The error

When you try to install Python cryptography, you get the following error:

    In file included from src/nclude/lxml.etree.c:320:0:
    src/lxml/includes/etree_defs.h:14:31: fatal error: libxml/xmlversion.h: No such file or directory

                       ^
    compilation terminated.
    error: command 'x86_64-linux-gnu-gcc' failed with exit status 1

Solution

If you are using Python 3:

sudo apt-get install python3 python-dev python3-dev \
     build-essential libssl-dev libffi-dev \
     libxml2-dev libxslt1-dev zlib1g-dev \
     python-pip

For Python 2 install the following dependencies:

sudo apt-get install python-dev  \
     build-essential libssl-dev libffi-dev \
     libxml2-dev libxslt1-dev zlib1g-dev \
     python-pip