.. title: Files generation in Python
.. slug: file-templating-python
.. date: 2019-08-12 10:02:35 UTC+02:00
.. tags: python
.. category: blog
.. link: 
.. description: 
.. type: text

It is fairly easy to generate text (therefore ASCII files, therefore source code) in Python using templates. I often see misuse of `print` to do so, whereas it seems much more elegant to use templates.
   
.. TEASER_END

Basic principle
================

The basic principle is very simple: you will create a source file with variables to substitute (using the usual Python formatting); this file will be loaded, then substitutions will be made and a modified file will be written.

Before going into technicalities, it is worth emphasizing that this way is much more pleasant and much less error-prone that a long list of `print`. Pleasant, because it is easy (and much less combersome) to start from a preexisting file; on top of it, keeping the file extension, we'll keep the text formatting in our favorite text editor. Less error-prone, for exactly the same reason: working from an existing file means that we can check that everything is working beforehand; we therefore avoid complicated debugging where Python bugs are intricated with bugs in the sources.

Example
===========

We can use it on a simple bash `hello world`:

- we start with a file containgin the `var` variable:

.. code-block:: bash

		$ more ex.sh
		#!/bin/bash
		echo "%(var)s"

- and we then open, substitute and write a new file in Python:
		
.. code-block:: python

		#!/usr/bin/env python

		template = open("ex.sh", "rt").read()
		data = {"var": "Hello world!"}
		with open("exm.sh", "wt") as output:
		    output.write(template %data)

- ... that we can launch:

.. code-block:: bash

		$ sh exm.sh
		Hello world!

A more complex example can be found on `GitHub <https://github.com/marcjoos-phd/pymisc/tree/master/template>`_, but it is basically the same. I also used this in `DUMSES-Hybrid <https://github.com/marcjoos-phd/dumses-hybrid>`_ for example (this example is actually interesting for tabs because I needed real tabs, not spaces, to generate makefiles).