Python None Comparison: When Should I Use 'is' vs. '=='?

Python None Comparison: When to Use "is" vs. ==

When working with the None value in Python, developers often use both "is" and "==" for comparison. Though both approaches are syntactically valid, the recommended choice depends on the intended purpose of the comparison.

Generally, "is" is preferred when checking for object identity, meaning whether two variables refer to the exact same object in memory. In contrast, "==" checks for equality, which can vary based on the object's implementation.

Consider the following custom class that overrides the eq method to define equivalence differently from object identity:

class Negator(object):
    def __eq__(self,other):
        return not other

thing = Negator()
print(thing == None)    # True
print(thing is None)    # False

In this example, "==" returns True because Negator's eq method overrides the default behavior. However, "is" returns False because the two variables do not refer to the same object.

When dealing with custom classes where object identity is not critical, "==" is the appropriate choice for checking equality. For example, if you want to compare the contents of two lists, you would use "==":

lst = [1,2,3]
lst == lst[:]  # This is True since the lists are "equivalent"
lst is lst[:]  # This is False since they're actually different objects

In contrast, if you want to specifically check if two variables point to the exact same object in memory, "is" is the preferred comparison operator. This is particularly important when working with mutable objects, such as dictionaries or lists, where changing one instance may affect the other if they are the same object:

a = [1, 2, 3]
b = a
b.append(4)
print(a, b)  # Output: [1, 2, 3, 4], [1, 2, 3, 4]

c = [1, 2, 3]
d = c
d is c  # True (same object in memory)

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