Python中的空元组是否为“常量"?

问题描述

我想使我的代码更高效(内存).现在，我们有很多函数都采用可迭代的参数，例如:

I want to make my code more (memory-)efficient. Right now we have a lot of functions that take an iterable as parameter like:

def foo(para,meter,iterable):
    #...
    pass

，有时我们必须为其提供一个空白列表才能正常工作:foo(14,25,[]).问题是每次构造一个新列表时:它需要在堆上分配，并且列表似乎有64字节的内存(在我自己的机器上，用sys.getsizeof([])测试)，而只有空元组需要(可能是一次)48个字节.

and sometimes we have to provide it an empty list to do its work properly: foo(14,25,[]). The problem is that each time a new list is constructed: it requires to allocate on the heap, and a list seems to 64 bytes of memory (on my own machine, tested with sys.getsizeof([])) whereas the empty tuple onlytakes a (potentially one time) 48 bytes.

因此，我想知道空元组是否为常数.由于元组是不可变的，因此可以轻松地将长度为0(因此())的元组在程序中设为常数.这样可以减少构造时间"(没有，因为它只会设置对常量的引用)，并减少了分配的内存量.

I was therefore wondering whether the empty tuple is a constant. Since tuples are immutable, one can easily make the tuple with length 0 (so ()) a constant in the program. This would decrease the "construction time" (well there is none since it only would set a reference to the constant) and reduce the amount of memory allocated.

我的问题是，关于Python解释器(即任何流行的解释器)，是否可以保证空元组确实是一个常量，使得()不需要构造时间或分配额外的内存.

My question is whether there are guarantees regarding the Python interpreter (that is any popular interpreter) that the empty tuple is indeed a constant such that () does not require construction time nor allocates additional memory.

用id(..)进行测试似乎支持这样的理论，即实际上只有一个零元组:

Testing it with id(..) seems to support the theory that there is indeed only one zero-tuple:

>>> id(())
140290183798856
>>> a = ()
>>> id(a)
140290183798856

但由于某些原因，在运行时Python解释器可能会分叉元组.

but it could be possible that at runtime the Python interpreter forks the tuple for some reason.

推荐答案

在CPython中，空元组是单例.永远只创建一个副本，然后在空生成器上使用()或tuple()时可以重复使用.

In CPython, the empty tuple is a singleton. Only one copy is created, ever, then reused whenever you use () or use tuple() on an empty generator.

PyTuple_new()函数基本上是这样做的:

The PyTuple_new() function essentially does this:

if (size == 0 && free_list[0]) {
    op = free_list[0];
    Py_INCREF(op);
    // ...
    return (PyObject *) op;
}

因此，如果元组大小为0(空)并且free_list[0]对象存在(现有的空元组单例)，则只需使用它.

So if the tuple size is 0 (empty) and free_list[0] object exists (the existing empty tuple singleton), just use that.

请参见如何在CPython中实现元组? free_list的更多详细信息； CPython还将重用长度为20的已经创建的tuple实例.

See How is tuple implemented in CPython? for more details on free_list; CPython will also re-use already-created tuple instances up to length 20.

这是实施细节.其他实现(Jython，IronPython，PyPy)不必这样做.

This is an implementation detail. Other implementations (Jython, IronPython, PyPy) do not have to do the same.

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