Declare batch input shape independently of first layer with Sequential API

I know the functional API supports declaring input shape in an Input layer, and then passing that to a Model() object, but what’s the equivalent for Sequential()? I’d expect something like this to work:

model = Sequential()
model.add(Input(batch_shape=(batch_size, height, width, channels)))
model.add(Convolutions/RNNs/Dense/Whatever)
model.compile(...)

but it doesn’t.

AttributeError: ‘TensorVariable’ object has no attribute ‘inbound_nodes’

What am I missing? Shouldn’t the API’s work similarly?

I’d even expect something like (my preferred API, because all models always have at least one input, right?):

model = Sequential((batch_size, height, width, channels))
model.add(Convolutions/RNNs/Dense/Whatever)
model.compile(...)

Also, why is it called batch_input_shape in some layers, and batch_shape in others? It’s a bit confusing, honestly.

[farizrahman4u]

I know the functional API supports declaring input shape in an Input layer, and then passing that to a Model() object, but what’s the equivalent for Sequential()?

InputLayer or just Layer.

model = Sequential()
model.add(InputLayer(batch_input_shape=(batch_size, height, width, channels)))
model.add(Convolutions/RNNs/Dense/Whatever)
model.compile(...)

What am I missing? Shouldn’t the API’s work similarly?

Input is a function which returns a tensor. You add layers to Sequential models. Not tensors

I’d even expect something like (my preferred API, because all models always have at least one input, right?):

model = Sequential([Layer(batch_input_shape=(batch_size, height, width, channels))])
model.add(...)
model.add(...)
...

Also, why is it called batch_input_shape in some layers, and batch_shape in others? It’s a bit confusing, honestly.

Its not. All layers take batch_input_shape and input_shape arguments. There is “input” in the argument names because they specify the shapes of the tensors which will be “input” to the layer.

The Input function takes batch_shape and shape arguments. No “input” here. Since here the arguments specify the shape of the tensor (placeholder to be more precise) itself. We are simply asking Keras to create a placeholder of a given shape.

[fchollet]

@farizrahman4u got it right.

Basically Input is the equivalent of instantiating an InputLayer and calling it in one go. Reading the guide “getting started with the functional API” should clear up any questions.