Update: 10.9.3 resolves the issue with system CPython.

This is caused by the latest clang update from Apple that came with Xcode 5.1 today and is affecting many, many people, so hopefully a fix will appear soon.

Update: Did not expect this to get so much attention, but here’s more detail: the clang 3.4 Apple is shipping defaults to erroring on unknown flags, but CPython builds modules using the same set of flags it was compiled with originally. The system CPython was compiled with several flags that are unknown by clang, thus causing this problem. The following are the current Mavericks (10.9.2) CPython compile flags:

-I/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -I/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -fno-strict-aliasing -fno-common -dynamic -arch x86_64 -arch i386 -g -Os -pipe -fno-common -fno-strict-aliasing -fwrapv -mno-fused-madd -DENABLE_DTRACE -DMACOSX -DNDEBUG -Wall -Wstrict-prototypes -Wshorten-64-to-32 -DNDEBUG -g -fwrapv -Os -Wall -Wstrict-prototypes -DENABLE_DTRACE

To resolve this issue you have a few options:

• Add ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future to your compile flags.
• Pass CFLAGS=""
• Strip the unsupported flags from the sysconfigdata.py (as is noted in another answer).
• Compile a new python using homebrew.
• Wait for the next OS X update, which is rumored to include recompiled Python/Ruby to remove this issue for system runtimes.

Here is a work around that involves removing the flag from the python installation.

In /System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/_sysconfigdata.py are several places where the -mfused-madd / -mno-fused-madd flag is set.

Edit this file and remove all of the references to that flag your compilation should work:

sudo sed -i '.old' 's/ -m(no-){0,1}fused-madd //g' /System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/_sysconfigdata.py

You need to delete the corresponding _sysconfigdata.pyc and _sysconfigdata.pyo files as well – at least on my system these files did not automatically get rebuilt:

cd /System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/
sudo rm _sysconfigdata.pyo _sysconfigdata.pyc

Note that have to use root access to make those changes.

I used homebrew python package.
Works well after re-compilling.

xCode 5.1

ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future pip install psycopg2

I also edited the system python’s notion of its original compile flags (as @user3405479 did). Instead of an editor I used command line tools to edit the file “in place” (the inode does change). I also compiled new pyo and pyc files instead of leaving them deleted.

The following commands are run as root, for example under sudo -i

pushd /System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7

sed 's/-mno-fused-madd//g' _sysconfigdata.py
| diff -p _sysconfigdata.py - | patch -b

python -m py_compile _sysconfigdata.py

python -OO -m py_compile _sysconfigdata.py

popd

I used homebrew to install postgresql, and then wanted to install psycopg2 into the system-provided python 2.7 on Mavericks. To get that to work I ended up running this:

sudo ARCHFLAGS="-arch x86_64" CFLAGS=-Wunused-command-line-argument-hard-error-in-future pip install psycopg2

None of the above solutions worked for me. My situation is slightly different. I was trying to install rdiff-backup via homebrew when I got this error message.

I apparently have both Apple’s default python (2.7.5 @ /usr/bin/python) and Homebrew’s python (2.7.6 @ /usr/local/Cellar/python/2.7.6/bin/python), but the error was coming from Apple’s python.

To fix the problem, basically I switched the python binaries.

I did this via sudo:

• cp /usr/bin/python /usr/bin/python_orig
• ln -s /usr/local/Cellar/python/2.7.6/bin/python /usr/bin/python

Then, ‘brew install rdiff-backup’ succeeded without any errors.

Finally, I did “mv /usr/bin/python_orig /usr/bin/python” to revert everything back.

Variation on the _sysconfigdata.py fix: use a virtual environment

cp /System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/_sysconfigdata.py ${VIRTUAL_ENV}/lib/python2.7/_sysconfigdata.py

Then edit your copy at ${VIRTUAL_ENV}/lib/python2.7/_sysconfigdata.py

I removed all the occurences of ‘-mno-fused-madd’ and then builds worked in that virtualenv.

No root needed!

At least Apple is aware of this as this issue is discussed in the Xcode 5.1 Release Notes (Compiler section). Expect an updated Python and Ruby in the next OS update (fingers crossed!). Also take note that the -Wno-error=unused-command-line-argument-hard-error-in-future flag is meant to be a temporary workaround and will go away in the future (hopefully after system Python is fixed!).

If you are a Python package maintainer like me and want to save your users the hassle of working around this themselves, here is a solution (besides providing binary wheels/eggs) that goes into your setup.py file:

from distutils.command.build_ext import build_ext
import subprocess
import sys

# Workaround for OS X 10.9.2 and Xcode 5.1+
# The latest clang treats unrecognized command-line options as errors and the
# Python CFLAGS variable contains unrecognized ones (e.g. -mno-fused-madd).
# See Xcode 5.1 Release Notes (Compiler section) and
# http://stackoverflow.com/questions/22313407 for more details. This workaround
# follows the approach suggested in http://stackoverflow.com/questions/724664.
class build_ext_subclass(build_ext):
    def build_extensions(self):
        if sys.platform == 'darwin':
            # Test the compiler that will actually be used to see if it likes flags
            proc = subprocess.Popen(self.compiler.compiler + ['-v'],
                                    stdout=subprocess.PIPE, stderr=subprocess.PIPE,
                                    universal_newlines=True)
            stdout, stderr = proc.communicate()
            clang_mesg = "clang: error: unknown argument: '-mno-fused-madd'"
            if proc.returncode and stderr.splitlines()[0].startswith(clang_mesg):
                for ext in self.extensions:
                    # Use temporary workaround to ignore invalid compiler option
                    # Hopefully -mno-fused-madd goes away before this workaround!
                    ext.extra_compile_args += ['-Wno-error=unused-command-line-argument-hard-error-in-future']
        build_ext.build_extensions(self)

setup(
    name="mypackage",
    ...
    cmdclass={'build_ext' : build_ext_subclass},
    ...
)

You’re probably on Python 2.7.5

$ python

Python 2.7.5 (default, Aug 25 2013, 00:04:04)  [GCC 4.2.1 Compatible
Apple LLVM 5.0 (clang-500.0.68)] on darwin

Easiest solution go to www.python.org and install Python 2.7.6 which is compatible with the LLVM 5.1.

Then toast your old virtual environment, rmvirtualenv {virtual_env_name}.

Then make a new virtual env..

mkvirtualenv --no-site-packages -p/Library/Frameworks/Python.framework/Versions/2.7/bin/python {virtual_env_name}

pip should work fine after this.

For me, the bad flags were explicitly given in the package’s Makefile. So I had to edit the makefile to remove the unrecognized flag from the erroring package.

Amazon Linux [AWS EC2]

Trying to pip install paramiko on Amazon Linux AMI release 2016.09 gave me an error that included a link to this SO post. So, even though it doesn’t fit the OP’s Title, I’m going to share the answer.

yum install libffi-devel
yum install gcc
yum install openssl-devel
pip install paramiko

Upgrading Homebrew and installing unixodbc solved it for me

brew upgrade
brew install unixodbc