DeepLIFT: Deep Learning Important FeaTures
This branch has been developed for keras 2.0 models and tensorflow 1.7. See the tags for older versions.
Algorithms for computing importance scores in deep neural networks. Implements the methods in "Learning Important Features Through Propagating Activation Differences" by Shrikumar, Greenside & Kundaje, as well as other commonly-used methods such as gradients, guided backprop and integrated gradients.
Please be aware that figuring out optimal references is still an unsolved problem and we are actively working on a principled solution. Suggestions on good heuristics for different applications are welcome.
Please feel free to follow this repository to stay abreast of updates.
Table of contents
- Under The Hood
DeepLIFT is on pypi, so it can be installed using pip:
pip install deeplift
If you want to be able to make edits to the code, it is recommended that you clone the repository and install using the
git clone https://github.com/kundajelab/deeplift.git #will clone the deeplift repository pip install --editable deeplift/ #install deeplift from the cloned repository. The "editable" flag means changes to the code will be picked up automatically.
While DeepLIFT does not require your models to be trained with any particular library, we have provided autoconversion functions to convert models trained using Keras into the DeepLIFT format. If you used a different library to train your models, you can still use DeepLIFT if you recreate the model using DeepLIFT layers.
This implementation of DeepLIFT was tested with tensorflow 1.7, and autoconversion was tested using keras 2.0.
These examples show how to autoconvert a keras model and obtain importance scores. Non-keras models can be converted to DeepLIFT if they are saved in the keras 2.0 format
#Convert a keras sequential model import deeplift from deeplift.conversion import kerasapi_conversion as kc #NonlinearMxtsMode defines the method for computing importance scores. #NonlinearMxtsMode.DeepLIFT_GenomicsDefault uses the RevealCancel rule on Dense layers #and the Rescale rule on conv layers (see paper for rationale) #Other supported values are: #NonlinearMxtsMode.RevealCancel - DeepLIFT-RevealCancel at all layers (used for the MNIST example) #NonlinearMxtsMode.Rescale - DeepLIFT-rescale at all layers #NonlinearMxtsMode.Gradient - the 'multipliers' will be the same as the gradients #NonlinearMxtsMode.GuidedBackprop - the 'multipliers' will be what you get from guided backprop #Use deeplift.util.get_integrated_gradients_function to compute integrated gradients #Feel free to email avanti [dot] email@example.com if anything is unclear deeplift_model =\ kc.convert_model_from_saved_files( saved_hdf5_file_path, nonlinear_mxts_mode=deeplift.layers.NonlinearMxtsMode.DeepLIFT_GenomicsDefault) #Specify the index of the layer to compute the importance scores of. #In the example below, we find scores for the input layer, which is idx 0 in deeplift_model.get_layers() find_scores_layer_idx = 0 #Compile the function that computes the contribution scores #For sigmoid or softmax outputs, target_layer_idx should be -2 (the default) #(See "3.6 Choice of target layer" in https://arxiv.org/abs/1704.02685 for justification) #For regression tasks with a linear output, target_layer_idx should be -1 #(which simply refers to the last layer) #If you want the DeepLIFT multipliers instead of the contribution scores, you can use get_target_multipliers_func deeplift_contribs_func = deeplift_model.get_target_contribs_func( find_scores_layer_idx=find_scores_layer_idx, target_layer_idx=-1) #You can also provide an array of indices to find_scores_layer_idx to get scores for multiple layers at once #compute scores on inputs #input_data_list is a list containing the data for different input layers #eg: for MNIST, there is one input layer with with dimensions 1 x 28 x 28 #In the example below, let X be an array with dimension n x 1 x 28 x 28 where n is the number of examples #task_idx represents the index of the node in the output layer that we wish to compute scores. #Eg: if the output is a 10-way softmax, and task_idx is 0, we will compute scores for the first softmax class scores = np.array(deeplift_contribs_func(task_idx=0, input_data_list=[X], batch_size=10, progress_update=1000))
This will work for sequential models involving dense and/or conv1d/conv2d layers and linear/relu/sigmoid/softmax or prelu activations. Please create a github issue or email avanti [dot] firstname.lastname@example.org readme if you are interested in support for other layer types.
