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Python interface and tools for FINESSE

Project description

PyKat is a wrapper for using FINESSE ( It aims to provide a Python toolset for automating more complex tasks as well as providing a GUI for manipulating and viewing simulation setups.


The easiest way to install PyKat is through PyPi:

pip install pykat

If you are a Windows user you also have the option to download the installer at

You should now be able to open up a new Python terminal and type import pykat, the output should be:

>>> import pykat
    PyKat 0.1              _                  '(
                          \`.|\.__...-""""-_." )
       ..+-----.._        /  ' `            .-'
   . '            `:      7/* _/._\    \   (
  (        '::;;+;;:      `-"' =" /,`"" `) /
  L.        \`:::a:f            c_/     n_'
  ..`--...___`.  .    ,
   `^-....____:   +.

You will also need to ensure that you have a fully working copy of FINESSE installed and setup on your machine. More details on this can be found at

You must setup 2 environment variables: ‘FINESSE_DIR’, whose value is the directory that the ‘kat’ executable is in; ‘KATINI’, which states the directory and name of the kat.ini file to use by default in FINESSE, more information in the FINESSE manual can be found about this.


This does not detail how to use FINESSE itself, just PyKat. FINESSE related queries should be directed at the FINESSE manual or the forum

We highly recommend running PyKat with IPython, it has so far provided the best way to explore the various PyKat objects and output data. Also of use is IPythons interactive matplotlib mode - or pylab mode - which makes displaying and interacting with multiple plots easy. You can start pylab mode from a terminal using:

ipython -pylab

Regardless of which interpreter you use, to begin using PyKat you first need to include the following:

from pykat import finesse
from pykat.detectors import *
from pykat.components import *
from pykat.commands import *
from pykat.structs import *

This provides all the various FINESSE components and commands you will typically need. Running a simulation requires you to already know how to code FINESSE files, which is beyond the scope of this readme. FINESSE commands can be entered in many ways: reading in a previous .kat file, creating pykat objects representing the various FINESSE commands or by writing blocks of FINESSE code as shown next:

import pylab as pl

# Here we write out any FINESSE commands we want to process
code = """
l l1 1 0 0 n1
s s1 10 1 n1 n2
m m1 0.5 0.5 0 n2 n3
s s2 10 1 n3 n4
m m2 0.5 0.5 0 n4 n5
s s3 10 1 n5 n6

pd pd_cav n4
xaxis m1 phi lin 0 360 360
yaxis abs:deg

# this kat object represents one single simulation, it containts
# all the objects and their various states.
kat = finesse.kat()

# Currently the kat object is empty. We can fill it using a block
# string of normal FINESSE commands by parsing them.

# Once we have some simulation built up we can run it simply by calling...
out =

# This out object contains the results from this run of the simulation.
# Parameters can then be changed and can be called again producing
# another output object. So if we wanted to change the reflectivity of m1 we can do
kat.m1.R = 0.2
kat.m1.T = 0.8
# now run it again...
out2 =

# We can plot the output simply enough using pylab plotting.
pl.plot(out.x, out["pd_cav"])
pl.ylabel("Intensity [W]")

The above demonstates a way of packaging up a FINESSE simulation - simple or complex - and including any post-processing and plotting in one Python script file. Or you can create kat files separately and produce Python scripts to run and process them, that choice is upto you, Pykat provides the means to be used in both ways.

To load in a separate FINESSE .kat file we can use the commands:

kat = finesse.kat()
# load in a separate file in the same directory...
# the kat object has now parsed all the commands in this file.

# We can alter and objects in there, e.g. if there was a mirror called m1
kat.m1.phi = 45

out =

Finesse Test Server

A Flask based website that runs the Finesse test suites is included in PyKat. This can be hosted in Apache or run as a development server for quick testing on a system. This is a developer tool for testing FINESSE against a selection of known test cases.


Flask Numpy CodernityDB

Command to start server:

python -m pykat.testing.web_server --path=[path to create website] --port=[HTTP port] --git-bin=[path to git binary]

The website can then be accessed in a web browser at the address: localhost:[port]

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