Python 函数变量类型注释会导致用 Cython 编译后执行与直接执行结果不一致

[qiwihui]

最近工作中遇到关于函数类型注释引起的错误，特此记录一下。

起因是公司的项目为了安全和执行速度，在发布时会使用 Cython 转为 C 语言并编译成动态连接库进行调用，但是有个函数在 Python 执行时正常，但是在动态连接库中却执行错误。

错误复现

测试用例 test.py：

def ip_str(ips: str):
    ips = [x for x in ips]

def ip(ips):
    ips = [x for x in ips]

编译 compile.py：

from setuptools import setup
from Cython.Build import cythonize

extensions = ["test.py",]

setup(name='test',
      ext_modules=cythonize(extensions)
)

运行编译：

python compile.py build

编译之后进入对应 so 文件目录 build/lib-{平台架构}，运行对比：

>>> import test
>>> test.ip("123")
>>> test.ip_str("123")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "test.py", line 5, in test.ip_str
TypeError: Expected str, got list

经过调查发现，当函数变量做了类型注释时，不能重新赋值为其他类型，否则会在 Cython 编译后执行时报错。

Cython 可以在编译时推断出部分简单的错误，比如：

def ip1():
    ips: str = '1'
    ips = ['1']

Error compiling Cython file:
------------------------------------------------------------
...

def ip1():
    ips: str = '1'
    ips = ['1']
         ^
------------------------------------------------------------

test.py:10:10: Cannot coerce list to type 'str object'

但如果代码比较复杂，则只能在运行时才会出错。所以上述错误只能在执行的时候才被抛出。

原因

Cython 将 Python 转为 C 代码比较后类型注释与否代码比较：

• 没有类型注释：

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   * 
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   *     ips = [x for x in ips]
   * 
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• 有类型注释：

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   * 
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   *     ips = [x for x in ips]
   * 
   */
  
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   * 
   * def ip(ips):             # <<<<<<<<<<<<<<
   *     ips = [x for x in ips]
   */
  
  /* Python wrapper */
  static PyObject *__pyx_pw_4test_3ip(PyObject *__pyx_self, PyObject *__pyx_v_ips); /*proto*/
  static PyMethodDef __pyx_mdef_4test_3ip = {"ip", (PyCFunction)__pyx_pw_4test_3ip, METH_O, 0};
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主要区别在于，类型注释增加了变量检测 __Pyx_ArgTypeTest ，以及之后赋值 ips 时的类型检测 PyString_CheckExact；没有变量类型注释则进行了变量推测，判断是否为List（ PyList_CheckExact）或者Tuple （ PyTuple_CheckExact），还是可迭代类型。

结论

1. 类型注释在编译后会简化处理流程；
2. 类型注释的变量不能赋值为其他类型。

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