The syntax for using functional models is similar:
deeplift_model =\ kc.convert_model_from_saved_files( saved_hdf5_file_path, nonlinear_mxts_mode=deeplift.layers.NonlinearMxtsMode.DeepLIFT_GenomicsDefault) #The syntax below for obtaining scores is similar to that of a converted graph model #See deeplift_model.get_name_to_layer().keys() to see all the layer names #As before, you can provide an array of names to find_scores_layer_name #to get the scores for multiple layers at once deeplift_contribs_func = deeplift_model.get_target_contribs_func( find_scores_layer_name="name_of_input_layer", pre_activation_target_layer_name="name_goes_here")
A notebook replicating the results in the paper on MNIST is at
examples/mnist/MNIST_replicate_figures.ipynb, and a notebook demonstrating use on a genomics model with 1d convolutions is at
Can you provide a brief intuition for how DeepLIFT works?
The 15-minute talk from ICML gives an intuition for the method. Here are links to the slides and the video (the video truncates the slides, which is why the slides are linked separately). Please file a github issue if you have questions.
Ancona et al., authors of the DeepExplain repository, leveraged overriding of the gradient operators in Tensorflow to implement the Rescale rule of DeepLIFT. Their implementation can work with a wider variety of architectures than the DeepLIFT implementation in this repository, and is potentially more computationally efficient, but it does not have the advantages of the RevealCancel rule (which deals with failure modes such as the min function). Note that their implementation can work with architectures that DeepLIFT was not designed for, such as LSTMs and GRUs (their implementation would use the standard gradient backpropagation rule in all cases where the gradient operator was not overridden; note that this may lead to unexpected results as the summation-to-delta property would be violated at the multiplicative units, i.e. the gates). We have not studied the appropriateness of this approach for architectures with multiplicative units, but the authors did find that “Integrated Gradients and DeepLIFT have very high correlation, suggesting that the latter is a good (and faster) approximation of the former in practice”.
Poerner et al. conducted a series of benchmarks comparing DeepLIFT to other explanation methods on NLP tasks. Their implementation differs from the canonical DeepLIFT implementation in two main ways. First, they considered only the Rescale rule of DeepLIFT (according to the implementation here). Second, to handle operations that involve multiplications with gating units (which DeepLIFT was not designed for), they treat the gating neuron as a weight (similar to the approach in Arras et al.) and assign all importance to the non-gating neuron. Note that this differs from the implementation in Ancona et al., which handles multiplications with gates using the standard gradient backpropagation rule (and thus would assign importance to the gating neuron). We have not studied the appropriateness of this approach, but the authors did find that "LIMSSE, LRP (Bach et al., 2015) and DeepLIFT (Shrikumar et al., 2017) are the most effective explanation methods (§4): LRP and DeepLIFT are the most consistent methods, while LIMSSE wins the hybrid document experiment."
Do you have support for non-keras models?
At the moment, we do not. However, if you are able to convert your model into the saved file format used by the Keras 2 API, then you can use this branch to load it into the DeepLIFT format. For inspiration on how to achieve this, you can look at
examples/convert_models/keras1.2to2 for a notebook demonstrating how to convert models saved in the keras1.2 format to keras 2. DeepLIFT conversion works directly from keras saved files without ever actually loading the models into keras.
What do negative scores mean?
A negative contribution score on an input means that the input contributed to moving the output below its reference value, where the reference value of the output is the value that it has when provided the reference input. A negative contribution does not mean that the input is "unimportant". If you want to find inputs that DeepLIFT considers "unimportant" (i.e. DeepLIFT thinks they don't influence the output of the model much), these would be the inputs that have contribution scores near 0.
How do I provide a reference argument?
Just as you supply
input_data_list as an argument to the scoring function, you can also supply
input_references_list. It would have the same dimensions as
input_data_list, but would contain reference images for each input.
What should I use as my reference?
