Python: Difference between is and equals (==)

Python: Difference between is and equals (==)

== operator checks the values behind the two names. It returns True if the two values are equal, False otherwise.

is operator checks the objects behind the two names. It returns True if both the names refer to the same object, False otherwise.

There’s a difference in meaning between equal and identical. And this difference is important when you want to understand how Python’s is and == comparison operators behave.

The == operator compares by checking for equality.

The is operator, however, compares identities.

Example1:

First, we’ll create a new list object and name it a, and then define another variable b that points to the same list object:

a = [1, 2, 3]

b = a

print(a)

print(b)

Let’s inspect these two variables. We can see they point to identical looking lists:

Output:

[1, 2, 3]

[1, 2, 3]

Because the two list objects look the same we’ll get the expected result when we compare them for equality using the == operator:

>>> a == b

True

Note: They both have same ids also.

>>> a = [1, 2, 3]

>>> b = a

>>> id(a)

4319913352

>>> id(b)

4319913352

>>> 

>>> 

However, that doesn’t tell us whether a and b are actually pointing to the same object (We can check with id() function).

Of course, we know they do because we assigned them earlier, but suppose we didn’t know—how might we find out?

The answer is comparing both variables with the is operator. This confirms both variables are in fact pointing to one list object:

>>> a is b

True

Let’s see what happens when we create an identical copy of our list object. We can do that by calling list() on the existing list to create a copy we’ll name c:

c = list(a)

Again you’ll see that the new list we just created looks identical to the list object pointed to by a and b:

>>> c

[1, 2, 3]

Now this is where it gets interesting—let’s compare our list copy c with the initial list a using the == operator. What answer do you expect to see?

>>> a == c

True

Okay, I hope this was what you expected. What this result tells us is that c and a have the same contents.

They’re considered equal by Python. But are they actually pointing to the same object? Let’s find out with the is operator:

>>> a is c

False

>>> id(a)

4319913352

>>> 

>>> id(c)

4319937160

>>> 

This is where we get a different result. Python is telling us that c and a are pointing to two different objects, even though their contents might be the same.

So, to recap let’s try and break the difference between is and == down to two short definitions:

• An is expression evaluates to True if two variables point to the same (identical) object.
• An == expression evaluates to True if the objects referred to by the variables are equal (have the same contents).

Example2:

>>> a = b = [1,2,3]

>>> c = [1,2,3]

>>> a == b

True

>>> a == c

True

>>> a is b

True

>>> a is c

False

>>> a = [1,2,3]

>>> b = [1,2]

>>> a == b

False

>>> a is b

False

>>> del a[2]

>>> a == b

True

>>> a is b

False

NOTE: Avoid using is operator for immutable types such as strings and numbers, the result is unpredictable.