, 2] and b = a and b.append(3), what does print(a) show?

b = a binds b to the same list object; nothing is copied. append changes that one object in place, so every name that refers to it sees [1, 2, 3].

02x = [1, 2, 3] and y = [1, 2, 3]. What are x == y and x is y?

Each list display builds a new object. The two lists have equal values, so == is True, but they are two different objects, so is is False.

03a and b refer to the same list. What does del a do?

del removes a name's binding from the namespace, not the object. The list's reference count drops by one, but b still refers to it, so it stays alive.

04In CPython, b holds the only reference to a list. Then the program runs b = None. When is the list freed?

CPython counts references. Rebinding b to None drops the list's count to 0, and CPython frees it straight away. The gc module is only needed for objects that refer to each other in a cycle.

05b = a, where a refers to a list. Which line moves only b to a new object and leaves a's list unchanged?

b + [4] builds a new list and = rebinds b to it. append, extend and += on a list all change the shared object in place, so a would see the 4 too.

المصطلحات الأساسية

المسرد

Object: A piece of data in memory with an identity, a type and a value. Every value in Python is an object.
Name: A label in a namespace that refers to one object. Assignment binds a name; it never copies the object.
Namespace: A table that maps names to objects. A module's global names live in a dict you can see with globals().
Reference: A pointer from a name (or a list slot, or an argument) to an object. Many references can point to one object.
Alias: One of two or more names that refer to the same object. A change made through one shows through the others.
Mutable: Able to change in place, like a list or a dict. Ints, strings and tuples are immutable: an operation makes a new object instead.
Identity, id(), is: Which object something is. id() gives a number unique to the object while it lives; is checks whether two references are the same object.
Reference count: CPython's count of the references to an object. When it falls to 0 the object is freed at once.

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الملاحظات والمصادر

The counts in the figure count only this program's names. CPython's real reference count also includes every other reference, such as a slot in a list or a function's argument; sys.getrefcount() shows it, usually one higher than you expect because the call itself holds a reference.
Freeing an object the moment its count reaches zero is a CPython implementation detail. Other implementations, such as PyPy, may free it later. Objects that refer to each other in a cycle never reach zero on their own; CPython's cycle collector (the gc module) finds them.
None is one object shared by the whole program. Since Python 3.12 it is immortal (PEP 683): its reference count never changes, so the figure shows no count for it.
id() is unique and constant for an object while it lives; in CPython it is the object's address in memory. After an object is freed, a new object may get the same id.
For immutable values such as small ints and short strings, CPython may reuse one object, so is can give surprising answers. Compare values with ==, and keep is for identity, as in x is None.
On macOS and Linux the command is often python3 rather than python; on Windows the py launcher also works. The $ is the terminal's own prompt.

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