alloc_scan.py

import theano
from theano import tensor
import numpy as np

def main():
    x = tensor.vector('x')
    y = tensor.alloc(np.float32(0), *x.shape)

    def step(x_elem, t, prev_y):
        new_y = tensor.set_subtensor(prev_y[t], x_elem)
        return t + 1, new_y

    (_, ys), _ = theano.scan(fn=step, outputs_info=[tensor.constant(0), y], sequences=[x])
    copy_of_x = ys[-1]

    x_val = np.random.rand(10).astype(np.float32)
    f = theano.function([x], copy_of_x)
    np.testing.assert_allclose(f(x_val), x_val)
    np.testing.assert_allclose(f(x_val), x_val)

if __name__ == "__main__":
    main()