CODEWITHSUNDEEP
python
David542
David542

Reputation: 110592

Difference between id and equality in python

How is the id of an object computed? https://docs.python.org/3/library/functions.html#id

It seems there is a place in a class to do equality, with __eq__ but where is the is operation done, and how is the id arrived at?

Upvotes: 0

Views: 80

Answers (3)

Jörg W Mittag
Jörg W Mittag

Reputation: 369633

How is the id of an object computed? https://docs.python.org/3/library/functions.html#id

You don't know. This is what the documentation has to say:

id(object)

Return the “identity” of an object. This is an integer which is guaranteed to be unique and constant for this object during its lifetime. Two objects with non-overlapping lifetimes may have the same id() value.

That is all the guarantees that Python makes about id():

  • it returns an int
  • no two objects can have the same id() at the same time
  • no object can have two different id()s

And that is all the guarantees that Python makes. In particular, the documentation explicitly points out one guarantee that it doesn't make:

  • id()s are not globally unique in space and time, i.e. two different objects may have the same id() at different times

You will also notice that the documentation makes no guarantees about how the id() is computed.

It does, however, say this:

CPython implementation detail: This is the address of the object in memory.

This actually re-inforces the fact there are no guarantees about how id()s are computed by explicitly stating that this

  • only applies to CPython and not any other implementation such as PyPy, Jython, IronPython, GraalPython, or PythonOMR
  • is an implementation detail, meaning that it can be changed at any time, for any reason, without warning

In fact, it is actually impossible for any implementation other than CPython to implement id() this way, since all other implementations have either moving garbage collectors (PyPy, PythonOMR), or don't even have a concept of "memory address" at all (Jython, IronPython, GraalPython).

It seems there is a place in a class to do equality, with __eq__ but where is the is operation done, and how is the id arrived at?

Let me break that down into two answers:

where is the is operation done

It is defined as part of the Python Language Specification, just like, say def. It does not translate into a method call, there is no way to modify its behavior.

how is the id arrived at

You don't know, can't know, and mustn't know. All you know is that it is an identifier, the actual value is opaque.

Upvotes: 1

shx2
shx2

Reputation: 64398

You can think of id(obj) as some sort of address of the object. The way it is computed, and what the value represents, is implementation-dependent, and you should not make any assumptions about the value.

What you need to know:

  1. Object's id will not change as long as the object exists
  2. Two co-existing objects will have different ids
  3. An object may have the same id as another object which has already been deallocated (since it is gone, its address may be reused).
  4. a is b is equivalent to id(a) == id(b).

You cannot override the way id is computed, nor the way is behaves, like you'd override operators such as __eq__.

Upvotes: 2

lenik
lenik

Reputation: 23546

From the page you have provided URL for: "CPython implementation detail: This is the address of the object in memory." -- so id is basically an address, not more not less.

Upvotes: 1

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