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YusufAdel
YusufAdel

Posted on

Object Comparisons: β€œis” vs β€œ==”

The == operator compares by checking for equality: if these Python objects we compared them with the == operator, we’d get both objects are equal as an answer.

The is operator, however, compares identities: if we compared our
objects with the is operator, we’d get these are two different objects as an answer.

let’s take a look at some real Python code.

>>> a = [1, 2, 3]
>>> b = a
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Let’s inspect these two variables. We can see that they point to
identical-looking lists:

>>> a
[1, 2, 3]
>>> b
[1, 2, 3]
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Because the two list objects look the same, we’ll get the expected result
when we compare them for equality by using the == operator:

>>> a == b
True
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However, that doesn’t tell us whether a and b are actually pointing to
the same object. Of course, we know they are because we assigned
them earlier, but suppose we didn’t knowβ€”how might we find out?

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

>>> a is b
True
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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)
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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]
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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
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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
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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.

Conclusion:

  • 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).

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