Usage of initializers

Initializations define the way to set the initial random weights of Keras layers.

The keyword arguments used for passing initializers to layers will depend on the layer. Usually it is simply kernel_initializer and bias_initializer:

model.add(Dense(64,
                kernel_initializer='random_uniform',
                bias_initializer='zeros'))

Available initializers

The following built-in initializers are available as part of the keras.initializers module:

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Initializer

keras.initializers.Initializer()

Initializer base class: all initializers inherit from this class.

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Zeros

keras.initializers.Zeros()

Initializer that generates tensors initialized to 0.

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Ones

keras.initializers.Ones()

Initializer that generates tensors initialized to 1.

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Constant

keras.initializers.Constant(value=0)

Initializer that generates tensors initialized to a constant value.

Arguments

value: float; the value of the generator tensors.

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RandomNormal

keras.initializers.RandomNormal(mean=0.0, stddev=0.05, seed=None)

Initializer that generates tensors with a normal distribution.

Arguments

mean: a python scalar or a scalar tensor. Mean of the random values to generate. stddev: a python scalar or a scalar tensor. Standard deviation of the random values to generate. seed: A Python integer. Used to seed the random generator.

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RandomUniform

keras.initializers.RandomUniform(minval=-0.05, maxval=0.05, seed=None)

Initializer that generates tensors with a uniform distribution.

Arguments

minval: A python scalar or a scalar tensor. Lower bound of the range of random values to generate. maxval: A python scalar or a scalar tensor. Upper bound of the range of random values to generate. Defaults to 1 for float types. seed: A Python integer. Used to seed the random generator.

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TruncatedNormal

keras.initializers.TruncatedNormal(mean=0.0, stddev=0.05, seed=None)

Initializer that generates a truncated normal distribution.

These values are similar to values from a RandomNormal except that values more than two standard deviations from the mean are discarded and re-drawn. This is the recommended initializer for neural network weights and filters.

Arguments

mean: a python scalar or a scalar tensor. Mean of the random values to generate. stddev: a python scalar or a scalar tensor. Standard deviation of the random values to generate. seed: A Python integer. Used to seed the random generator.

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VarianceScaling

keras.initializers.VarianceScaling(scale=1.0, mode='fan_in', distribution='normal', seed=None)

Initializer capable of adapting its scale to the shape of weights.

With distribution="normal", samples are drawn from a truncated normal distribution centered on zero, with stddev = sqrt(scale / n) where n is:

• number of input units in the weight tensor, if mode = "fan_in"
• number of output units, if mode = "fan_out"
• average of the numbers of input and output units, if mode = "fan_avg"

With distribution="uniform", samples are drawn from a uniform distribution within [-limit, limit], with limit = sqrt(3 * scale / n).

Arguments

• scale: Scaling factor (positive float).
• mode: One of "fan_in", "fan_out", "fan_avg".
• distribution: Random distribution to use. One of "normal", "uniform".
• seed: A Python integer. Used to seed the random generator.

Raises

ValueError: In case of an invalid value for the "scale", mode" or "distribution" arguments.

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Orthogonal

keras.initializers.Orthogonal(gain=1.0, seed=None)

Initializer that generates a random orthogonal matrix.

Arguments

• gain: Multiplicative factor to apply to the orthogonal matrix.
• seed: A Python integer. Used to seed the random generator.

References

• Exact solutions to the nonlinear dynamics of learning in deep linear neural networks

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Identity

keras.initializers.Identity(gain=1.0)

Initializer that generates the identity matrix.

Only use for 2D matrices. If the long side of the matrix is a multiple of the short side, multiple identity matrices are concatenated along the long side.

Arguments

gain: Multiplicative factor to apply to the identity matrix.

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lecun_uniform

keras.initializers.lecun_uniform(seed=None)

LeCun uniform initializer.

It draws samples from a uniform distribution within [-limit, limit] where limit is sqrt(3 / fan_in) where fan_in is the number of input units in the weight tensor.

Arguments

• seed: A Python integer. Used to seed the random generator.

Returns

An initializer.

References

• Efficient BackProp

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glorot_normal

keras.initializers.glorot_normal(seed=None)

Glorot normal initializer, also called Xavier normal initializer.

It draws samples from a truncated normal distribution centered on 0 with stddev = sqrt(2 / (fan_in + fan_out)) where fan_in is the number of input units in the weight tensor and fan_out is the number of output units in the weight tensor.

Arguments

• seed: A Python integer. Used to seed the random generator.

Returns

An initializer.

References

• Understanding the difficulty of training deep feedforward neural networks

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glorot_uniform

keras.initializers.glorot_uniform(seed=None)

Glorot uniform initializer, also called Xavier uniform initializer.

It draws samples from a uniform distribution within [-limit, limit] where limit is sqrt(6 / (fan_in + fan_out)) where fan_in is the number of input units in the weight tensor and fan_out is the number of output units in the weight tensor.

Arguments

• seed: A Python integer. Used to seed the random generator.

Returns

An initializer.

References

• Understanding the difficulty of training deep feedforward neural networks

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he_normal

keras.initializers.he_normal(seed=None)

He normal initializer.

It draws samples from a truncated normal distribution centered on 0 with stddev = sqrt(2 / fan_in) where fan_in is the number of input units in the weight tensor.

Arguments

• seed: A Python integer. Used to seed the random generator.

Returns

An initializer.

References

• Delving Deep into Rectifiers: Surpassing Human-Level Performance on ImageNet Classification

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lecun_normal

keras.initializers.lecun_normal(seed=None)

LeCun normal initializer.

It draws samples from a truncated normal distribution centered on 0 with stddev = sqrt(1 / fan_in) where fan_in is the number of input units in the weight tensor.

Arguments

• seed: A Python integer. Used to seed the random generator.

Returns

An initializer.

References

• Self-Normalizing Neural Networks
• Efficient Backprop

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he_uniform

keras.initializers.he_uniform(seed=None)

He uniform variance scaling initializer.

It draws samples from a uniform distribution within [-limit, limit] where limit is sqrt(6 / fan_in) where fan_in is the number of input units in the weight tensor.

Arguments

• seed: A Python integer. Used to seed the random generator.

Returns

An initializer.

References

• Delving Deep into Rectifiers: Surpassing Human-Level Performance on ImageNet Classification

An initializer may be passed as a string (must match one of the available initializers above), or as a callable:

from keras import initializers

model.add(Dense(64, kernel_initializer=initializers.random_normal(stddev=0.01)))

# also works; will use the default parameters.
model.add(Dense(64, kernel_initializer='random_normal'))

Using custom initializers

If passing a custom callable, then it must take the argument shape (shape of the variable to initialize) and dtype (dtype of generated values):

from keras import backend as K

def my_init(shape, dtype=None):
    return K.random_normal(shape, dtype=dtype)

model.add(Dense(64, kernel_initializer=my_init))