def compose(self, other, qargs=None, front=False):
"""Return the composed operator.
Args:
other (BaseOperator): an operator object.
qargs (list or None): a list of subsystem positions to apply
other on. If None apply on all
subsystems [default: None].
front (bool): If True compose using right operator multiplication,
instead of left multiplication [default: False].
Returns:
BaseOperator: The operator self @ other.
Raises:
QiskitError: if other has incompatible dimensions for specified
subsystems.
Additional Information:
Composition (``@``) is defined as `left` matrix multiplication for
matrix operators. That is that ``A @ B`` is equal to ``B * A``.
Setting ``front=True`` returns `right` matrix multiplication
``A * B`` and is equivalent to the :meth:`dot` method.
"""
if qargs is None:
qargs = getattr(other, 'qargs', None)
if not isinstance(other, BaseOperator):
other = Operator(other)
new_shape = self._op_shape.compose(other._op_shape, qargs, front)
# If other is also an ScalarOp we only need to
# update the coefficient and dimensions
if isinstance(other, ScalarOp):
ret = copy.copy(self)
ret._coeff = self.coeff * other.coeff
ret._op_shape = new_shape
return ret
# If we are composing on the full system we return the
# other operator with reshaped dimensions
if qargs is None:
ret = copy.copy(other)
ret._op_shape = new_shape
# Other operator might not support scalar multiplication
# so we treat the identity as a special case to avoid a
# possible error
if self.coeff == 1:
return ret
return self.coeff * ret
# For qargs composition we initialize the scalar operator
# as an instance of the other BaseOperators subclass. We then
# perform subsystem qargs composition using the BaseOperator
# subclasses compose method.
# Note that this will raise an error if the other operator does
# not support initialization from a ScalarOp or the ScalarOps
# `to_operator` method).
return other.__class__(self).compose(other, qargs=qargs, front=front)
def compose(self, other, qargs=None, front=False):
if qargs is None:
qargs = getattr(other, "qargs", None)
if not isinstance(other, BaseOperator):
other = Operator(other)
new_shape = self._op_shape.compose(other._op_shape, qargs, front)
# If other is also an ScalarOp we only need to
# update the coefficient and dimensions
if isinstance(other, ScalarOp):
ret = copy.copy(self)
ret._coeff = self.coeff * other.coeff
ret._op_shape = new_shape
return ret
# If we are composing on the full system we return the
# other operator with reshaped dimensions
if qargs is None:
ret = copy.copy(other)
ret._op_shape = new_shape
# Other operator might not support scalar multiplication
# so we treat the identity as a special case to avoid a
# possible error
if self.coeff == 1:
return ret
return self.coeff * ret
# For qargs composition we initialize the scalar operator
# as an instance of the other BaseOperators subclass. We then
# perform subsystem qargs composition using the BaseOperator
# subclasses compose method.
# Note that this will raise an error if the other operator does
# not support initialization from a ScalarOp or the ScalarOps
# `to_operator` method).
return other.__class__(self).compose(other, qargs=qargs, front=front)
