tf.cond

tf.cond(
    pred,
    true_fn=None,
    false_fn=None,
    strict=False,
    name=None,
    fn1=None,
    fn2=None
)

Defined in tensorflow/python/ops/control_flow_ops.py.

See the guide: Control Flow > Control Flow Operations

Return true_fn() if the predicate pred is true else false_fn(). (deprecated arguments)

SOME ARGUMENTS ARE DEPRECATED. They will be removed in a future version. Instructions for updating: fn1/fn2 are deprecated in favor of the true_fn/false_fn arguments.

true_fn and false_fn both return lists of output tensors. true_fn and false_fn must have the same non-zero number and type of outputs.

Note that the conditional execution applies only to the operations defined in true_fn and false_fn. Consider the following simple program:

z = tf.multiply(a, b)
result = tf.cond(x < y, lambda: tf.add(x, z), lambda: tf.square(y))

If x < y, the tf.add operation will be executed and tf.square operation will not be executed. Since z is needed for at least one branch of the cond, the tf.multiply operation is always executed, unconditionally. Although this behavior is consistent with the dataflow model of TensorFlow, it has occasionally surprised some users who expected a lazier semantics.

Note that cond calls true_fn and false_fn exactly once (inside the call to cond, and not at all during Session.run()). cond stitches together the graph fragments created during the true_fn and false_fn calls with some additional graph nodes to ensure that the right branch gets executed depending on the value of pred.

tf.cond supports nested structures as implemented in tensorflow.python.util.nest. Both true_fn and false_fn must return the same (possibly nested) value structure of lists, tuples, and/or named tuples. Singleton lists and tuples form the only exceptions to this: when returned by true_fn and/or false_fn, they are implicitly unpacked to single values. This behavior is disabled by passing strict=True.

Args:

  • pred: A scalar determining whether to return the result of true_fn or false_fn.
  • true_fn: The callable to be performed if pred is true.
  • false_fn: The callable to be performed if pred is false.
  • strict: A boolean that enables/disables 'strict' mode; see above.
  • name: Optional name prefix for the returned tensors.

Returns:

Tensors returned by the call to either true_fn or false_fn. If the callables return a singleton list, the element is extracted from the list.

Raises:

  • TypeError: if true_fn or false_fn is not callable.
  • ValueError: if true_fn and false_fn do not return the same number of tensors, or return tensors of different types.

Example:

x = tf.constant(2)
y = tf.constant(5)
def f1(): return tf.multiply(x, 17)
def f2(): return tf.add(y, 23)
r = tf.cond(tf.less(x, y), f1, f2)
# r is set to f1().
# Operations in f2 (e.g., tf.add) are not executed.