test for on_unused_input flag

test_on_unused_input.py

import theano
import theano.tensor as T

a = T.vector(name='a')
b = T.vector(name='b')
x = T.vector(name='x')
x = a*2

try:
    f = theano.function((a,b), outputs=x)
except ValueError as ex:
    expected_error = \
    "theano.function was asked to create a function computing outputs given " + \
    "certain inputs, but one of the provided input variables is not part of the " + \
    "computational graph needed to compute the outputs: b.\n" + \
    "To make this error into a warning, you can pass the parameter "+ \
    "on_unused_input='warn' to theano.function. To disable it completely, "+ \
    "use on_unused_input='ignore'."
    assert(str(ex)==expected_error)

# 'ignore' works fine ..
f = theano.function((a,b), outputs=x, on_unused_input='ignore')

# 'warn' not so much
f = theano.function((a,b), outputs=x, on_unused_input='warn')

# gives this output:

# Traceback (most recent call last):
#  File "test_on_unused_input.py", line 25, in <module>
#     f = theano.function((a,b), outputs=x, on_unused_input='warn')
#   File "/Users/utunga/dev/Theano/theano/compile/function.py", line 119, in function
#     profile=profile)
#   File "/Users/utunga/dev/Theano/theano/compile/pfunc.py", line 469, in pfunc
#     on_unused_input=on_unused_input)
#  File "/Users/utunga/dev/Theano/theano/compile/function_module.py", line 1345, in orig_function
#     on_unused_input=on_unused_input).create(
#   File "/Users/utunga/dev/Theano/theano/compile/function_module.py", line 1016, in __init__
#     self._check_unused_inputs(inputs, outputs, on_unused_input)
#   File "/Users/utunga/dev/Theano/theano/compile/function_module.py", line 1111, in _check_unused_inputs
#     warnings.warn(msg % (i.variable, warn_msg), stacklevel=5)
# NameError: global name 'warnings' is not defined