UNIX and Python - a marriage of convenience

Recently we were having a discussion about how many Friday 13ths there were in a particular year (or which year had the most, or something).

I thought it should be easy to write a solution in Python, so I started with the following code:

import datetime
from datetime import timedelta

start = datetime.date(2009, 1, 1)
end = datetime.date(2013, 1, 1)

while start < end:
        print start.strftime('%Y-%m-%d %A')
        start = start + timedelta(1)

This little program prints out all the dates from 2009-01-01 to 2012-12-31 in the following format:

2009-01-01 Thursday
2009-01-02 Friday
2009-01-03 Saturday
2009-01-04 Sunday
2009-01-05 Monday
...

I could then have added some more code to determine if a date was a Friday 13th and printed it - but decided not to. I had a general purpose tool that printed a calendar, so all I needed to do was the following:

python printcal | grep '13 Friday'

Which then gives the following:

2009-02-13 Friday
2009-03-13 Friday
2009-11-13 Friday
2010-08-13 Friday
2011-05-13 Friday
2012-01-13 Friday
2012-04-13 Friday
2012-07-13 Friday

So, why did I prefer to do this rather than modify my Python script. Let's look at the Unix Philosophy

This is the Unix philosophy: Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface.

Doug McIlroy

A second part of the philosophy from Mike Gancarz is "Look for the 90-percent solution."

So, I have created a small tool that prints all the dates in a period (this is the 90-percent solution). It is easy to verify that it is doing what I want to do. I use the output of this program as the input to the standard unix program grep that finds all the output that contains Friday 13. Suppose I now want to find all years that have at 3 months where the first of the month is a Monday, I can do it easily by modifying the grep part of the pipeline. If I want to see how many Monday 5ths we had in 2008, I can pipe the output of grep to wc -l which will print the number of lines output.