def freeze(model):
    """ Set all layers in a model to non-trainable.

    The weights for these layers will not be updated during training.

    This function modifies the given model in-place,
    but it also returns the modified model to allow easy chaining with other functions.
    """
    for layer in model.layers:
        layer.trainable = False
    return model
 Set all layers in a model to non-trainable.

    The weights for these layers will not be updated during training.

    This function modifies the given model in-place,
    but it also returns the modified model to allow easy chaining with other functions.
    
keras_retinanet/utils/model.py