[Fixed] installing Python cryptography package error "ffi.h: No such file or directory"

When installing Scrapy, it'll install Python cryptography package, but getting the following error:

raise DistutilsError("Setup script exited with %s" % (v.args[0],))    distutils.errors.DistutilsError: Setup script exited with error: command 'i686-linux-gnu-gcc' failed with exit status 1    Complete output from command /home/ubuntu/.virtual_envs/env/bin/python -c "import setuptools, tokenize;__file__='/home/ubuntu/.virtual_envs/env/build/cryptography/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /tmp/pip-SVMn92-record/install-record.txt --single-version-externally-managed --compile --install-headers /home/ubuntu/.virtual_envs/env/include/site/python2.7:    c/_cffi_backend.c:13:17: fatal error: ffi.h: No such file or directory
 #include
                 ^
compilation terminated....distutils.errors.DistutilsError: Setup script exited with error: command 'i686-linux-gnu-gcc' failed with exit status 1

How to fix this error?

For Debian or Ubuntu, install required dependencies by using the following command:

$ sudo apt-get install build-essential libssl-dev libffi-dev python-dev

For Fedora:

$ sudo yum install gcc libffi-devel python-devel openssl-devel

And install cryptography:

$ pip install cryptography

It works.