Searching Files

Python is an excellent language to use when searching within files. Searching is very useful, for example you could imagine using Python to search an output file to find the results of a calculation. Searching in Python is straight-forward. Open a new Python script nano search.py and type the following;

from __future__ import print_function
import sys
import re

lines = open( sys.argv[1], "r" ).readlines()

for line in lines:
    if re.search( r"the", line ):
        print(line, end="")

This script will search a file and print out all of the lines that contain the word the. Try it out!

The key line of this script is if (re.search( r"the", line )):. This is a condition that uses Python’s regular expression pattern matching module, re together with a match string, r"the". The match string is just like a normal string. The r in front of the string is a label to tell Python not to expand slashes. While there are no slashes in this search string, I prefer to add the r by default to search strings as this prevents confusion when using more complicated searches.

For example;

import re

#does the line contain a lowercase a?
re.search( r"a", line )

#does the line contain an uppercase A?
re.search( r"A", line )

#does the line contain the word "cat"
re.search( r"cat", line )

To make the search case-insensitive, you must add re.IGNORECASE after the string to search, e.g.

import re

#search for an upper case or lower case a
re.search( r"a", line, re.IGNORECASE )

#search for "cat", "CAT", "CaT", "caT" etc.
re.search( r"cat", line, re.IGNORECASE )

The combination of search with split provides a powerful tool to help you process simulation output files. Imagine you have run a simulation that calculates the energy of a molecule. Lets imagine that the output file from the simulation looks something like this;

Starting program...
Loading molecule...
Initialising variables...
Starting the calculation - this could take a while!
Molecule energy = 2432.6 kcal mol-1
Calculation finished. Bye!

You can get the energy by searching for lines that contain Molecule energy =, and then using split to break this line into words. The value of the energy is the fourth word. Here is an example script that does just this;

import sys
import re

lines = open( sys.argv[1], "r" ).readlines()

for line in lines:
    if re.search( r"Molecule energy =", line ):
        words = line.split()

        energy = float( words[3] )

        print("The energy of the molecule is %f kcal mol-1" % energy)
        break

Try copying this example output to a file (e.g. logfile.txt) and copying the above Python script (e.g. search_log.py) to see that this works. Or try to write a similar Python script that processes an output file from one of the programs that you use.

Python’s text search is very flexible. For example, you can search for the contents of a variable, e.g.

import re

search_string = "the"

re.search( search_string, line )

This will match if line contains the value of search_string (namely the).


Exercise

grep is a useful UNIX program that lets you print out lines in a file that match some passed text, for example

grep the file.txt

will print out all of the lines that contain the word the.

Write a Python script (grep.py) that acts like grep. (Answer)


Compare with Perl


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