Cython“不允许在常量表达式中使用”，boundscheck False不起作用 [英] Cython "Not allowed in a constant expression", boundscheck False doesn't work

问题描述

我对Cython比较陌生，遇到一个错误，我的研究使我失败了（我在spyder中使用Python3，而我的Sython版本是0.26）

I am relatively new to Cython and have encountered an error that my research has failed me on (I am using Python3 in spyder and my Sython version is 0.26)

我试过了：

import cython

@cython.boundscheck(False)
def boundtest():
     cdef int r=4
     cdef double l[3]

，效果很好。但是后来我尝试了这个：

and it works fine. But then I tried this:

import cython

@cython.boundscheck(False)
def boundtest():
     cdef int r=4
     cdef double l[r]

并且收到错误

[1/1] Cythonizing test.pyx

Error compiling Cython file:
------------------------------------------------------------
...
import cython

@cython.boundscheck(False)
def boundtest():
     cdef int r=4
     cdef double l[r]
                   ^
------------------------------------------------------------

test.pyx:13:20: Not allowed in a constant expression

添加修饰符找到与此相关的stackexchange帖子并阅读Cython由Kurt W. Smith所著。据我所知，这应该告诉Cython不要担心动态索引变量可能导致的超出范围的错误，但出于某种原因却不会。我也尝试过在编译器选项和全局范围内更改boundscheck，但无济于事。

The decorator was added due to finding this related stackexchange post and reading the Cython book by Kurt W. Smith. As far as I can tell this should work to tell Cython not to worry about out of bounds errors that may result from having a dynamic indexing variable but for some reason it does not. I have also tried changing boundscheck in the compiler options and globally to no avail.

如果不是Cython文档声称是最新的，我认为boundscheck已被贬值。

If it weren't for the Cython documentation claiming to be up to date I would think boundscheck has been depreciated.

我意识到我已经使用 import Cython 代替了 cimport cython 。我再次尝试使用

I realized I have used import Cython instead of cimport cython. I tried again with

cimport cython

但是会得到相同的错误。

But get the same error.

类似注意代码

cdef int N = 3
cdef double[:] lout = array.array('d', N)

抛出错误

TypeError: 'int' object is not iterable

我认为与C无法处理（可能）动态数组分配的原因相同。相反，我们必须使用

I assume for the same reason that C cannot deal with (possibly) dynamic array allocation. Instead we must use

cdef double[:] lout = numpy.empty(N, 'd')

我假设有一行在将N放入C数组之前将N转换为静态类型

and I assume there is a line that converts N into a static type somewhere before putting it into the C-array

推荐答案

故障与 cython.boundscheck 无关。

边界检查只是检查您是否尝试访问数组中不存在的元素。例如，如果您有一个大小为4的数组，并尝试访问元素5-使用 boundscheck（True），它将给您一个异常，使用 boundscheck（错）会导致未定义的行为（可能导致分段错误）。

Boundchecking is just checking if you try to access an element of an array that isn't there. For example if you have a size 4 array and try to access element 5 - with boundscheck(True) it will give you an Exception, with boundscheck(False) it will result in undefined behavior (probably causing a segmentation fault).

编译失败的原因是另一个：用动态长度创建一个静态数组！在编译时需要知道元素的数量，这只是 c 强制执行（我想）。

The reason for the compilation failure is another: You can't create a static array with a dynamic length! The number of elements need to be known at compile time, that's just something c enforces (I guess).

不过，您可以定义 r 以便在编译时间：

However you could define r to be known at compile time:

DEF r=4

cimport cython

@cython.boundscheck(False)
def boundtest():
    cdef double l[r]

不过，您可以简单地创建一个NumPy数组并将其存储在memoryview变量中：

You could however simply create a NumPy array and store it in a memoryview variable:

cimport cython
import numpy as np

@cython.boundscheck(False)
def boundtest():
    cdef int r=4
    cdef double[:] l = np.empty(r, dtype=np.double)

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