checking theano l1 gradient == sign

theano_l1.py

import theano
import theano.tensor as T

import numpy as np

# 0 and some arbitrarily small positive and negative numbers
test_vector = np.array(
    [0., 1., -1., .1, -.1, 10., -10., 1e-8, -1e-8,
     1e-10, -1e-10, 1e-16, -1e-16]).astype(np.float32)

input_expr = T.fvector()
l1_norm = abs(input_expr).sum()

l1_norm_grad = T.grad(l1_norm, wrt=input_expr)

f_grad = theano.function([input_expr], l1_norm_grad)

from numpy.testing import assert_array_equal

# show that on all examples the gradient of the l1 norm is identical to the sign of the vector
assert_array_equal(f_grad(test_vector), np.sign(test_vector))