Getting Started With Python for Data Science

By Chris Clark, 07/20/2012, in Code & tutorials

As the product manager at Kaggle I'm fortunate enough to hang around with some of the world's top machine learning experts. And working at a place that specializes in running predictive modelling competitions means that I inevitably got the itch to learn some of this stuff myself.

I don't have a super-strong math or machine learning background, but I figured I'm a decent-enough programmer and have all these smart people around me, so I figured I should be able to figure out how to use some off-the-shelf tools to build some predictive models of my own.

I wrote the following tutorial after my experience hacking away on a Kaggle competition for a weekend. I hope it serves to get other programming-literate but machine learning-ignorant people like me up and running, building their own models, and learning data science. So without further ado...

Who is this for and what will I learn?

(Cross posted on Kaggle's blog, No Free Hunch.)

This tutorial assumes some knowledge of Python and programming, but no knowledge whatsoever of data science, machine learning, or predictive modeling (or, heck, even statistics). To the extent there is a target audience, it's probably hacker types who learn best by doing.

All the code from this tutorial is available on github.

You might encounter terms you're not familiar with, but that shouldn't stop you from completing the tutorial. By the end, you won't know a heck of a lot more about data science per se, but you'll have a nice environment set up where you can easily play with lots of different data science tools and even make credible entries to Kaggle competitions. Most importantly you'll be in a great position to experiment and learn more data science.

Here's what you'll learn:

Excited? I thought so!

1. Environment Setup

First thing, we'll need a Python environment suitable for scientific and statistical computing. Assuming you already have Python installed (no? Well then get it! Python 2.7 is recommended), we'll need three packages. You should install each in the order they appear here:

Note: 64 bit versions of these libraries can be found here.

Click through the links above for the home pages of each project and get the installation for your operating system or, if you're running Linux, you can install from a package manager (pip). If you're on a Windows machine, it's easiest to install using the setup executables for scipy and scikit-learn rather than installing from a package manager.

I'd also highly recommend to setting up a decent Python development environment. You can certainly execute Python scripts from the command line, but it's a heck of a lot easier to use a proper environment with debugging support. I use PyDev, but even something like IPython is better than nothing.

Now you're ready for machine learning greatness!

2. Your First Submission

The Biological Response competition provides a great data set to get started with because the value to be predicted is a simple binary classifier (0 or 1) and the data is just a bunch of numbers, so feature extraction and selection aren't as important as in some other Kaggle competitions. Download the training and test data sets now. Even though this competition is over, you can still make submissions and see how you compare to the world's best data scientists.

In the code below, we'll use an ensemble classifier called a random forest that often performs very well as-is, without much babysitting and parameter-tweaking. Although a random forest is actually a pretty sophisticated classifier, it's a piece of cake to get up and running with one thanks to sklearn.

Remember: You don't have to understand all of the underlying mathematics to use these techniques. Experimentation is a great way to start getting a feel for how this stuff works. Understanding the models is important, but it's not necessary to get started, have fun, and compete.

Here's the code:

from sklearn.ensemble import RandomForestClassifier
from numpy import genfromtxt, savetxt

def main():
    #create the training & test sets, skipping the header row with [1:]
    dataset = genfromtxt(open('Data/train.csv','r'), delimiter=',', dtype='f8')[1:]    
    target = [x[0] for x in dataset]
    train = [x[1:] for x in dataset]
    test = genfromtxt(open('Data/test.csv','r'), delimiter=',', dtype='f8')[1:]

    #create and train the random forest
    #multi-core CPUs can use: rf = RandomForestClassifier(n_estimators=100, n_jobs=2)
    rf = RandomForestClassifier(n_estimators=100)
    rf.fit(train, target)
    predicted_probs = [x[1] for x in rf.predict_proba(test)]

    savetxt('Data/submission.csv', predicted_probs, delimiter=',', fmt='%f')

if __name__=="__main__":
    main()

At this point you should go ahead and actually get this running by plopping the into a new Python script and saving it as makeSubmission.py. You should now have directories and files on your computer like this:

"My Kaggle Folder")
 |
 |---"Data"
 |   |
 |   |---train.csv
 |   |
 |   |---test.csv
 |
 |---makeSubmission.py

Once you've run makeSubmission, you'll also have my_first_submission.csv in your data folder. Now it is time to do something very important:

Submit this file to the bio-response competition

Did you do it? You did it! Great! You could stop here - you know how to make a successful Kaggle entry - but if you're the curious type, I'm sure you'll want to play around with other types of models, different parameters, and other shiny things. Keep reading to learn an important technique that will let you test models locally, without burning through your daily Kaggle submission limit.

