Very simple template substitutions using Python and Perl

Very Simple Template Substitutions Using Python Perl

2020-03-15T20:50:05Z

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Contents

• Introduction
• Using Python
• Using Shell and Perl

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Introduction

It is quite common to treat text files as templates which contains place holders that will be replaced by some real values. Python jinja or Ruby erb template engines are there to resolve for these kind of issues. They are quite feature rich. Following is a very simple Template substitution program in Python and Perl that does similar kind of job. Its very basic but works for simple scenarios.

Using Python

Consider following text file test_file.txt with place holders in it.

This is a %TEST1% . Another %TEST1%  %TEST1%
Now this is a %TEST2%.

Following is the file var.py containing values of above place holder.

TEST1="test_string1"
TEST2="test_string2"

Following is the python program that will replace above values in test_file.txt from var.py file.

#! /usr/bin/env python

import re
import fileinput
import sys
from var import *

def print_data_vars(matchObj):
    return globals()[matchObj.group(1)]

for line in fileinput.input('test_file', inplace=True):
    line = re.sub(r'%(.+?)%', print_data_vars, line)
    sys.stdout.write(line)

when above program is run , it will replace placeholders with values in existing test_file.txt file.

Using Shell and Perl

Assuming var.py and test_file.txt contents are same as above, same effect can also be achieved using shell and perl combination as follows.

#! /usr/bin/env bash
set -a
source ./var.py
set +a
perl -i -wpl -e 's!%(.+?)%!$ENV{$1}!g' test_file