jax-ml/jax · error · ValueError

Either both or neither of the x and y arguments should be pr

Error message

Either both or neither of the x and y arguments should be provided to jax.numpy.where, got {x} and {y}.

What it means

jnp.where requires either only condition (one-arg form) or both x and y (three-arg form). Passing exactly one of x or y is ambiguous and rejected.

Source

Thrown at jax/_src/numpy/lax_numpy.py:2801

    >>> jnp.nonzero(x > 4)
    (Array([5, 6, 7, 8, 9], dtype=int32),)

    When ``x`` and ``y`` are provided, ``where`` selects between them based on
    the specified condition:

    >>> jnp.where(x > 4, x, 0)
    Array([0, 0, 0, 0, 0, 5, 6, 7, 8, 9], dtype=int32)
  """
  if x is None and y is None:
    util.check_arraylike("where", condition)
    return nonzero(condition, size=size, fill_value=fill_value)
  else:
    util.check_arraylike("where", condition, x, y)
    if size is not None or fill_value is not None:
      raise ValueError("size and fill_value arguments cannot be used in "
                       "three-term where function.")
    if x is None or y is None:
      raise ValueError("Either both or neither of the x and y arguments "
                       "should be provided to jax.numpy.where, got "
                       f"{x} and {y}.")
    return util._where(condition, x, y)


@export
def select(
    condlist: Sequence[ArrayLike],
    choicelist: Sequence[ArrayLike],
    default: ArrayLike = 0,
) -> Array:
  """Select values based on a series of conditions.

  JAX implementation of :func:`numpy.select`, implemented in terms
  of :func:`jax.lax.select_n`

  Args:
    condlist: sequence of array-like conditions. All entries must be mutually

View on GitHub (pinned to 1e1c6a8fc0)

Solutions

1. Supply both x and y: jnp.where(cond, x, 0)
2. Or drop both to use the index form: jnp.where(cond)

Example fix

// before
jnp.where(cond, x)
// after
jnp.where(cond, x, 0)

if (x is None) != (y is None):
    y = 0 if y is None else y  # or raise

When it happens

Trigger: Calling jnp.where(cond, x) or jnp.where(cond, y=0) where the other branch is omitted (e.g. left as default None).

Common situations: Coming from APIs where the else branch defaults to 0 or False; partial refactors that delete one branch; kwargs confusion where x is passed but y forgotten.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Context manager for {state.__name__} config option requires
• size and fill_value arguments cannot be used in three-term w
• No input was provided to the clip function.
• Missing required keyword argument: 'in_sharding'
• Missing required keyword argument: 'in_layouts'

AI-assisted analysis of jax-ml/jax@1e1c6a8fc0 (2026-08-27). Data as JSON: /api/errors/4152c54dddd06e13. Report an issue: GitHub.