## DEV Community

Marius Borcan

Posted on • Originally published at programmerbackpack.com

# Decision Tree Classifier Tutorial in Python and Scikit-Learn

Decision Tree Classifier is a classification model that can be used for simple classification tasks where the data space is not huge and can be easily visualized. Despite being simple, it is showing very good results for simple tasks and outperforms other, more complicated models.

Interested in more stories like this? Follow me on Twitter at @b_dmarius and I'll post there every new article.

## Dataset for the Decision Tree Classifier

Recently I've created a small dummy dataset to use for simple classification tasks. I'll paste the dataset here again for your convenience.

The purpose of this data is, given 3 facts about a certain moment(the weather, whether it is a weekend or a workday or whether it is morning, lunch or evening), can we predict if there's a traffic jam in the city?

## Decision Tree Classifier in Python with Scikit-Learn

We have 3 dependencies to install for this project, so let's install them now. Obviously, the first thing we need is the scikit-learn library, and then we need 2 more dependencies which we'll use for visualization.

``````pip3 install scikit-learn
pip3 install matplotlib
pip3 install pydotplus
``````

Now let's import what we need from these packages.

``````from sklearn import preprocessing
from sklearn import tree
from IPython.display import Image
import pydotplus
``````

Now let's load our dataset. In a previous article on the Naive Bayes Classifier I've defined a few helper methods to easily load our simple dataset. Let's reuse that.

``````def getWeather():
return ['Clear', 'Clear', 'Clear', 'Clear', 'Clear', 'Clear',
'Rainy', 'Rainy', 'Rainy', 'Rainy', 'Rainy', 'Rainy',
'Snowy', 'Snowy', 'Snowy', 'Snowy', 'Snowy', 'Snowy']

def getTimeOfWeek():
return ['Workday', 'Workday', 'Workday',
'Weekend', 'Weekend', 'Weekend',
'Workday', 'Workday', 'Workday',
'Weekend', 'Weekend', 'Weekend',
'Workday', 'Workday', 'Workday',
'Weekend', 'Weekend', 'Weekend']

def getTimeOfDay():
return ['Morning', 'Lunch', 'Evening',
'Morning', 'Lunch', 'Evening',
'Morning', 'Lunch', 'Evening',
'Morning', 'Lunch', 'Evening',
'Morning', 'Lunch', 'Evening',
'Morning', 'Lunch', 'Evening',
]

def getTrafficJam():
return ['Yes', 'No', 'Yes',
'No', 'No', 'No',
'Yes', 'Yes', 'Yes',
'No', 'No', 'No',
'Yes', 'Yes', 'Yes',
'Yes', 'No', 'Yes'
]
``````

## Decision Tree Classifier - preprocessing

We know that computers have a really hard time when dealing with text and we can make their lives easier by converting the text to numerical values.

### Label Encoder

We will use this encoder provided by scikit to transform categorical data from text to numbers. If we have n possible values in our dataset, then LabelEncoder model will transform it into numbers from 0 to n-1 so that each textual value has a number representation.

For example, let's encode our time of day values.

`````` timeOfDay = ['Morning', 'Lunch', 'Evening',
'Morning', 'Lunch', 'Evening',
'Morning', 'Lunch', 'Evening',
'Morning', 'Lunch', 'Evening',
'Morning', 'Lunch', 'Evening',
'Morning', 'Lunch', 'Evening',
]
labelEncoder = preprocessing.LabelEncoder()
encodedTimeOfDay = labelEncoder.fit_transform(timeOfDay)
print (encodedTimeOfDay)

# Prints [2 1 0 2 1 0 2 1 0 2 1 0 2 1 0 2 1 0]
``````

## Training the Decision Tree Classifier model

Now let's train our model. So remember, since all our features are textual values, we need to encode all our values and only then we can jump to training.

``````if __name__=="__main__":
# Get the data
weather = getWeather()
timeOfWeek = getTimeOfWeek()
timeOfDay = getTimeOfDay()
trafficJam = getTrafficJam()

labelEncoder = preprocessing.LabelEncoder()

# Encode the features and the labels
encodedWeather = labelEncoder.fit_transform(weather)
encodedTimeOfWeek = labelEncoder.fit_transform(timeOfWeek)
encodedTimeOfDay = labelEncoder.fit_transform(timeOfDay)
encodedTrafficJam = labelEncoder.fit_transform(trafficJam)

# Build the features
features = []
for i in range(len(encodedWeather)):
features.append([encodedWeather[i], encodedTimeOfWeek[i], encodedTimeOfDay[i]])

classifier = tree.DecisionTreeClassifier()
classifier = classifier.fit(features, encodedTrafficJam)
``````

## Using our model for predictions

Now we can use the model we have trained to make predictions about the traffic jam.

`````` # ["Snowy", "Workday", "Morning"]
print(classifier.predict([[2, 1, 2]]))
# Prints [1], meaning "Yes"
# ["Clear", "Weekend", "Lunch"]
print(classifier.predict([[0, 0, 1]]))
# Prints [0], meaning "No"
``````

And it seems to be working! It correctly predicts the traffic jam situations given our data.

## Visualizing our model

Scikit also provides us with a way of visualizing a Decision Tree model. Here's a quick helper method I wrote to generate a .png image from our decision tree.

``````def printTree(classifier):
feature_names = ['Weather', 'Time of Week', 'Time of Day']
target_names = ['Yes', 'No']
# Build the daya
dot_data = tree.export_graphviz(classifier, out_file=None,
feature_names=feature_names,
class_names=target_names)
# Build the graph
graph = pydotplus.graph_from_dot_data(dot_data)

# Show the image
Image(graph.create_png())
graph.write_png("tree.png")
``````

And here's the result from that.

Yes, so I'm very happy with the results we got today. We used a really small dataset and a really small model, but I think we've learned a lot about this type of classifier.

If you want to explore more about the logic behind the Decision Tree Classifier, then please don't forget to read the other article in this mini-series: Decision Tree Classifiers Explained.

Thank you so much for reading this! Interested in more stories like this? Follow me on Twitter at @b_dmarius and I'll post there every new article.