Python_Dev_Notes.md

# Python Dev Notes

Each Python version uses a specific compiler version. In most cases, you should use the same compiler version for building python extensions.

## Which Microsoft Visual C++ compiler to use with a specific Python version ?

| Visual C++  | CPython                 |
|-------------|:-----------------------:|
|2015, 2017   | 3.7                     |
|2015         | 3.5,3.6                 |
|2010         | 3.3,3.4                 |
|2008         | 2.6, 2.7, 3.0, 3.1, 3.2 |

Currently, the official ONNXRuntime Python wheel (v1.3.0 onwards) hosted on PyPi requires [Visual C++ 2019 runtime ](https://support.microsoft.com/en-us/help/2977003/the-latest-supported-visual-c-downloads) installed on the target machine. 

If the Python wheel is built from source using the build toolset provided with Visual Studio 2017, it will work with the Visual C++ 2017 runtime.

CPython 3.7 is distributed with a VC++ 2017 runtime. Unlike the earlier VC++ version, VC++ 2017 Runtime is binary backward compatible with VC++ 2015. Which means you could build your application with VC++ 2015 then run it with VC++ 2017 runtime.