3. Evaluation and Cross-Validation

Let's say we wanted to try out sklearn's gradient boosting machine instead of a random forest. Or maybe some simple linear models. It's easy enough to important these things from sklearn and generate submission files, but it's not so easy to compare their performance. It's not practical to upload a new submission every time we make a change to the model - we'll need a way to test things out locally, and we'll need two things in order to do that:

  1. An evaluation function
  2. Cross validation

You'll always need some kind of evaluation function to determine how your models are performing. Ideally, these would be identical to the evaluation metric that Kaggle is using to score your entry. Competition participants often post evaluation code in the forums, and Kaggle has detailed descriptions of the metrics available on the wiki. In the case of the bio-response competition, the evaluation metric is log-loss and user Grunthus has posted a Python version of it. We won't spend too much time on this (read the forum post for more information), but go ahead and save the following into your working directory as logloss.py.

import scipy as sp
def llfun(act, pred):
    epsilon = 1e-15
    pred = sp.maximum(epsilon, pred)
    pred = sp.minimum(1-epsilon, pred)
    ll = sum(act*sp.log(pred) + sp.subtract(1,act)*sp.log(sp.subtract(1,pred)))
    ll = ll * -1.0/len(act)
    return ll

Finally, we'll need data to test our models against. When you submitted your first Kaggle competition entry earlier in this tutorial, Kaggle compared (using log-loss) your answers to the actual real world results (the "ground truth") associated with the test data set. Without access to those answers, how can we actually test our models locally? Cross-validation to the rescue!

Cross-validation is a simple technique that basically grabs a chunk of the training data and holds it in reserve while the model is trained on the remainder of the data set. In case you haven't realized it yet, sklearn it totally awesome and is here to help. It has built in tools to generate cross validation sets. The sklearn documentation has a lot of great information on cross-validation.

The code below creates 10 cross-validation sets (called folds), each with 10% of the training data set held in reserve, and tests our random forest model against that withheld data.

from sklearn.ensemble import RandomForestClassifier
from sklearn import cross_validation
import logloss
import numpy as np

def main():
    #read in  data, parse into training and target sets
    dataset = np.genfromtxt(open('Data/train.csv','r'), delimiter=',', dtype='f8')[1:]    
    target = np.array([x[0] for x in dataset])
    train = np.array([x[1:] for x in dataset])

    #In this case we'll use a random forest, but this could be any classifier
    cfr = RandomForestClassifier(n_estimators=100)

    #Simple K-Fold cross validation. 5 folds.
    cv = cross_validation.KFold(len(train), k=5, indices=False)

    #iterate through the training and test cross validation segments and
    #run the classifier on each one, aggregating the results into a list
    results = []
    for traincv, testcv in cv:
        probas = cfr.fit(train[traincv], target[traincv]).predict_proba(train[testcv])
        results.append( logloss.llfun(target[testcv], [x[1] for x in probas]) )

    #print out the mean of the cross-validated results
    print "Results: " + str( np.array(results).mean() )

if __name__=="__main__":
    main()

Note that your cross-validated results might not exactly match the score Kaggle gives you on the model. This could be for a variety of (legitimate) reasons: random forests have a random component and won't yield identical results every time; the actual test set might deviate from the training set (especially when the sample size is fairly low, like in the bio-response competition); the evaluation implementation might differ slightly. Even if you are getting slightly different results, you can compare model performance locally and you should know when you have made an improvement.

That's a wrap!

That's it! We're done! You now have great tools at your disposal, and I expect to see you at the top of the leaderboard in no time!

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