From bea44ba8cdbd0a7766084c8aebbd9c5737c54daf Mon Sep 17 00:00:00 2001
From: Mudit Pandey <mudit.pandey@xanadu.ai>
Date: Mon, 3 Jun 2024 17:04:28 -0400
Subject: [PATCH] Found a way to raise error with diff_method=None

---
 pennylane/workflow/execution.py | 62 ++++++++++++++++++++++-----------
 1 file changed, 41 insertions(+), 21 deletions(-)

diff --git a/pennylane/workflow/execution.py b/pennylane/workflow/execution.py
index c87ffe2600d..a3f7dc845d9 100644
--- a/pennylane/workflow/execution.py
+++ b/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":