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I've been programming in Python for a long time, but I still can't understand why classes base their attribute lookup on the __dict__ dictionary by default instead of the faster __slots__ tuple.

Wouldn't it make more sense to use the more efficient and less flexible __slots__ method as the default implementation and instead make the more flexible, but slower __dict__ method optional?

Also, if a class uses __slots__ to store its attributes, there's no chance of mistakenly creating new attributes like this:

class Object:
    __slots__ = ("name",)
    def __init__(self, name):
        self.name = name

obj = Object()
# Note the typo here
obj.namr = "Karen"

So, I was wondering if there's a valid reason why Python defaults to accessing instance attributes through __dict__ instead of through __slots__.

Nicholas Obert
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    Does this answer your question? [Are there disadvantages of using \_\_slots\_\_?](https://stackoverflow.com/questions/55767040/are-there-disadvantages-of-using-slots) – VPfB Aug 07 '22 at 07:58
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    Just historically speaking, `__slots__` was an optimization added *later*. Perhaps this might have made sense to do from the beginning - it seems to be a matter of opinion to me - but in any case, that is simply not how the language was designed – juanpa.arrivillaga Aug 07 '22 at 09:28

1 Answers1

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Python is designed to be an extremely flexible language, and allows objects to modify themselves in many interesting ways at runtime. Making a change to prevent that kind of flexibility would break a massive amount of other people's code, so for the sake of backwards compatibility I don't think it will happen any time soon (if at all).

As well as this, due to the way Python code is interpreted, it is very difficult to design a system that can look ahead and determine exactly what variables a particular class will use ahead of time, especially given the existence of setattr() and other similar functions, which can modify the state of other objects in unpredictable ways.

In summary, Python is designed to value flexibility over performance, and as such, having __slots__ be an optional technique to speed up parts of your code is a trade-off that you choose to make if you wish to write your code in Python. I can't answer whether this is a worthwhile design decision for you, since it's entirely based on opinion.

If you wish to have a bit more safety to prevent issues such as the one you described, there are tools such as mypy and pylint which can catch that sort of error.

Miguel Guthridge
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