What is the difference between == and is in Python?

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In Python, == and is answer two different questions. Mixing them up is one of the most common sources of subtle bugs. == — equality of value. Calls the left operand’s __eq__ method; asks “do these two objects compare equal?” is — identity. Compares the memory addresses (returned by id() ); asks “are these two names pointing at the exact same object?” Example: a = [1, 2, 3] b = [1, 2, 3] c = a a == b # True — same values a is b # False — different list objects a is c # True — same object, two names When to use which: Use == for value comparisons : numbers, strings, collections, domain objects. Use is only for checking against singletons or known sentinels — most commonly None : if x is None: . Also is True , is False , and sentinel patterns like a module-level _MISSING = object() . The None check (is None vs == None) is the most practical point. Always use is for None. The integer caching gotcha (-5 to 256) is a classic interview question. Never use is for general value comparison.