tf.while_loop( cond, body, loop_vars, shape_invariants=None, parallel_iterations=10, back_prop=True, swap_memory=False, name=None, maximum_iterations=None )
See the guide: Control Flow > Control Flow Operations
body while the condition
cond is true.
cond is a callable returning a boolean scalar tensor.
body is a callable
returning a (possibly nested) tuple, namedtuple or list of tensors of the same
arity (length and structure) and types as
loop_vars is a
(possibly nested) tuple, namedtuple or list of tensors that is passed to both
body both take as many arguments as there are
In addition to regular Tensors or IndexedSlices, the body may accept and return TensorArray objects. The flows of the TensorArray objects will be appropriately forwarded between loops and during gradient calculations.
body exactly once (inside the
while_loop, and not at all during
stitches together the graph fragments created during the
calls with some additional graph nodes to create the graph flow that
cond returns false.
tf.while_loop() strictly enforces shape invariants for
the loop variables. A shape invariant is a (possibly partial) shape that
is unchanged across the iterations of the loop. An error will be raised
if the shape of a loop variable after an iteration is determined to be more
general than or incompatible with its shape invariant. For example, a shape
of [11, None] is more general than a shape of [11, 17], and [11, 21] is not
compatible with [11, 17]. By default (if the argument
not specified), it is assumed that the initial shape of each tensor in
loop_vars is the same in every iteration. The
allows the caller to specify a less specific shape invariant for each loop
variable, which is needed if the shape varies between iterations. The
function may also be used in the
body function to indicate that
the output loop variable has a particular shape. The shape invariant for
SparseTensor and IndexedSlices are treated specially as follows:
a) If a loop variable is a SparseTensor, the shape invariant must be TensorShape([r]) where r is the rank of the dense tensor represented by the sparse tensor. It means the shapes of the three tensors of the SparseTensor are ([None], [None, r], [r]). NOTE: The shape invariant here is the shape of the SparseTensor.dense_shape property. It must be the shape of a vector.
b) If a loop variable is an IndexedSlices, the shape invariant must be a shape invariant of the values tensor of the IndexedSlices. It means the shapes of the three tensors of the IndexedSlices are (shape, [shape], [shape.ndims]).
while_loop implements non-strict semantics, enabling multiple iterations
to run in parallel. The maximum number of parallel iterations can be
parallel_iterations, which gives users some control over
memory consumption and execution order. For correct programs,
should return the same result for any parallel_iterations > 0.
For training, TensorFlow stores the tensors that are produced in the forward inference and are needed in back propagation. These tensors are a main source of memory consumption and often cause OOM errors when training on GPUs. When the flag swap_memory is true, we swap out these tensors from GPU to CPU. This for example allows us to train RNN models with very long sequences and large batches.
cond: A callable that represents the termination condition of the loop.
body: A callable that represents the loop body.
loop_vars: A (possibly nested) tuple, namedtuple or list of numpy array,
shape_invariants: The shape invariants for the loop variables.
parallel_iterations: The number of iterations allowed to run in parallel. It must be a positive integer.
back_prop: Whether backprop is enabled for this while loop.
swap_memory: Whether GPU-CPU memory swap is enabled for this loop.
name: Optional name prefix for the returned tensors.
maximum_iterations: Optional maximum number of iterations of the while loop to run. If provided, the
condoutput is AND-ed with an additional condition ensuring the number of iterations executed is no greater than
The output tensors for the loop variables after the loop. When the length
loop_vars is 1 this is a Tensor, TensorArray or IndexedSlice and when
the length of
loop_vars is greater than 1 it returns a list.
bodyis not callable.
i = tf.constant(0) c = lambda i: tf.less(i, 10) b = lambda i: tf.add(i, 1) r = tf.while_loop(c, b, [i])
Example with nesting and a namedtuple:
import collections Pair = collections.namedtuple('Pair', 'j, k') ijk_0 = (tf.constant(0), Pair(tf.constant(1), tf.constant(2))) c = lambda i, p: i < 10 b = lambda i, p: (i + 1, Pair((p.j + p.k), (p.j - p.k))) ijk_final = tf.while_loop(c, b, ijk_0)
Example using shape_invariants:
i0 = tf.constant(0) m0 = tf.ones([2, 2]) c = lambda i, m: i < 10 b = lambda i, m: [i+1, tf.concat([m, m], axis=0)] tf.while_loop( c, b, loop_vars=[i0, m0], shape_invariants=[i0.get_shape(), tf.TensorShape([None, 2])])
Example which demonstrates non-strict semantics: In the following
example, the final value of the counter
i does not depend on
while_loop can increment the counter parallel to updates of
However, because the loop counter at one loop iteration depends
on the value at the previous iteration, the loop counter itself cannot
be incremented in parallel. Hence if we just want the final value of the
counter (which we print on the line
x will never be incremented, but the counter will be updated on a
single thread. Conversely, if we want the value of the output (which we
print on the line
print(sess.run(out).shape)), then the counter may be
incremented on its own thread, while
x can be incremented in
parallel on a separate thread. In the extreme case, it is conceivable
that the thread incrementing the counter runs until completion before
x is incremented even a single time. The only thing that can never
happen is that the thread updating
x can never get ahead of the
counter thread because the thread incrementing
x depends on the value
of the counter.
import tensorflow as tf
n = 10000 x = tf.constant(list(range(n))) c = lambda i, x: i < n b = lambda i, x: (tf.Print(i + 1, [i]), tf.Print(x + 1, [i], "x:")) i, out = tf.while_loop(c, b, (0, x)) with tf.Session() as sess: print(sess.run(i)) # prints  ... 
# The following line may increment the counter and x in parallel. # The counter thread may get ahead of the other thread, but not the # other way around. So you may see things like #  x: # meaning that the counter thread is on iteration 9996, # while the other thread is on iteration 9987 print(sess.run(out).shape)