Citation? This doesn't appear to be true: https://stackoverflow.com/questions/14118564/how-does-slots-avoid-a-dictionary-lookup https://stackoverflow.com/ques

Citation? This doesn't appear to be true: https://stackoverflow.com/questions/14118564/how-does-slots-avoid-a-dictionary-lookup https://stackoverflow.com/questions/14118564/how-does-slots-... The goal of using __slots__ is to save memory; instead of using a .__dict__ mapping on the instance, the class has descriptors objects for each and every attribute So python still has to look at the class for each attribute access on an instance of Foo (to find the descriptor). Any unknown attribute (say, Foo.ham) will still result in Python looking through the class MRO to search for that attribute, and that includes dictionary searches. If I have time I'll write an example program to test this. Since I've been curious for awhile. I've actually forked the Python interpreter to make my highly compatible bash shell faster and smaller, while not completely rewriting it [1]. ----- Even if __slots__ did what you think it does, it doesn't completely solve the problem. A lookup like "return os.path.exists(foo)" can't use slots, because it's a module and not a class. In principle, local variable lookups are also slower in Python than say Dalvik (a JVM without a JIT). LOAD_FAST still involves a dict lookup (as opposed to the even slower LOAD_ATTR). It just happens at the beginning of the function once, not on say every iteration of a loop inside the function. In contrast, a Java compiler will resolve the attribute access at compile time. PEP 509 might help with avoiding repeated dictionary lookups in this case, but it's very new: https://www.python.org/dev/peps/pep-0509/ https://www.python.org/dev/peps/pep-0509/ Also, if slots did what you think it does, it would be 10x faster than normal attribute lookup, but it's not (according to the benchmark above, and at least one other one I saw.) [1] http://www.oilshell.org/blog/2017/04/25.html http://www.oilshell.org/blog/2017/04/25.html EDIT: My last two points might seem like a tangent, since they're not really related to structs. But my overall point is that Python doesn't do very much at compile time, so some of that work has to be done at runtime. slots could easily be faster than it is; it's mainly a memory optimization.