[BugFix] KeyError when running QNode(qfun, dev)(graph) with graph…

… an instance of `networkx` Graph object (#6600)

**Context:** Issue #6585 indicates that we don't currently allow
arguments with types defined in libraries not listed in the supported
interfaces.

In #6225, the convention of treating internally as `interface=None`
parameters of the `numpy` interface has been introduced.
The idea of this PR is to allow non-trainable `qnode` inputs to "just
work" as arguments, without any special consideration or treatment. We
want to rely on the default `numpy` interface for libraries we know
nothing about.

**Description of the Change:** By default, we set `None` as the value of
the interface if the latter is not found in the list of supported
interfaces. According to the convention introduced in #6225, this should
automatically map the interface to `numpy`.

**Benefits:** Now the `qnode` accepts arguments with types defined in
libraries that are not necessarily in the list of supported interfaces,
such as the `Graph` class defined in `networkx`.

**Possible Drawbacks:** None that I can think of.

**Related GitHub Issues:** #6585 

**Related Shortcut Stories:** [sc-78344]

---------

Co-authored-by: Christina Lee <[email protected]>

doc/releases/changelog-dev.md

@@ -23,7 +23,6 @@
 
 * Added `qml.devices.qubit_mixed` module for mixed-state qubit device support [(#6379)](https://github.com/PennyLaneAI/pennylane/pull/6379). This module introduces an `apply_operation` helper function that features:
 
-
   * Two density matrix contraction methods using `einsum` and `tensordot`
 
   * Optimized handling of special cases including: Diagonal operators, Identity operators, CX (controlled-X), Multi-controlled X gates, Grover operators
@@ -132,7 +131,6 @@ following 4 sets of functions have been either moved or removed[(#6588)](https:/
 * The `expand_depth` argument for `qml.compile` has been removed.
   [(#6531)](https://github.com/PennyLaneAI/pennylane/pull/6531)
 
-
 * The `qml.shadows.shadow_expval` transform has been removed. Instead, please use the
   `qml.shadow_expval` measurement process.
   [(#6530)](https://github.com/PennyLaneAI/pennylane/pull/6530)
@@ -165,14 +163,19 @@ same information.
   [(#6549)](https://github.com/PennyLaneAI/pennylane/pull/6549)
 
 <h3>Documentation 📝</h3>
+
 * Add reporting of test warnings as failures.
   [(#6217)](https://github.com/PennyLaneAI/pennylane/pull/6217)
 
-* Add a warning message to Gradients and training documentation about ComplexWarnings
+* Add a warning message to Gradients and training documentation about ComplexWarnings.
   [(#6543)](https://github.com/PennyLaneAI/pennylane/pull/6543)
 
 <h3>Bug fixes 🐛</h3>
 
+* `qml.QNode` now accepts arguments with types defined in libraries that are not necessarily 
+  in the list of supported interfaces, such as the `Graph` class defined in `networkx`.
+  [(#6600)](https://github.com/PennyLaneAI/pennylane/pull/6600)
+
 * `qml.math.get_deep_interface` now works properly for autograd arrays.
   [(#6557)](https://github.com/PennyLaneAI/pennylane/pull/6557)
 

pennylane/workflow/execution.py

@@ -19,7 +19,7 @@
 # pylint: disable=import-outside-toplevel,too-many-branches,not-callable,unexpected-keyword-arg
 # pylint: disable=unused-argument,unnecessary-lambda-assignment,inconsistent-return-statements
 # pylint: disable=invalid-unary-operand-type,isinstance-second-argument-not-valid-type
-# pylint: disable=too-many-arguments,too-many-statements,function-redefined,too-many-function-args
+# pylint: disable=too-many-arguments,too-many-statements,function-redefined,too-many-function-args,too-many-positional-arguments
 
 import inspect
 import logging
@@ -269,7 +269,7 @@ def _get_interface_name(tapes, interface):
             params.extend(tape.get_parameters(trainable_only=False))
         interface = qml.math.get_interface(*params)
         if interface != "numpy":
-            interface = INTERFACE_MAP[interface]
+            interface = INTERFACE_MAP.get(interface, None)
     if interface == "tf" and _use_tensorflow_autograph():
         interface = "tf-autograph"
     if interface == "jax":

pennylane/workflow/qnode.py

@@ -14,7 +14,7 @@
 """
 This module contains the QNode class and qnode decorator.
 """
-# pylint: disable=too-many-instance-attributes,too-many-arguments,protected-access,unnecessary-lambda-assignment, too-many-branches, too-many-statements, unused-argument
+# pylint: disable=too-many-instance-attributes,too-many-arguments,protected-access,unnecessary-lambda-assignment, too-many-branches, too-many-statements, unused-argument, too-many-positional-arguments
 import copy
 import functools
 import inspect
@@ -75,7 +75,7 @@ def _convert_to_interface(res, interface):
         "tf-autograph": "tensorflow",
     }
 
-    interface_name = interface_conversion_map[interface]
+    interface_name = interface_conversion_map.get(interface, None)
 
     return qml.math.asarray(res, like=interface_name)
 
@@ -983,7 +983,7 @@ def _impl_call(self, *args, **kwargs) -> qml.typing.Result:
                 else qml.math.get_interface(*args, *list(kwargs.values()))
             )
             if interface != "numpy":
-                interface = INTERFACE_MAP[interface]
+                interface = INTERFACE_MAP.get(interface, None)
             self._interface = interface
 
         try:

tests/test_qnode.py

@@ -1030,6 +1030,47 @@ def circuit(x):
         res = circuit(x)
         assert qml.math.get_interface(res) == "numpy"
 
+    def test_qnode_default_interface(self):
+        """Tests that the default interface is set correctly for a QNode."""
+
+        # pylint: disable=import-outside-toplevel
+        import networkx as nx
+
+        @qml.qnode(qml.device("default.qubit"))
+        def circuit(graph: nx.Graph):
+            for a in graph.nodes:
+                qml.Hadamard(wires=a)
+            for a, b in graph.edges:
+                qml.CZ(wires=[a, b])
+            return qml.expval(qml.PauliZ(0))
+
+        graph = nx.complete_graph(3)
+        res = circuit(graph)
+        assert qml.math.get_interface(res) == "numpy"
+
+    def test_qscript_default_interface(self):
+        """Tests that the default interface is set correctly for a QuantumScript."""
+
+        # pylint: disable=import-outside-toplevel
+        import networkx as nx
+
+        dev = qml.device("default.qubit")
+
+        # pylint: disable=too-few-public-methods
+        class DummyCustomGraphOp(qml.operation.Operation):
+            """Dummy custom operation for testing purposes."""
+
+            def __init__(self, graph: nx.Graph):
+                super().__init__(graph, wires=graph.nodes)
+
+            def decomposition(self) -> list:
+                return []
+
+        graph = nx.complete_graph(3)
+        tape = qml.tape.QuantumScript([DummyCustomGraphOp(graph)], [qml.expval(qml.PauliZ(0))])
+        res = qml.execute([tape], dev)
+        assert qml.math.get_interface(res) == "numpy"
+
 
 class TestShots:
     """Unit tests for specifying shots per call."""