Understanding Python's "is" operator

Question

You misunderstood what the is operator tests. It tests if two variables point the same object, not if two variables have the same value.

From the documentation for the is operator:

The operators is and is not test for object identity: x is y is true if and only if x and y are the same object.

Use the == operator instead:

print(x == y)

This prints True. x and y are two separate lists:

x[0] = 4
print(y)  # prints [1, 2, 3]
print(x == y)   # prints False

If you use the id() function you'll see that x and y have different identifiers:

>>> id(x)
4401064560
>>> id(y)
4401098192

but if you were to assign y to x then both point to the same object:

>>> x = y
>>> id(x)
4401064560
>>> id(y)
4401064560
>>> x is y
True

and is shows both are the same object, it returns True.

Remember that in Python, names are just labels referencing values; you can have multiple names point to the same object. is tells you if two names point to one and the same object. == tells you if two names refer to objects that have the same value.

Another duplicate was asking why two equal strings are generally not identical, which isn't really answered here:

>>> x = 'a' 
>>> x += 'bc'
>>> y = 'abc'
>>> x == y
True
>>> x is y
False

So, why aren't they the same string? Especially given this:

>>> z = 'abc'
>>> w = 'abc'
>>> z is w
True

---

Let's put off the second part for a bit. How could the first one be true?

The interpreter would have to have an "interning table", a table mapping string values to string objects, so every time you try to create a new string with the contents 'abc', you get back the same object. Wikipedia has a more detailed discussion on how interning works.

And Python has a string interning table; you can manually intern strings with the sys.intern method.

In fact, Python is allowed to automatically intern any immutable types, but not required to do so. Different implementations will intern different values.

CPython (the implementation you're using if you don't know which implementation you're using) auto-interns small integers and some special singletons like False, but not strings (or large integers, or small tuples, or anything else). You can see this pretty easily:

>>> a = 0
>>> a += 1
>>> b = 1
>>> a is b
True
>>> a = False
>>> a = not a
>>> b = True
a is b
True
>>> a = 1000
>>> a += 1
>>> b = 1001
>>> a is b
False

---

OK, but why were z and w identical?

That's not the interpreter automatically interning, that's the compiler folding values.

If the same compile-time string appears twice in the same module (what exactly this means is hard to define—it's not the same thing as a string literal, because r'abc', 'abc', and 'a' 'b' 'c' are all different literals but the same string—but easy to understand intuitively), the compiler will only create one instance of the string, with two references.

In fact, the compiler can go even further: 'ab' + 'c' can be converted to 'abc' by the optimizer, in which case it can be folded together with an 'abc' constant in the same module.

Again, this is something Python is allowed but not required to do. But in this case, CPython always folds small strings (and also, e.g., small tuples). (Although the interactive interpreter's statement-by-statement compiler doesn't run the same optimization as the module-at-a-time compiler, so you won't see exactly the same results interactively.)

---

So, what should you do about this as a programmer?

Well… nothing. You almost never have any reason to care if two immutable values are identical. If you want to know when you can use a is b instead of a == b, you're asking the wrong question. Just always use a == b except in two cases:

• For more readable comparisons to the singleton values like x is None.
• For mutable values, when you need to know whether mutating x will affect the y.

is only returns true if they're actually the same object. If they were the same, a change to one would also show up in the other. Here's an example of the difference.

>>> x = [1, 2, 3]
>>> y = [1, 2, 3]
>>> print x is y
False
>>> z = y
>>> print y is z
True
>>> print x is z
False
>>> y[0] = 5
>>> print z
[5, 2, 3]

Prompted by a duplicate question, this analogy might work:

# - Darling, I want some pudding!
# - There is some in the fridge.

pudding_to_eat = fridge_pudding
pudding_to_eat is fridge_pudding
# => True

# - Honey, what's with all the dirty dishes?
# - I wanted to eat pudding so I made some. Sorry about the mess, Darling.
# - But there was already some in the fridge.

pudding_to_eat = make_pudding(ingredients)
pudding_to_eat is fridge_pudding
# => False