Python 101: Equality vs Identity

People who are new to the Python programming language can get a bit confused about the difference between "==" (equality) and Python's keyword "is" (identity). I have even seen experienced programmers who will find the difference subtle enough that they will introduce logic errors in their code do to a misunderstanding between the two. In this article, we will look at this interesting topic.

Equality in Python

Many programming languages have the concept of equality and several use the double equals sign ("==") to designate this concept. Let;s take a look at equality in action:

>>> num = 1
>>> num_two = num
>>> num == num_two
True

Here we create a variable that we call num and assign it to the integer 1. Next we create a second variable called num_two and assign it to the value of num. Finally we ask Python if num and num_two are equal. In this case, Python tells us that this expression is True.

Another way to think of equality is that we are asking Python if two variables contain the same thing. In the example above, they both contain the integer 1. Let's see what happens when we create two lists with the same values:

>>> list_one = [1, 2, 3]
>>> list_two = [1, 2, 3]
>>> list_one == list_two
True

This worked out the way we expected.

Now let's see what happens if we ask Python about their identity:

>>> num is num_two
True
>>> list_one is list_two
False

What happened here? The first example returned True, but the second returned False! We will look into that in the next section.

Identity in Python

When you ask Python about whether one object is the same as another object, you are asking if they have the same identity. Are they actually the same object? In the case of num and num_two, the answer is yes. Python provides an easy way to prove it via it's built-in id() function:

>>> id(num)
10914368
>>> id(num_two)
10914368

The reason that these two variables share the same identity is because we told Python they should back when we assigned num to num_two (i.e. num_two = num). If you come from C or C++, you can think of the identity as a pointer where num and num_two are both pointing to the same place in memory. If you use Python's id() function on the two list objects, you will quickly see that they have different identities:

>>> id(list_one)
140401050827592
>>> id(list_two)
140401050827976

Thus when you ask Python the question "list_one is list_two", you receive False. Note that you can also ask Python if one object is not another object:

>>> list_one = [1, 2, 3]
>>> list_two = [1, 2, 3]
>>> list_one is not list_two
True

Let's take a moment to find out what happens when you mix equality and identity up.

Mixing it up

I know when I was starting out as a Python programmer, these sorts of things would lead to silly mistakes. The reason is that I would see recommended statements like this one:

if obj is None: 
    # do something
    call_function()

So I would assume naively that you could do something like this:

>>> def func():
    return [1, 2, 3]

>>> list_one = [1, 2, 3]
>>> list_two = func()
>>> list_one is list_two
False

Of course, that doesn't work as I now have two different objects with different identities. What I wanted to do here was this:

>>> list_one == list_two
True

Another issue that is tangential to this one is when you create two variables that point to the same object, but you think you can work on them independently of each other:

>>> list_one = list_two = [1, 2, 3]
>>> list_one == list_two
True
>>> list_one is list_two
True
>>> list_two.append(5)
>>> list_one
[1, 2, 3, 5]

In this example, I created two variables that both pointed at one object. Then I tried adding an element to just list_two. What a lot of beginners don't realize is that they just added that element to list_one as well. The reason for this is that both list_one and list_two are pointing at the exact same object. This is proven when we asked Python is list_one is list_two and it returned True.

Wrapping Up

Hopefully by now you understand the differences between equality (==) and identity (is) in Python. Equality is basically just asking if the contents of the two object are the same and in the case of lists, it needs to be in the same order as well. Identity in Python refers to the object you are referring to. In Python, the identity of an object is a unique, constant integer (or long integer) that exists for the length of the object's life.

Additional Reading

• Python documentation on the id() function
• Python documentation of "is"
• StackOverflow: Understanding Python's “is” operator