Theano shared variable update causes `ValueError: length not known`

Question

Minimal example code:

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

x = T.dscalars('x')
z = th.shared(np.zeros(2))
updates = [z, z+x]

f1 = th.function(inputs=[x], updates=updates) 
f1(3)
print z.get_value()

Error message:

Traceback (most recent call last):
  File "/home/temp2/theano.test.py", line 9, in <module>
    f1 = th.function(inputs=[x], updates=updates) 
  File "/usr/local/lib/python2.7/dist-packages/theano/compile/function.py", line 205, in function
    profile=profile)
  File "/usr/local/lib/python2.7/dist-packages/theano/compile/pfunc.py", line 460, in pfunc
    no_default_updates=no_default_updates)
  File "/usr/local/lib/python2.7/dist-packages/theano/compile/pfunc.py", line 191, in rebuild_collect_shared
    for (store_into, update_val) in iter_over_pairs(updates):
  File "/usr/local/lib/python2.7/dist-packages/theano/tensor/basic.py", line 1610, in __iter__
    for i in xrange(get_vector_length(self)):
  File "/usr/local/lib/python2.7/dist-packages/theano/tensor/basic.py", line 5210, in get_vector_length
    raise ValueError("length not known")
ValueError: length not known

What is the cause of this error?

Answer 1

Updates must contain a list of pairs. See official tutorial on using shared variables.

Correct code:

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

x = T.dscalars('x')
z = th.shared(np.zeros(2))
updates = [(z, z+x)]

f1 = th.function(inputs=[x], updates=updates) 
f1(3)
print z.get_value()