Python Slots Conflicts With Class Variable
Python Slots Conflicts With Class Variable 9,4/10 7168 reviews
Pure python equivalent of the __slots__ implementation using descriptors and a metaclass.
In CPython, when class A defines __slots__=('x','y') then A.x is a 'member_descriptor' with __get__ and __set__ methods which directly access memory allocated to each instance.
As an illustration, the above code shows a rough equivalent using pure Python. In the example, when the metaclass sees that _slots_ have been defined for 'x' and 'y', it creates two additional class variables, x=Member(0) and y=Member(1). Then, it wraps the __init__() method so that new instances get created with an initialized _slotvalues list.
In Python, if a variable is a numeric zero or empty, or a None object then it is considered as False, otherwise True. In that case, as x = 10 so it is True. In that case, as x = 10 so it is True. As x is True, so not operator evaluated as False and else part executed. Now this is something that's easy to get wrong because of the many slight differences to old-style objects in Python 2. Assuming Class is the class and instance is an instance of Class, evaluating instance.foobar roughly equates to this: Call the type slot for Class.getattribute (tpgetattro). The default does this. Analytics cookies. We use analytics cookies to understand how you use our websites so we can make them better, e.g. They're used to gather information about the pages you visit and how many clicks you need to accomplish a task.
In Python 3, this was triggering this error: builtins.ValueError: 'offset' in slots conflicts with class variable In Python 2, if you defined slots and after that defined a property with the same name the slots descriptor was replaced silently. In Python 3 this isn't allowed anymore. I've got conflicts between Python's typing system and slots. Here is a small reproducible example. From typing import TypeVar, Generic, Sequence T = TypeVar('T') class TestGeneric(Sequence.
To make it obvious that the illustrative code is running, _slots_ is spelled with single underscores (to differentiate it from the CPython version which is spelled with double underscores).
The CPython version differs in that:
Instead of a _slotvalues pointer to an external list, CPython allocatesmemory directly inside each instance. Accordingly, the member descriptoraccesses that memory directly instead of using a list lookup.
Whenever __slots__ are present, the __new__ method prevents an instancedictionary from being created. That makes __slots__ useful for creatingvery lightweight instances.
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