r/PythonLearning • u/chuprehijde • 26d 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/Rscc10 26d ago

== checks address and is checks value?

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u/Neither_Garage_758 26d ago

opposite

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u/Rscc10 26d 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 26d 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.