Why can’t I build python extensions with a different version of Visual Studio?

The short answer: you probably can if you are careful enough to limit your use of Python API calls.

For example, you cannot use PyFile_AsFile, the reason is that C runtime objects like FILE* created in one runtime e.g. msvcrt.dll cannot be used in another, e.g. msvcrt70.dll

In addition, malloc() allocated memory from one runtime cannot be freed from another runtime.

Here’s a rundown:

• Potential Errors Passing CRT Objects Across DLL Boundaries
• Also read PEP 384 – maintaining a stable ABI, where it mentions
  

  In addition, functions expecting FILE* are not part of the ABI, to avoid depending on a specific version of the Microsoft C runtime DLL on Windows.

Interestingly, there are techniques called forwarder DLLs that redirect DLL calls to another version.

• See Exported functions that are really forwarders.
• DLL forwarding in Python (StackOverflow).

More interestingly though is the possibility of using Visual Studio to link against the older runtime directly

• linking against the old MSVCRT with newer versions of visual studio, also
• Dynamically linking with MSVCRT.DLL using Visual C++ 2005.