Python通过SWIG调用C++时出现的ImportError问题解析

摘要 win10系统，编译器为mingw，按照教程封装C++的一个类并用python调用，一步步进行直到最后一步运行python代码时，在python代码中import example时报错ImportError: DLL load failed while importing _example: The specified module could not be found.本文记录我的解决方法和其它一些解决思路。

先测试C语言

官方文档有完整教程，命令分别为

• swig -python example.i
• gcc -c -fpic example.c example_wrap.c -I “C:/Users/xd15zhn/AppData/Local/Programs/Python/Python310/include”
• gcc -shared example.o example_wrap.o -o _example.pyd -L “C:/Users/xd15zhn/AppData/Local/Programs/Python/Python310/libs” -lpython310

注意包含自己的头文件目录和库目录，gcc和g++的具体参数见文末的参考链接。
example.c

#include <stdio.h>
double My_variable = 3.0;
int my_sum(double a, double b) {
    return(a + b);
}
void my_print() {
    printf("Hello World!\n");
}

example.i

%module example
%{
    extern double My_variable;
    extern int my_sum(int n, int m);
    extern void my_print();
%}
extern double My_variable;
extern int my_sum(int n, int m);
extern void my_print();

main.py

import example
example.my_print()
print(example.my_sum(1,2))

再测试C++

• swig -c++ -python example.i
• g++ -c -fPIC example.cpp example_wrap.cxx -I “C:/Users/xd15zhn/AppData/Local/Programs/Python/Python310/include”
• g++ -shared example.o example_wrap.o -o _example.pyd -L “C:/Users/xd15zhn/AppData/Local/Programs/Python/Python310/libs” -lpython310

example.cpp

#include <iostream>
#include "example.hpp"
void Example::say_hello() {
    std::cout << "Hello world!" << std::endl;
}
double Example::my_sum(double a, double b) {
    return a+b;
}

example.hpp

class Example {
public:
    void say_hello();
    double my_sum(double a, double b);
};

example.i

%module example
%{
#include "example.hpp"
%}
%include "example.hpp"

main.py

import example
e = example.Example()
e.say_hello()
print(e.my_sum(1,2))

运行本节开头的3条命令后，不出意外的话，运行3条命令后依次分别生成：

1. example.py, example_wrap.cxx
2. example.o example_wrap.o
3. _example.pyd

问题描述与解决

最后运行main.py后在import example时报错ImportError: DLL load failed while importing _example: The specified module could not be found. stackoverflow上有人提到了这个问题，一个解决方法是

I met exactly the same problem after upgraded python to 3.9 on windows . After struggling for hours, I managed to solve it by manually copying some dlls from ***/mingw/bin/ where mingw32-g++ is found to where my ***.pyd is located. I’m sure that ***/mingw/bin/ has been appended to %PATH%, but don’t know why python3.9 couldn’t find it.

按照这个方法我把/mingw/bin/目录下的所有.dll文件全都复制到当前项目目录下确实解决了问题，但文中说只复制了一部分，想到使用静态编译需要用到两个命令-static-libstdc++和-static-libgcc，然后测试了一下确实是只需要这两个文件。除了复制文件这一不太优雅的方法以外，只需要在python代码中加上/mingw/bin/目录即可，完整的python代码如下

import os
os.add_dll_directory('C:/Users/xd15zhn/Documents/mingw64/bin')
import example
e = example.Example()
e.say_hello()
print(e.my_sum(1,2))

但这种方法还是不够优雅，能不能把这两个动态库直接链接到_example.pyd文件里？我尝试在上面第3条命令后加上-llibgcc_s_sjlj-1 -llibstdc++-6，但没有用。不知道有没有更优雅的解决方法。

cmake

cmake_minimum_required(VERSION 3.14)
project(example)
set(CMAKE_BUILD_TYPE release)
list(APPEND CMAKE_PREFIX_PATH "C:/Users/xd15zhn/Documents/cpplibraries")
# set(MinGW_PATH "C:/Users/xd15zhn/Documents/mingw64/bin")
# set(CMAKE_SHARED_LINKER_FLAGS "-static-libgcc -static-libstdc++")
find_package(Python3 COMPONENTS Interpreter Development REQUIRED)
find_package(Python3 COMPONENTS NumPy REQUIRED)
find_package(SWIG REQUIRED)
message(STATUS "Python3_LIBRARIES: ${Python3_LIBRARIES}")
message(STATUS "Python3_INCLUDE_DIRS: ${Python3_INCLUDE_DIRS}")
message(STATUS "Python3_NumPy_INCLUDE_DIRS: ${Python3_NumPy_INCLUDE_DIRS}")

include(UseSWIG)
set(example_INTERFACE
    ${PROJECT_SOURCE_DIR}/cppsrc/example.i
)
set(example_SOURCES
    ${PROJECT_SOURCE_DIR}/cppsrc/example.cpp
)
set_source_files_properties(${example_INTERFACE} PROPERTIES CPLUSPLUS ON)
swig_add_library(example LANGUAGE python SOURCES ${example_INTERFACE} ${example_SOURCES})

target_include_directories(example PUBLIC
    ${PROJECT_SOURCE_DIR}/cppsrc
    ${Python3_INCLUDE_DIRS}
    ${Python3_NumPy_INCLUDE_DIRS}
)
target_link_libraries(example PUBLIC
    ${Python3_LIBRARIES}
)

其它

下面的代码用于单独生成example.cpp的动态库来测试自己写的代码是否有问题。

g++ example.cpp -fpic -shared -o example.dll
g++ main.cpp example.dll -o untitled

参考

SWIG doesn’t work on Windows with MinGW-w64 when binding C++ and Python: DLL load failed while importing: The specified module could not be found -stackoverflow
gcc静态编译之-static-libstdc++、-static-libgcc、-static -简书
g++编译详解 -CSDN博客
gcc&g++链接动态库或静态库方法 -CSDN博客
SWIG:Python调用C++(新手保姆级示范) -知乎
The specified module could not be found的解决办法 -CSDN博客