.. title: Create Sequence Diagrams Using seqdiag
.. slug: create-sequence-diagrams-using-seqdiag
.. date: 2014-10-01 18:06:44
.. tags: python, seqdiag
.. category:
.. link:
.. description:
.. type: text

Have you ever needed to create `sequence diagrams <https://en.wikipedia.org/wiki/Sequence_diagram>`_? If you're like me
you constantly have to. Up to now I have always used a manual process
using pen/paper or Visio. But I wanted an easier way and found
`seqdiag <http://blockdiag.com/en/seqdiag/index.html>`_.

.. TEASER_END: Read more

I installed it in a virtualenv on Ubuntu 14.04.

::

    codeghar@host~$ virtualenv --system-site-packages virt

I used `--system-site-packages` because seqdiag relies on Python Imaging
Library (PIL). Of course, make sure you have python-pil package
installed in your OS. I ran into this problem (`About the PIL Error —
IOError: decoder zip not available <http://stackoverflow.com/questions/3544155/about-the-pil-error-ioerror-decoder-zip-not-available>`_) when I didn't use `--system-
site-packages`. The impression I got was it's `hard to get PIL
installed in a virtualenv <http://stackoverflow.com/a/10864383>`_ and easier to just use the system-wide
PIL.

::

    codeghar@host~$ source virt/bin/activate

    (virt)codeghar@host~$ pip install seqdiag

To create a diagram you need to provide a dialog
in a .diag file. You can look at the `sample diagrams <http://blockdiag.com/en/seqdiag/examples.html>`_ for more
information. The following will create a new file (or overwrite an
existing file) called example.png.

::

    (virt)codeghar@host~$ seqdiag -Tpng --no-transparency example.diag

Hat tip to `generate uml sequence diagrams with python or perl <http://stackoverflow.com/a/13136890>`_ for bringing seqdiag to my
attention.

Note: I published this post originally at `Create Sequence Diagrams Using seqdiag <https://codeghar.wordpress.com/2014/10/01/create-sequence-diagrams-using-seqdiag/>`_. Migrated to Nikola in February 2015.