Compiling Python extensions on Windows

Sun 21 December 2014

Extensions usually need to be built with the same compiler version as the interpreter. This is because each compiler version uses a different and incompatible CRT, until VS 2015 that is. You can read more about that here.

If you're missing the required compiler you usually get a vague error: Unable to find vcvarsall.bat.

For Python 2.7 *

For Python 2.7 you need to get Microsoft Visual C++ Compiler for Python 2.7. It's a special package made by Microsoft that has all the stuff. It is supported since setuptools 6.0 [1].

Unfortunately the latest virtualenv, 1.11.6 as of now, still bundles setuptools 3.6. This means that if you try to run python build_ext in an virtualenv it will fail, because setuptools can't detect the compiler. The solution is to force upgrade setuptools, example: pip install "setuptools>=6.0".

If you're using tox then just add it to your deps. Example:

deps =

This seems to work fine for 64bit extensions.


Probably works for Python 3.3 too.

For Python 3.4 *

This one gave me a headache. I've tried to follow this guide but had some problems with getting 64bit extensions to compile. In order to get it to work you need to jump through these hoops:

  1. Install Visual C++ 2010 Express (download).

  2. Before installing the Windows SDK v7.1 (these gave me a bad time):

    • Do not install Microsoft Visual Studio 2010 Service Pack 1 yet. If you did then you have to reinstall everything. Uninstalling the Service Pack removes components so you have to reinstall Visual C++ 2010 Express again.
    • Remove all the Microsoft Visual C++ 2010 Redistributable packages from Control Panel\Programs and Features.

    If you don't do those then the install is going to fail with an obscure "Fatal error during installation" error.

  3. Install Windows SDK for Visual Studio 2010 (v7.1) (download). This is required for 64bit extensions.

    ⚠ On Windows 10 the web installer gets confused:

    Some Windows SDK components require the RTM .NET Framework 4. Setup detected a pre-release version of the .NET Framework 4.

    You certainly don't need any of the .NET stuff so you can work around by getting the offline version of the SDK (ISO) (download).

    • Make sure you download GRMSDKX_EN_DVD.iso (X means 64bit).
    • Windows has builtin mounting for ISOs. Just mount the ISO and run Setup\SDKSetup.exe instead of setup.exe.
  4. Create a vcvars64.bat file in C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\bin\amd64 that contains [2]:

    CALL "C:\Program Files\Microsoft SDKs\Windows\v7.1\Bin\SetEnv.cmd" /x64

    If you don't do this you're going to get a mind-boggling error like this:

    Installing collected packages: whatever
      Running develop for whatever
        building 'whatever.cext' extension
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "C:/whatever\", line 112, in <module>
        for root, _, _ in os.walk("src")
      File "c:\python34\Lib\distutils\", line 148, in setup
      File "c:\python34\Lib\distutils\", line 955, in run_commands
      File "c:\python34\Lib\distutils\", line 974, in run_command
      File "C:\whatever\.tox\3.4\lib\site-packages\setuptools\command\", line 32, in run
      File "C:\whatever\.tox\3.4\lib\site-packages\setuptools\command\", line 117, in install_for_development
      File "c:\python34\Lib\distutils\", line 313, in run_command
      File "c:\python34\Lib\distutils\", line 974, in run_command
      File "C:/whatever\", line 32, in run
      File "C:\whatever\.tox\3.4\lib\site-packages\setuptools\command\", line 54, in run
      File "c:\python34\Lib\distutils\command\", line 339, in run
      File "c:\python34\Lib\distutils\command\", line 448, in build_extensions
      File "C:/whatever\", line 39, in build_extension
        build_ext.build_extension(self, ext)
      File "C:\whatever\.tox\3.4\lib\site-packages\setuptools\command\", line 187, in build_extension
        _build_ext.build_extension(self, ext)
      File "c:\python34\Lib\distutils\command\", line 503, in build_extension
      File "c:\python34\Lib\distutils\", line 460, in compile
      File "c:\python34\Lib\distutils\", line 371, in initialize
        vc_env = query_vcvarsall(VERSION, plat_spec)
      File "C:\whatever\.tox\3.4\lib\site-packages\setuptools\", line 52, in query_vcvarsall
        return unpatched['query_vcvarsall'](version, *args, **kwargs)
      File "c:\python34\Lib\distutils\", line 287, in query_vcvarsall
        raise ValueError(str(list(result.keys())))
    ValueError: ['path']
    Complete output from command C:\whatever\.tox\3.4\Scripts\python.exe -c "import setuptools, tokenize; __file__='C:/whatever\\'; exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" develop --no-deps:
 will run vcvarsall.bat amd64:

    c:\Program Files (x86)\Microsoft Visual Studio 10.0\VC>vcvarsall.bat amd64
    The specified configuration type is missing.  The tools for the
    configuration might not be installed.

    Basically that is caused by vcvarsall.bat not being able to run vcvar64.bat because, surprise, the Windows SDK is missing that file.

  5. Now everything should work, go and try py -3 clean --all build_ext --force.

  6. Install Microsoft Visual Studio 2010 Service Pack 1 (download). This is optional, however, if you do this you also have to do the following too [3]:

  7. Install Microsoft Visual C++ 2010 Service Pack 1 Compiler Update for the Windows SDK 7.1 (download).

  8. Remove all those SQL Server packages that you don't need from Control Panel\Programs and Features.

Alternative: Victor Stinner argues that just installing the Windows SDKs (v7.0 for Python 2.7, v7.1 for Python 3.3 and 3.4) is enough if you prepare the environment (inconvenient in my opinion):

setenv /x64 /release
set MSSDK=1

See his notes or the Python wiki. You still need to install those fiddly SDKs :-)

For Python 3.5 *

It seems Visual Studio 2015 Community Edition just works without any fuss. Get it here. It's quite big and doesn't let you skip the irrelevant SQL Server and .NET tools so I had to manually remove them from Control Panel\Programs and Features.

Alternative: Get the Visual C++ 2015 Build Tools instead. It doesn't come with Visual Studio and all the useless cruft but make sure you select the Windows 8.1 SDK during the install.

[1]Support added in
[3]Installing the Microsoft Visual Studio 2010 Service Pack 1 removes some components from the SDK. The solution is to also install the Microsoft Visual C++ 2010 Service Pack 1 Compiler Update for the Windows SDK 7.1. See:

This entry was tagged as pip python windows