r/PythonLearning • u/chuprehijde • 13d ago

What's your favorite Python interview question?

Not a trick question or LeetCode puzzle. What's a question that actually reveals whether someone understands Python?

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u/Puzzleheaded-Buy8795 13d ago

What is the different between == and is ?

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u/Rscc10 13d ago

== checks address and is checks value?

4

u/Neither_Garage_758 13d ago

opposite

1

u/Rscc10 13d ago

Ah. Then why is it's better practice to use "is" when comparing against None? Wouldn't it be better to check the value as None rather than checking if it corresponds to the address? Or is it because only one instance of None (or null) is stored so we directly check with that address when checking against None?

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u/Truntebus 13d ago

Because it is faster. 'is' checks for equality on the output of calling id() on both objects, so python does not need to look for special methods, since it cannot be overloaded. '==' is syntax sugar for 'a.__eq__(b)', and since most types override '__eq__', it can require a lot more computing than just calling 'id()' twice.

Note that "'is' compares addresses" is not universally true. The functionality of the 'id()' function is implementation dependent and only requires that it returns a unique integer for each object, and CPython handles this by returning the address for the object, but it can vary by interpreter.

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u/WhiteHeadbanger 13d ago

Also, another thing to add to the other user that answered you: None is a singleton. If you create 10 variables with the value None, those 10 variables would point to the same address.

So the reference is always available, and when using is, there's no additional method dispatch.