The choice of reference depends on the question you wish to ask of the data. Generally speaking, the reference should retain the properties you don't care about and scramble the properties you do care about. In the supplement of the DeepLIFT paper, Appendix L looks at the results on a CIFAR10 model with two different choices of the reference. You'll notice that when a blurred version of the input is used as a reference, the outlines of objects stand out. When a black reference is used, the results are more confusing, possibly because the net is also highlighting color. One thing to consider is using multiple different references to interpret a single image and averaging the results over all the different references. We use this approach in genomics; we generate a collection of references per input sequence by shuffling the sequence (this is demonstrated in the genomics example notebook).
How can I get a sense of how much an input contributes across all examples?
It is fine to average the DeepLIFT contribution scores across examples. Be aware that there might be considerable heterogeneity in your data (i.e. some inputs may be very important for some subset of examples but not others) so clustering may prove more insightful than averaging.
Can I have multiple input modes?
Yes. Rather than providing a single numpy array to input_data_list, provide a list of numpy arrays containing the input to each mode. You can also provide a dictionary to input_data_list where the key is the mode name and the value is the numpy array. Each numpy array should have the first axis be the sample axis.
Can I get the contribution scores on multiple input layers at once?
Also yes. Just provide a list to
find_scores_layer_name rather than a single argument.
How does DeepLIFT compare to integrated gradients?
As illustrated in the DeepLIFT paper, the RevealCancel rule of DeepLIFT can allow DeepLIFT to properly handle cases where integrated gradients may give misleading results. Independent researchers have found that DeepLIFT with just the Rescale rule performs comparably to Integrated Gradients (they write: “Integrated Gradients and DeepLIFT have very high correlation, suggesting that the latter is a good (and faster) approximation of the former in practice”). Their finding was consistent with our own personal experience. The speed improvement of DeepLIFT relative to Integrated Gradients becomes particularly useful when using a collection of references (since having a collection of references per example increases runtime).
What's the license?
MIT License. While we had originally filed a patent on some of our interpretability work, we have since disbanded the patent as it appears this project has enough interest from the community to be best distributed in open-source format.
I have heard DeepLIFT can do pattern discovery - is that right?
Please email avanti [dot] shrikumar [at] gmail.com with questions, ideas, feature requests, etc. If I don't respond, keep emailing me until I feel guilty and respond. Also feel free to email my adviser (anshul [at] kundaje [dot] net), who can further guilt me into responding. I promise I do actually want to respond; I'm just busy with other things because the incentive structure of academia doesn't reward maintenance of projects.
Under the hood
This section explains finer aspects of the deeplift implementation
The layer (
deeplift.layers.core.Layer) is the basic unit.
deeplift.layers.convolution.Conv2D are both examples of layers.
Layers implement the following key methods:
Returns symbolic variables representing the activations of the layer. For an understanding of symbolic variables, refer to the documentation of symbolic computation packages like theano or tensorflow.
get_pos_mxts() and get_neg_mxts()
Returns symbolic variables representing the positive/negative multipliers on this layer (for the selected output). See paper for details.
Returns symbolic variables representing the importance scores. This is a convenience function that returns
self.get_pos_mxts()*self._pos_contribs() + self.get_neg_mxts()*self._neg_contribs(). See paper for details.
The Forward Pass
Here are the steps necessary to implement a forward pass. If executed correctly, the results should be identical (within numerical precision) to a forward pass of your original model, so this is definitely worth doing as a sanity check. Note that if autoconversion (as described in the quickstart) is an option, you can skip steps (1) and (2).
