Cannot “pip install cryptography” in Docker Alpine Linux 3.3 with OpenSSL 1.0.2g and Python 2.7

For those who are still experiencing problems installing cryptography==2.1.4 in Alpine 3.7 like this:

writing manifest file 'src/cryptography.egg-info/SOURCES.txt'
running build_ext
generating cffi module 'build/temp.linux-x86_64-2.7/_padding.c'
creating build/temp.linux-x86_64-2.7
generating cffi module 'build/temp.linux-x86_64-2.7/_constant_time.c'
generating cffi module 'build/temp.linux-x86_64-2.7/_openssl.c'
building '_openssl' extension
creating build/temp.linux-x86_64-2.7/build
creating build/temp.linux-x86_64-2.7/build/temp.linux-x86_64-2.7
gcc -fno-strict-aliasing -Os -fomit-frame-pointer -g -DNDEBUG -Os -fomit-frame-pointer -g -DTHREAD_STACK_SIZE=0x100000 -fPIC -I/usr/include/python2.7 -c build/temp.linux-x86_64-2.7/_openssl.c -o build/temp.linux-x86_64-2.7/build/temp.linux-x86_64-2.7/_openssl.o -Wconversion -Wno-error=sign-conversion
build/temp.linux-x86_64-2.7/_openssl.c:493:30: fatal error: openssl/opensslv.h: No such file or directory
 #include <openssl/opensslv.h>
                              ^
compilation terminated.
error: command 'gcc' failed with exit status 1

Solution

Install these dependencies in the Alpine container:

$ apk add --no-cache libressl-dev musl-dev libffi-dev

To install these dependencies using a Dockerfile:

RUN apk add --no-cache \
        libressl-dev \
        musl-dev \
        libffi-dev && \
    pip install --no-cache-dir cryptography==2.1.4 && \
    apk del \
        libressl-dev \
        musl-dev \
        libffi-dev

Reference

Installation instructions for cryptography on Alpine can be found here:

• https://cryptography.io/en/latest/installation/#building-cryptography-on-linux
• A version from the time of writing is available on github

Here is the relevant portion:

Building cryptography on Linux

[skipping over the part for non-Alpine Linux] …

$ pip install cryptography

If you are on Alpine or just want to compile it yourself then
cryptography requires a compiler, headers for Python (if you’re not
using pypy), and headers for the OpenSSL and libffi libraries
available on your system.

Alpine

Replace python3-dev with python-dev if you’re using Python 2.

$ sudo apk add gcc musl-dev python3-dev libffi-dev openssl-dev

If you get an error with openssl-dev you may have to use libressl-dev.