Aaron Maenpaa

Python for Software Development

Python for Software Development which was initially a port of How to Think Like a Computer Scientist from Java to Python. It is forthcoming from Cambridge University Press and will be published in the first quarter of 2009. The manuscript is available for free as a PDF under the title Think Python.

This book introduces computer science from first pricipals and introduces:

• Variables
• Functions
• Recursion
• Fundamental Data structures (Lists and Maps)

The Python Tutorial

Python has an excellent official tutorial. It covers:

• The Python Interpreter
• Python's Flow Control Statements
• Python's Built-in Data Structures (Lists, Sets, Dictionaries)
• Python's Modules
• I/O
• Errors, Exceptions, Classes, etc.

Dive Into Python

Dive Into Python is an excellent book for learning Python provided you already know how to program. It is published by Apress, but is also freely available online. It covers more advanced topics such as:

• Introspection
• Regular Expressions
• HTML and XML Processing
• HTTP and SOAP Web Services
• Unit Testing and Test First Programming
• Refactoring
• Functional Programming

In Python, Take I

A first crack written in python:

date_pattern = re.compile(r'^(?P<day>\d\d)/(?P<month>\d\d)/(?P<year>\d\d\d\d)$')

def parse_date(input):
    if not date_pattern.match(input):
        raise ValueError("'%s' is not in DD/MM/YYYY format" % input)

    day, month, year = map(int, input.split('/'))
    d = datetime.date(year, month, day)
    if d > datetime.date.today():
        raise ValueError("'%s' is in the future" % input)

    return d

1. We validate the date against a regex so that we know what we're dealing with.
2. We split up the input string and construct the date.
3. We test that the date is not in the future, and return the result.

This implementation is better than the CSOD in a number of ways:

1. It uses a regex to validate the format of the input string with is so much more faster/expressive/productive that writing our own validation code.
2. We use the platform's built in Date object rather than storing and manipulating the year/month/day ourselves which helps to avoid all kinds of silly bugs.

Unfortunately, we still parse and construct the date ourselves, duplicating functionality present in the standard library.

In Python, Take II

A second attempt, this time we're going to rely on strptime rather than parsing the string ourselves:

def parse_date(input):
    d = datetime.datetime.strptime(input, "%d/%m/%Y").date()
    if d > datetime.date.today():
        raise ValueError("'%s' is in the future" % input)
    return d

This implementation is even better as it relyies on strptime to handle the parsing/validating and the only real code that we write is testing if the date is in the future which is our logic.

In JavaScript

An implementation in JavaScript because the CSOD was submitted in JS. This is essentially a transcription of the first Python implementation as none of the JS date parsing utilities seem to take a formate string:

date_pattern = new RegExp('^\\d\\d/\\d\\d/\\d\\d\\d\\d$');

function parse_date(input){
    if(!date_pattern.test(input)){
        alert("'" + input + "' does not conform to the dd/mm/yyyy format");
    }

    ordinals = input.split('/');
    d = new Date(ordinals[2], ordinals[1] - 1, ordinals[0]);
    if(d > new Date()){
        alert("'" + input + "' is in the future");
    }

    return d;
}

It would be preferable to try and achieve the simplicity of the second Python implementation but that would require writing (or including third party code) comparable to strptime.

Aside

It is very, very, strange that months are 0 indexed while day and year are not in the Date constructor:

js> new Date('12/02/2008')
Tue Dec 02 2008 00:00:00 GMT-0500 (EST)
js> new Date(2008, 12, 02)
Fri Jan 02 2009 00:00:00 GMT-0500 (EST)
js> new Date(2008, 11, 02)
Tue Dec 02 2008 00:00:00 GMT-0500 (EST)

Seriously, what's up with that?