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 random_normal_initializer 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

Saxe et al., http://arxiv.org/abs/1312.6120

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Identity

keras.initializers.Identity(gain=1.0)

Initializer that generates the identity matrix.

Only use for square 2D matrices.

Arguments

  • gain: Multiplicative factor to apply to the identity matrix.

lecun_uniform

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

LeCun 98, Efficient Backprop, - http://yann.lecun.com/exdb/publis/pdf/lecun-98b.pdf

glorot_normal

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

Glorot & Bengio, AISTATS 2010 - http://jmlr.org/proceedings/papers/v9/glorot10a/glorot10a.pdf

glorot_uniform

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

Glorot & Bengio, AISTATS 2010 - http://jmlr.org/proceedings/papers/v9/glorot10a/glorot10a.pdf

he_normal

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

He et al., http://arxiv.org/abs/1502.01852

he_uniform

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

He et al., http://arxiv.org/abs/1502.01852

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, init=my_init))