cython的setup.py中的language_level有什么作用？ [英] What does language_level in setup.py for cython do?

问题描述

如果我在 ext_modules = cythonize（extensions，language_level = 2）中设置 language_level = 2 更改？仅仅是我编写的代码应该解释为Python2？

If I set language_level=2 in ext_modules = cythonize(extensions, language_level=2), what does that change? Is it just that the code I have written should be interpreted as Python2?

最终结果是否完全相同？

Is the end result exactly the same?

推荐答案

建立cython扩展程序分为两个步骤：

Building a cython extension is a two-step proccess:

1. 创建<$使用PythonX + cython-module的 foo.pyx 文件中的c $ c> foo.c 文件。 X 可以是2.7、3.7，也可以是您喜欢的任何版本。


2. 使用以下命令创建相应的so-file（或Windows上的pyd）编译器的帮助，包括PythonY和相应的共享库。在这里 Y 不必是 X ，但在大多数情况下是 Y 和 X 相同。

1. creating the foo.c-file from foo.pyx file using PythonX+cython-module. X could be here 2.7, 3.7 or whatever version you prefer.
2. creating the corresponding so-file (or pyd on Windows) with help of compiler and includes of PythonY and corresponding shared library. Here Y doesn't have to be X, but in most cases Y and X are the same.

生成的扩展名可以与PythonY一起使用（ X 不起作用）。

The resulting extension can be used with PythonY (it doesn't play a role what X was).

但是，仍然有问题：原始的 pyx 文件写在哪个Python版本中？如果未设置 language_level ，则当前的Cython版本将pyx文件假定为版本 2 （btw对于IPython-%% cython-magic来说不是这种情况，其中文件 foo.c 的版本被cythonized）。

However, there is still the question: In which Python-version was the original pyx-file written? If language_level is not set, current Cython-versions assume that the pyx-file was written in the version 2 (btw. this is not the case for IPython-%%cython-magic, where the version with which the file foo.c is cythonized).

这种行为将来会改变，这就是如果您使用 cython> = 0.29 进行构建时会看到有点恼人的警告的原因：

This behavior will change in the future, this is the reason you see the somewhat irritating warning, if you build with cython>=0.29:

/Main.py:367：FutureWarning：未设置Cython指令'language_level'，现在使用2（Py2）。这将在以后的版本中更改！文件：
XXXXXX.pyx

tree = Parsing.p_module（s，pxd，full_module_name）

因此您可以显式设置 language_level ，以便您的扩展名具有与Python版本无关的相同行为，

So you can explicitly set the language_level, so that your extension has the same behavior independent of the Python-version with which it was cythonized.

有关一些不同行为的示例，请参见下面的示例。

For some examples of different behavior see, the follwoing example.

使用 language_level = 3 ：

%%cython -3
print("I'm", "not a tuple")
print(5/4) 

结果

I'm not a tuple
1.25  

，但使用 language_level = 2 ：

%%cython -2
print("I'm", "not a tuple")
print(5/4) 

结果

("I'm", 'not a tuple')   # yet a tuple!
1                        # integer division in Python2!

显然，以上只是两个示例，差异更大（例如 str 和 unicode 的东西）。

Obviously the above are only two examples, there are much more differences (e.g. str & unicode stuff).

其他值得注意的区别之一是 Python3禁用 隐式相对导入，这意味着在包内部，我们不再使用隐式cimport相对导入

One of other notable differences is that Python3 disables implicit relative imports, that means inside of a package, we no longer cimport using implicit relative import

# works with language_level=2
cimport other_local_cymodule

，但使用显式相对导入

 # works with language_level=3,3str
 from . cimport other_local_cymodule

或绝对导入

 # works with language_level=3,3str
 cimport package.other_local_cymodule

---

通常，我会尽量避免混合使用不同的 language_level 和Python-interpreter-version ，因为它可能导致违反直觉的行为。

---

In general I would try to avoid mixing different language_level and Python-interpreter-version, as it can lead to counter-intuitive behavior.

例如，在下面的示例中，混合 language_level = 2 和Python3：

For example in the following example mixing language_level=2 and Python3:

%%cython -2
def divide2(int a, int b):
    return a/b

def divide3(a, b):
    return a/b

>>> divide2(2,3), divide3(2,3)
# (0, 0.66666666) 

对于函数 divide2 ，Cython可以确保Python2行为正确，但是如何执行除法取决于 int的行为 -object，具有正常的Python3行为。

For the function divide2 Cython can ensure the "right" Python2-behavior, but how the division is performed depends on the behavior of int-object, which has the normal Python3-behavior.

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