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What is "is" in Python?

Updated: Sep 23, 2019

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 (object location in memory is same). Object identity is determined using the id() function. x is not y yields the inverse truth value.

It’s easy to understand if I compare this operator with equality operator.

  • is and is not compares the object reference. Check for identity.

  • == and != compares the object value. Check for equality.

For example, if you consider integer objects (excluding integers from -5 to 256),

>>> A=9999 
>>> B=9999 
>>> A == B, A is B
(True, False
>>> A, B
(9999, 9999) 
>>> id(A), id(B)
(4452685328, 4452686992)

Python stores integer objects as single object between range -5 to 256 so the identity is same.

>>> A=99
>>> B=99 
>>> A == B, A is B
(True, True
>>> A, B
(99, 99) 
>>> id(A), id(B)
(4404392064, 4404392064)

Lets see behavior of other immutable objects like int & float, string, tuples and boolean:

>>> 1 == 1.0, 1 is 1.0
(True, False
>>> 1 == 1, 1 is 1
(True, True
>>> 'A' == 'A', 'A' is 'A'
(True, True
>>> (1,2) == (1,2), (1,2) is (1,2)
(True, True
>>> True == True, True is True
(True, True)

What about mutable objects - list, set, dict? Behavior is totally different.

>>> [1,2] == [1,2], [1,2] is [1,2]
(True, False
>>> {1,2} == {1,2}, {1,2} is {1,2}
(True, False
>>> {'k1':1} == {'k1':1}, {'k1':1} is {'k1':1}
(True, False)

is operator is used only when you want to compare the object identity, for regular comparison equality operator is used.


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