- Create a layer object for every layer in the network
- Tell each layer what its inputs are via the
set_inputsfunction. The argument to
set_inputsdepends on what the layer expects
- If the layer has a single layer as its input (eg: Dense layers), then the argument is simply the layer that is the input
- If the layer takes multiple layers as its input, the argument depends on the specific implementation - for example, in the case of a Concat layer, the argument is a list of layers
- Once every layer is linked to its inputs, you may compile the forward propagation function with
- If you are working with a model produced by autoconversion, you can access individual layers via
model.get_layers()for sequential models (where this function would return a list of layers) or
model.get_name_to_layer()for Graph models (where this function would return a dictionary mapping layer names to layers)
- The first argument is a list of symbolic tensors representing the inputs to the net. If the net has only one input layer, then this will be a list containing only one tensor
- The second argument is the output of the function. In the example above, it is a single tensor, but it can also be a list of tensors if you want the outputs of more than one layer
- Once the function is compiled, you can use
deeplift.util.run_function_in_batches(func, input_data_list)to run the function in batches (which would be advisable if you want to call the function on a large number of inputs that wont fit in memory)
funcis simply the compiled function returned by
input_data_listis a list of numpy arrays containing data for the different input layers of the network. In the case of a network with one input, this will be a list containing one numpy array
- Optional arguments to
The Backward Pass
Here are the steps necessary to implement the backward pass, which is where the importance scores are calculated. Ideally, you should create a model through autoconversion (described in the quickstart) and then use
model.get_target_multipliers_func. Howver, if that is not an option, read on (please also consider sending us a message to let us know, as if there is enough demand for a feature we will consider adding it). Note the instructions below assume you have done steps (1) and (2) under the forward pass section.
For the layer(s) that you wish to compute the importance scores for, call
reset_mxts_updated(). This resets the symbolic variables for computing the multipliers. If this is the first time you are compiling the backward pass, this step is not strictly necessary.
For the output layer(s) containing the neuron(s) that the importance scores will be calculated with respect to, call
- Briefly, this is the scoring mode that is used when we want to find scores with respect to a single target neuron. Other kinds of scoring modes may be added later (eg: differences between neurons).
- A point of clarification: when we eventually compile the function, it will be a function which computes scores for only a single output neuron in a single layer every time it is called. The specific neuron and layer can be toggled later, at runtime. Right now, at this step, you should call
set_scoring_modeon all the target layers that you might conceivably want to find the scores with respect to. This will save you from having to recompile the function to allow a different target layer later.
- For Sigmoid/Softmax output layers, the output layer that you use should be the linear layer (usually a Dense layer) that comes before the final nonlinear activation. See "3.6 Choice of target layer" in the paper for justification. If there is no final nonlinearity (eg: in the case of many regression tasks), then the output layer should just be the last linear layer.
- For Softmax outputs, you should may want to subtract the average contribution to all softmax classes as described in "Adjustments for softmax layers" in the paper (section 3.6). If your number of softmax classes is very large and you don't want to calculate contributions to each class separately for each example, contact me (avanti [dot] email@example.com) and I can implement a more efficient way to do the calculation (there is a way but I haven't coded it up yet).
For the layer(s) that you wish to compute the importance scores for, call
update_mxts(). This will create the symbolic variables that compute the multipliers with respect to the layer specified in step 2.
Compile the importance score computation function with
deeplift.backend.function([input_layer.get_activation_vars()..., input_layer.get_reference_vars()...], layer_to_find_scores_for.get_target_contrib_vars())
- The first argument represents the inputs to the function and should be a list of one symbolic tensor for the activations of each input layer (as for the forward pass), followed by a list of one symbolic tensor for the references of each input layer
- The second argument represents the output of the function. In the example above, it is a single tensor containing the importance scores of a single layer, but it can also be a list of tensors if you wish to compute the scores for multiple layers at once.
- Instead of
get_target_contrib_vars()which returns the importance scores (in the case of
NonlinearMxtsMode.DeepLIFT, these are called "contribution scores"), you can use
get_neg_mxts()to get the multipliers.
Now you are ready to call the function to find the importance scores.
- Select a specific output layer to compute importance scores with respect to by calling
set_active()on the layer.
- Select a specific target neuron within the layer by calling
update_task_index(task_idx)on the layer. Here
task_idxis the index of a neuron within the layer.
- Call the function compiled in step 4 to find the importance scores for the target neuron. Refer to step 4 in the forward pass section for tips on using
deeplift.util.run_function_in_batchesto do this.
- Deselect the output layer by calling
set_inactive()on the layer. Don't forget this!
- (Yes, I will bundle all of these into a single function at some point)
- Select a specific output layer to compute importance scores with respect to by calling