Tuesday, March 25, 2014

Parameter files in Python for Python programs

I often write and use Python programs that require input parameters to tell them what to do. When there are a few parameters I usually pass them through the command line (my current favorite framework for command line argument parsing is the amazing docopt module). When there are many parameters, however, command line arguments can get cumbersome.

In the past I have looked to configuration markup languages (YAML comes to mind) and have also used microsoft excel format files (using the excellent xlrd package). Using proprietary formats makes my stomach upset, but there are no good solutions for the odf counterparts.

Recently I have started to experiment with storing parameter files in python and reading them into my code.

__import__(filename) works and puts the parameters into a separate name space, so you can access them as filename.x, filename.y etc. However, the file needs to be in the module search path, and it feels a bit of a misuse.

A better solution, I find, is to use execfile to read the parameter file into the code. There are no path restrictions, execfile IS meant to execute code (so no misuse there) and the variables from the Python file are loaded into a separate dictionary so they are contained and do not spill all over your code.

#Main file
pars = {}
execfile('params.py', {}, pars)

With params.py looking like

a = 43
b = 'a string'
c = {'d': 23}

We get:

pars --> {'a': 43, 'b': 'a string', 'c': {'d': 23}}

We can put in comments in the file and structure however we need which I find very convenient.

execfile has disappeared in Python 3 but you can use

with open(filename) as f:
    exec(compile(f.read(), filename, "exec"))

UPDATE: However, see a funny issue with execfile here.

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