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points by hermitdev 2 days ago | 0 comments

> Python is harder to JIT because of the way __dict__ works.

Heh, how about how `__class__` works? Not that I recommend actually doing this, but you can literally change the type of an already instantiated object by reassigning `__class__`:

    >>> class Dog:
    ...     def hi(self):
    ...             print("woof")
    ...
    >>> class Cat:
    ...     def hi(self):
    ...             print("meow")
    ...
    >>> dog = Dog()
    >>> dog.hi()
    woof
    >>> dog.__class__ = Cat
    >>> dog.hi()
    meow
    >>> isinstance(dog, Dog)
    False
    >>> isinstance(dog, Cat)
    True
    >>>

[–] samatman 2 days ago

Lua has the same property with metatables. LuaJIT, which is very fast, has a solution: if you change a metatable after assigning it, or reassign a metatable, Mike Pall turns his back on you. Your code dies slowly, in the outer darkness of the interpreter loop, never to be JIT’ted upon again.

[–] sakesun 2 days ago

I've been doing things in Python for over 20 years. Only recently I learn that I can do this.

    >>> Dog.__bases__ = (Cat,)
    >>> isinstance(dog, Dog)
    True
    >>> isinstance(dog, Cat)
    True
    >>>

[–] pjmlp 1 day ago

Smalltalk, Self, Common Lisp, Ruby as well, no big deal in regards to JIT code.

[–] gregdeon 2 days ago

This is wild. Is it part of Python's specification, or an implementation detail of CPython?

[–] sakesun 2 days ago

As far as I understand, CPython implementation is THE specification. Some billions dollar ERP system firmly rely on this behavior, btw.

[–] gregdeon 1 day ago

Yeah, I was afraid you would say that.

[–] sakesun 1 day ago

Just check with pypy and Graalpy. Both support this as well.

[–] dcrazy 1 day ago

Is the question whether multiple base classes are part of Python’s spec, or if the ability to assign to `__class__` is part of the spec?

[–] esprehn 2 days ago

That's also possible in JS with __proto__. The prototypes of the classes in the chain can be mutated at runtime too. How to handle these cases is well understood for JITs though and python could do the same.