How may I override the compiler (GCC) flags that setup.py uses by default?

I understand that setup.py uses the same CFLAGS that were used to build Python. I have a single C extension of ours that is segfaulting. I need to build it without -O2 because -O2 is optimizing out some values and code so that the core files are not sufficient to pin down the problem.

I just need to modify setup.py so that -O2 is not used.

I've read distutils documentation, in particular distutils.ccompiler and distutils.unixccompiler and see how to add flags and libs and includes, but not how to modify the default GCC flags.

Specifically, this is for a legacy product on Python 2.5.1 with a bunch of backports (Fedora 8, yes, I know...). No, I cannot change the OS or Python version and I cannot, without great problems, recompile Python. I just need to build a one off of the C extension for one customer whose environment is the only one segfaulting.

• Prepend CFLAGS="-O0" before you run setup.py:

  % CFLAGS="-O0" python ./setup.py
  

  The -O0 will be appended to CFLAGS while compiling, therefore will override previous -O2 setting.

• Another way is add -O0 to extra_compile_args in setup.py:

  moduleA = Extension('moduleA', .....,
          include_dirs = ['/usr/include', '/usr/local/include'], 
          extra_compile_args = ["-O0"], 
          )
  

• If you want to remove all default flags, use:

  % OPT="" python ./setup.py
  

I ran into this problem when I needed to fully remove a flag (-pipe) so I could compile SciPy on a low-memory system. I found that, as a hack, I could remove unwanted flags by editing /usr/lib/pythonN.N/_sysconfigdata.py to remove every instance of that flag, where N.N is your Python version. There are a lot of duplicates, and I'm not sure which are actually used by setup.py.

distutils/​setuptools allows any compiler/​linker flags to be specified with extra_compile_args/​extra_link_args argument when defining a Python extension in setup.py script. These extra flags will be added after default ones and will override any mutually exclusive flags present earlier.

For regular use, however, this is not much useful as a package you distribute through PyPI can be built by different compilers having an incompatible options.
The following code allows you to specify these options in extension- and compiler-specific way:

from setuptools import setup
from setuptools.command.build_ext import build_ext


class build_ext_ex(build_ext):

    extra_compile_args = {
        'extension_name': {
            'unix': ['-O0'],
            'msvc': ['/Od']
        }
    }

    def build_extension(self, ext):
        extra_args = self.extra_compile_args.get(ext.name)
        if extra_args is not None:
            ctype = self.compiler.compiler_type
            ext.extra_compile_args = extra_args.get(ctype, [])

        build_ext.build_extension(self, ext)


setup(
    ...
    cmdclass = {'build_ext': build_ext_ex},
    ...
)

Of course you could simplify it if you want all extensions to use the same (but still compiler-specific) options.

Here is a list of supported compiler types (as returned by setup.py build_ext --help-compiler):

--compiler=bcpp     Borland C++ Compiler
--compiler=cygwin   Cygwin port of GNU C Compiler for Win32
--compiler=mingw32  Mingw32 port of GNU C Compiler for Win32
--compiler=msvc     Microsoft Visual C++
--compiler=unix     standard UNIX-style compiler