Found a way to raise error with diff_method=None

pennylane/workflow/execution.py

@@ -368,6 +368,36 @@ def execution_function_with_caching(tapes):
  return execution_function_with_caching
 
 
+def _get_interface_name(tapes, interface):
+ """Helper function to get the interface name of a list of tapes
+
+ Args:
+ tapes (list[.QuantumScript]): Quantum tapes
+ interface (Optional[str]): Original interface to use as reference.
+
+ Returns:
+ str: Interface name"""
+ if interface == "auto":
+ params = []
+ for tape in tapes:
+ params.extend(tape.get_parameters(trainable_only=False))
+ interface = qml.math.get_interface(*params)
+ if INTERFACE_MAP.get(interface, "") == "tf" and _use_tensorflow_autograph():
+ interface = "tf-autograph"
+ if interface == "jax":
+ try: # pragma: no-cover
+ from .interfaces.jax import get_jax_interface_name
+ except ImportError as e: # pragma: no-cover
+ raise qml.QuantumFunctionError( # pragma: no-cover
+ "jax not found. Please install the latest " # pragma: no-cover
+ "version of jax to enable the 'jax' interface." # pragma: no-cover
+ ) from e # pragma: no-cover
+
+ interface = get_jax_interface_name(tapes)
+
+ return interface
+
+
 def execute(
  tapes: Sequence[QuantumTape],
  device: device_type,
@@ -522,26 +552,10 @@ def cost_fn(params, x):
 
  ### Specifying and preprocessing variables ####
 
- if interface == "auto":
- params = []
- for tape in tapes:
- params.extend(tape.get_parameters(trainable_only=False))
- interface = qml.math.get_interface(*params)
- if INTERFACE_MAP.get(interface, "") == "tf" and _use_tensorflow_autograph():
- interface = "tf-autograph"
- if interface == "jax":
- try: # pragma: no-cover
- from .interfaces.jax import get_jax_interface_name
- except ImportError as e: # pragma: no-cover
- raise qml.QuantumFunctionError( # pragma: no-cover
- "jax not found. Please install the latest " # pragma: no-cover
- "version of jax to enable the 'jax' interface." # pragma: no-cover
- ) from e # pragma: no-cover
-
- interface = get_jax_interface_name(tapes)
- # Only need to calculate derivatives with jax when we know it will be executed later.
- if interface in {"jax", "jax-jit"}:
- grad_on_execution = grad_on_execution if isinstance(gradient_fn, Callable) else False
+ interface = _get_interface_name(tapes, interface)
+ # Only need to calculate derivatives with jax when we know it will be executed later.
+ if interface in {"jax", "jax-jit"}:
+ grad_on_execution = grad_on_execution if isinstance(gradient_fn, Callable) else False
 
  if (
  device_vjp
@@ -558,7 +572,13 @@ def cost_fn(params, x):
  gradient_fn, grad_on_execution, interface, device, device_vjp, mcm_config
  )
 
- if interface == "jax-jit" and config.mcm_config.mcm_method == "deferred":
+ # Mid-circuit measurement configuration validation
+ if interface is None:
+ mcm_interface = "auto"
+ mcm_interface = _get_interface_name(tapes, mcm_interface)
+ else:
+ mcm_interface = interface
+ if mcm_interface == "jax-jit" and config.mcm_config.mcm_method == "deferred":
  # This is a current limitation of defer_measurements. "hw-like" behaviour is
  # not yet accessible.
  if config.mcm_config.postselect_mode == "hw-like":