scan_missing_input.py

import numpy as np
import theano
import theano.tensor as T


floatX = theano.config.floatX


def scan1(a, y, x=None):
    """
    works
    """
    def step(a_, y_):
        return 0.9 * a_, theano.scan_module.until(y_ <= 0)

    return theano.scan(step, outputs_info=[a], non_sequences=[y], n_steps=10)


def scan11(a, y, x=None):
    """
    does not work

    This case is somewhat close to what I am doing in line search.
    """
    def step(a_, y_):
        y_ = theano.clone(y_, replace={x: 2*x})
        return 0.9 * a_, theano.scan_module.until(y_ <= 0)

    return theano.scan(step, outputs_info=[a], non_sequences=[y], n_steps=10)


def scan2(a, y, x=None):
    """
    does not work
    """
    def step(a_):
        return 0.9 * a_, theano.scan_module.until(y <= 0)

    return theano.scan(step, outputs_info=[a], n_steps=10)


def run(a, y, givens, scan_fn, x=None):
    rval, updates = scan_fn(a, y, x)
    f = theano.function([], rval, givens=givens, updates=updates)
    print f()


if __name__ == '__main__':
    a = T.constant(1.0, name='a')
    x = T.vector('x')
    y = x.sum()

    X = theano.shared(np.random.uniform(size=10).astype(floatX), borrow=True)
    givens = {x: X}

    run(a, y, givens, scan1) # works
    run(a, y, givens, scan11, x) # does not work
    run(a, y, givens, scan2) # does not work