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"""Modified version of Driverless AI's internal LightGBM implementation with a custom objective function (used for tree split finding).
"""
from h2oaicore.models import BaseCustomModel, LightGBMModel
import numpy as np
# The custom objective function will be pickled along with the underlying LightGBM model for persistance purposes
# as a result it can't a lambda function or a method of the custom model object
# The only option is to make the function global in the following manner
def custom_asymmetric_objective(y_true, y_pred):
"""Asymetric MSE loss
A custom loss has to return the gradient and the hessian of the loss or objective function
This is not an evaluation but the loss LightGBM will optimize during training
"""
residual = (y_true - y_pred).astype("float")
grad = np.where(residual < 0, -2 * 10.0 * residual, -2 * residual)
hess = np.where(residual < 0, 2 * 10.0, 2.0)
return grad, hess
class MyLGBMAsymMSE(BaseCustomModel, LightGBMModel):
"""Custom model class that re-uses DAI LightGBMModel
The class inherits :
- BaseCustomModel that really is just a tag. It's there to make sure DAI knows it's a custom model and not
its inner LightGBM Model
- LightGBMModel object so that the custom model inherits all the properties and methods, especially for params
mutation
"""
# The loss is a regression loss
_regression = True
# The loss is not for classification
_binary = False
_multiclass = False # WIP
_mojo = True
# Give the display name and description that will be shown in the UI
_display_name = "MyLGBMAsymMSE"
_description = "LightGBM with custom asymetric loss/objective"
_is_reproducible = False # not be exactly reproducible on GPUs
def set_default_params(self,
accuracy=None, time_tolerance=None, interpretability=None,
**kwargs):
# Define the global loss
# global custom_asymmetric_objective
# First call the LightGBM set_default_params
# This will input all model parameters just like DAI would do.
LightGBMModel.set_default_params(
self,
accuracy=accuracy,
time_tolerance=time_tolerance,
interpretability=interpretability,
**kwargs
)
# Now we just need to tell LightGBM that it has to optimize for our custom objective
# And we are done
self.params["objective"] = custom_asymmetric_objective
def mutate_params(
self, get_best=False, time_tolerance=10, accuracy=10, interpretability=1,
imbalance_ratio=1.0,
train_shape=(1, 1), ncol_effective=1,
time_series=False, ensemble_level=0,
score_f_name: str = None, **kwargs):
# If we don't override the parent mutate_params method, DAI would have the opportunity
# to modify the objective and select the winner
# For demonstration purposes we purposely make sure that the objective
# is the one we want
# So first call the parent method to mutate parameters
super().mutate_params(
get_best=get_best, time_tolerance=time_tolerance, accuracy=accuracy,
interpretability=interpretability,
imbalance_ratio=imbalance_ratio, train_shape=train_shape, ncol_effective=ncol_effective,
time_series=time_series, ensemble_level=ensemble_level,
score_f_name=score_f_name, **kwargs)
# Now set the objective
self.params["objective"] = custom_asymmetric_objective
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