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WekaPy v1.3.6

A simple Python module to provide a wrapper for some of the basic functionality of the Weka toolkit. The project focuses on the classification side of Weka, and does not consider clustering, distributions or any visualisation functions at this stage.

Weka is a machine learning tool, allowing you to classify data based on a set of its attributes and for generating predictions for unseen feature instances.

This module abstracts the use of ARFF files, making Weka much easier to use programmatically in Python.

Please note that this project is in very early stages of development and probably will not work in some cases.

Installation: pip install wekapy


  • Working Python installation
  • Working Java installation
  • weka.jar
    • Download the JAR file from Weka's website
    • Place the weka.jar file in your system's Java classpath or in your project's directory (or, at least, somewhere you can import it into your project from)

Current features:

  • Create a classifier instance
    • Tested with Bayesian Networks and Trees, but should be compatible with most classifier types
  • Train the classifier with a training dataset
    • Provide an ARFF file containing the instances you with to train the model with
    • Or, provide a list of Instance objects
  • Produce predictions for a test dataset
    • Provide an ARFF file containing the instances you wish to test on the trained model
    • Or, provide a list of Instance objects
    • Generates a list of Prediction objects which can then be retrieved from the Model object
  • Save / load models
    • Save a model trained using WekaPy
    • Load a pre-trained model (from Weka, previous uses of WekaPy, etc.)
  • Exporting data to ARFF format
    • WekaPy can generate ARFF files for your training and/or test data
    • Useful on its own if you'd rather use the GUI for making classifications
  • Filter data
    • WekaPy can be used to filter input data prior to training/testing models
    • Can use any of the weka.filters classes to filter specified input data

Example usage

Please see the examples/ directory for full example uses. These include:

  • Programmatically training and testing a model (
  • Training and testing a model using ARFF files (
  • Training and saving a model to file for testing with later (
  • Loading a trained model from file and testing against it (
  • Filtering an input data set (

For more detailed documentation, please read on.

The module can be very easily used as shown in the following example. The functionality is built around the Model class, which requires a classifier type when constructed.

from wekapy import *

model = Model(classifier_type = "trees.J48")
model.train(training_file = "train.arff")
model.test(test_file = "test.arff")

The list of predictions can then be retrieved like so:

prediction = model.predictions[0]
observed_value = prediction.observed_value
predicted_value = prediction.predicted_value
probability = prediction.probability

More detailed documentation

  1. Model construction

1.1 Standard construction

Construct the Model object very simply:

model = Model(classifier_type = "trees.J48")

1.2 Optional arguments

Currently, the Model class also supports the following additional arguments during construction:

  • verbose (True by default)
    • set verbose = False to prevent the module from printing out status reports to STDOUT (fatal errors will still be printed)
  • max_memory (1500 by default)
    • change the value of max_memory to set the maximum allowable memory (in MB) for the Java Virtual Machine to run with Weka
    • The Model itself does not use this memory - only Weka when it is training and testing
    • You may need to increase this if you get stack overflow Exceptions
    • You may need to decrease this if your machine does not have enough RAM to support this

For example, to instantiate the Model object with additional arguments, you could use:

model = Model(classifier_type = "bayes.BayesNet", verbose = False, max_memory = 1000)
  1. Training the model

Currently, there are two methods for training the model - either through ARFF files or by providing a list of instances. Both require the Model object to be instantiated first (see above).

When training the model, any models trained previously with this Model object will be replaced by the new model.

You will need to provide either an ARFF file or a list of Instances in order for the train to be successful.

2.1 Using ARFF Files

Generate an ARFF file and pass this to the train() function:

model.train(training_file = "train.arff")

2.2 Using the Instance object

If you would rather carry this out programmatically, then you can instead provide a list of Instance objects.

An Instance simply contains a list of Features, and can be instantiated as follows:

instance1 = Instance()

feature1 = Feature(name="num_milkshakes",value=46,possible_values="real")
feature2 = Feature(name="is_sunny",value=True,possible_values="{False, True}")
feature3 = Feature(name="boys_in_yard",value=True,possible_values="{False ,True}")


instance2  = Instance()

The final feature in each instance will be the that predictions are made against (the 'class').

When you've created all of your training instances, add them to your untrained model:


Finally, train the model:


In the background, WekaPy generates an ARFF file and saves this and the trained model in its data directories. If desired, these can be found using:

training_arff = model.training_file
trained_model = model.model_file

** 2.3 Optional arguments**

To configure the training more precisely, you can also set a different directory for the model and specify the number of cross-validation folds the training algorithm will carry out:

  • save_as (hidden, by default, to improve seamlessness of use)
    • Set save_as = "path/to/model" to save the model in a different directory and with your own name.
    • This saved model can then be used later by passing it to the test() method as described later.
  • folds (10 by default)
    • Set folds = x to specify the number of cross-validation folds you want the algorithm to carry out.
    • If your Instance list is short, you may need to reduce this.
  • instances
    • Pass a list of instances to train() instead of using add_train_instance(), if desired.
  • training_file
    • Pass a training ARFF file to train() instead of programmatically adding features. This method is covered in section 2.1.
  1. Testing with the trained model

As with the training, there are two methods for testing with the model. You must provide either an ARFF file or a list of Instances for the testing to be successful.

3.1 Using ARFF files

Test using your own ARFF file as follows:

model.test(test_file = "test.arff")

3.2 Using a list of Instances

Generate a list of Instances as described earlier. When testing, if the outcome feature is unknown, then use a "?" to signify this. For example:

test_instance1 = Instance()

test_feature1 = Feature(name="num_milkshakes",value=5,possible_values="real")
test_feature2 = Feature(name="is_sunny",value=False,possible_values="{False, True}")
test_feature3 = Feature(name="boys_in_yard",value="?",possible_values="{False, True}")


test_instance2 = Instance()

Now add the testing instances to the model and test them:



As before, an ARFF file is generated and this is used to test against the model.

3.3 Optional arguments

You can specify the use of a different model for testing against, and thus skip out the train() section, if you desire. This could be useful if you have already used train() and chose to save the model elsewhere, you have trained the model using Weka's GUI, using someone else's model, etc.

  • model_file (None by default)
    • Set model_file = "path/to/model.model" to test with this model instead.
    • Any models trained previously will be discarded by the current Model object and replaced by this one.
  • instances
    • Pass a list of Instances to test() instead of using the add_test_instance() method demonstrated in 3.2.
  • test_file
    • Pass a test file to test() as demonstrated in 3.1.

4 Accessing the predictions

If the testing is successful, a list of Predictions will be generated, containing a Prediction object for each Instance in the test ARFF file or the list of test Instances.

Below is an example demonstrating how to access the Predictions:

predictions = model.predictions
for prediction in predictions:
    print prediction

4.1 Further information

For each Prediction object, these fields are available:

  • index - integer representing the number of that Prediction. This equates to that Instance in the test Instance set or ARFF file. For example, the Prediction with index = 1 is the prediction for the first instance in the test set.
  • observed_category - integer representing the category number of the observed value (will not be available if observed value is unknown)
  • observed_value - the observed outcome feature for this Instance
  • predicted_category - integer representing the category number of the predicted value
  • predicted_value - the predicted outcome feature for this Instance
  • error - this will be True if the predicted value differs from the observed value. Therefore, this will be unavaialble if the observed value is unknown.
  • probability - the probability with which the classifier believes the predicted value to be correct.

5 Filtering Input data

Occasionally it may be necessary to carry out some filtering on input data prior to training/testing a model. For example, it may be necessary to reduce the number of attributes in the input data, or split the data into training/testing instances.


Simple Python wrapper for the Weka toolkit






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