Glmnet Wrappers for Python

This package provides a convenient python interface to Jerome Friedman's Fortran implementation of glmnet. It provides Elastic and Logistic net models, cross validation, and plotting utilities.

It is very much a work in progress.

Authors

• David Warde-Farley (2010)
• Matthew Drury (2014)

Use

To create an elastic net model:

from glmnet import ElasticNet
enet = ElasticNet(alpha=.1)

(Here, alpha is the relative weighting between the L1 and L2 regularization terms). The model can then be fit using a design matrix X and a response vector y:

enet.fit(X, y)

glmnet also accepts sparse design matrices, using the compressed sparse column format:

from scipy.sparse import csc_matrix
Xsp = csc_matrix(X)
enet.fit(Xsp, y)

Fitting a glmnet automatically fits models for multiple values of lambda, the overall amount of regularization.

After fitting, the model can be used to generate predictions on new data:

enet.predict(X')

note that this generates predictions for each lambda glmnet chose during fitting.

or get a summary of the model:

enet.describe()

A elastic net model with alpha = 0.1.
The model was fit on 100 observations and 5 parameters.     
The model was fit in 169 passes over the data.                
There were 77 values of lambda resulting in non-zero models. 
There were 4 non-zero coefficients in the largest model.    

Passing a lambda index into describe will display the fit coefficients:

enet.describe(lidx=20)

Parameter Estiamtes for elastic net model with lambda = 7.12026e-02
Varaible Name                           Coefficent Estiamte
C(catagorical_var)[A]                   -2.17143e-01
C(catagorical_var)[B]                   2.13176e-01
first_var                               5.43638e-01
another_var                             0.00000e+00
the_last_var                            2.72075e-01

The parameter paths can also be visualized, that is, the values of the model parameters for each lambda:

enet.plot_paths()

To select a value of lambda cross-validation is provided:

from glmnet import ElasticNet, CVGlmNet
enet = ElasticNet(alpha=.1)
enet_cv = CVGlmNet(enet, folds=10, n_jobs=10)
enet_cv.fit(X, y)

Glmnet then fits ten models for each value of lambda and chooses the best model by observing which optimizes the out of fold deviance.

Once the cross validation is fit, the mean out of fold deviances for each value of lambda can be viewed, along with their standard deviations:

enet_cv.plot_oof_devs()

The cross validation object can then be used to generate predictions at the optimal value of lambda:

enet_cv.predict(X')

Note: glmnet uses the joblib.Parallel function to parallelize its fitting across folds, there is a known bug in some versions of OSX where using this causes a race condition and the fitting will hang. Setting n_jobs=1 will disable the cross validation, at the expense of fitting the models in series. If using the anaconda distribution of python, enabling the mkl optimizations will allow the models to be fit in parallel. The parallelization has also been tested on various linux boxes with no issues. See this sklearn issue for more information.

Building

A build script has been provided in the glmnet/glmnet directory, so to build the fortran extension:

cd glmnet/glmnet
source build.sh

Once the build is complete, you should have a _glmnet.so file in the glmnet directory. Dropping into a python shell:

import _glmnet

Should work without error.

AFAIK, this package requires gfortran to build. g77 will not work as it does not support -fdefault-real-8.

In order to get double precision working without modifying Friedman's code, some compiler trickery is required. The wrappers have been written such that everything returned is expected to be a real*8 i.e. a double-precision floating point number, and unfortunately the code is written in a way Fortran is often written with simply real specified, letting the compiler decide on the appropriate width. f2py assumes real are always 4 byte/ single precision, hence the manual change in the wrappers to real*8, but that change requires the actual Fortran code to be compiled with 8-byte reals, otherwise bad things will happen (the stack will be blown, program will hang or segfault, etc.).

Planned Enhancements

• Improved interface with cross validation.
• Pretty graphs with better knobs and dials.
• Wrapper classes for the Poisson and Cox models.

License

Friedman's code in glmnet.f is released under the GPLv2, necessitating that any code that uses it (including my wrapper, and anyone using my wrapper) be released under the GPLv2 as well. See LICENSE for details.

That said, to the extent that they are useful in the absence of the GPL Fortran code (i.e. not very), the other portions may be used under the 3-clause BSD license.

Thanks

• To Jerome Friedman for the fantastically fast and efficient Fortran code.
• To Pearu Peterson for writing f2py and answering my dumb questions.
• To Dag Sverre Seljebotn for his help with f2py wrangling.
• To Kevin Jacobs for showing me his wrapper which helped me side-step some problems with the auto-generated .pyf.

References

• J Friedman, T Hastie, R Tibshirani (2010). "Regularization Paths for Generalized Linear Models via Coordinate Descent".
• J Friedman, T Hastie, H Hofling, R Tibshirani (2007). "Pathwise Coordinate Optimization".