Use inspect module to get argument counts (#1151)

Using `qml.cond` on PennyLane operators raises an exception because the
target does not have a `__code__` attribute:
```py
@qjit
@qml.qnode(qml.device("lightning.qubit", wires=1))
def f(x: int):
    qml.cond(x < 5, qml.Hadamard)(0)
    return qml.probs()
```
```
AttributeError: type object 'Hadamard' has no attribute '__code__'
```

[sc-74292]

doc/releases/changelog-dev.md

@@ -53,7 +53,8 @@
   Array([[1], [0], [1], [1], [0], [1],[0]], dtype=int64))
   ```
 
-* A new function `catalyst.passes.pipeline` allows the quantum circuit transformation pass pipeline for QNodes within a qjit-compiled workflow to be configured.
+* A new function `catalyst.passes.pipeline` allows the quantum circuit transformation pass pipeline
+  for QNodes within a qjit-compiled workflow to be configured.
   [(#1131)](https://github.com/PennyLaneAI/catalyst/pull/1131)
 
   ```python
@@ -92,10 +93,12 @@
         return jnp.abs(circuit_pipeline(x) - circuit_other(x))
   ```
 
-  For a list of available passes, please see the [catalyst.passes module documentation](https://docs.pennylane.ai/projects/catalyst/en/stable/code/__init__.html#module-catalyst.passes).
+  For a list of available passes, please see the
+  [catalyst.passes module documentation](https://docs.pennylane.ai/projects/catalyst/en/stable/code/__init__.html#module-catalyst.passes).
 
-  The pass pipeline order and options can be configured *globally* for a
-  qjit-compiled function, by using the `circuit_transform_pipeline` argument of the :func:`~.qjit` decorator.
+  The pass pipeline order and options can be configured *globally* for a qjit-compiled
+  function, by using the `circuit_transform_pipeline` argument of the :func:`~.qjit`
+  decorator.
 
   ```python
     my_passes = {
@@ -114,11 +117,13 @@
   Available MLIR passes are now documented and available within the
   [catalyst.passes module documentation](https://docs.pennylane.ai/projects/catalyst/en/stable/code/__init__.html#module-catalyst.passes).
 
-* Catalyst Autograph now supports updating a single index or a slice of JAX arrays using Python's array assignment operator syntax.
+* Catalyst Autograph now supports updating a single index or a slice of JAX arrays using Python's
+  array assignment operator syntax.
   [(#769)](https://github.com/PennyLaneAI/catalyst/pull/769)
   [(#1143)](https://github.com/PennyLaneAI/catalyst/pull/1143)
 
-  Using operator assignment syntax in favor of `at...op` expressions is now possible for the following operations:
+  Using operator assignment syntax in favor of `at...op` expressions is now possible for the
+  following operations:
   * `x[i] += y` in favor of `x.at[i].add(y)`
   * `x[i] -= y` in favor of `x.at[i].add(-y)`
   * `x[i] *= y` in favor of `x.at[i].multiply(y)`
@@ -154,8 +159,9 @@
   in the program in tensors, including scalars, leading to unnecessary memory allocations for
   programs compiled to CPU via MLIR to LLVM pipeline.
 
-* Bufferization of `gradient.ForwardOp` and `gradient.ReverseOp` now requires 3 steps: `gradient-preprocessing`, 
-  `gradient-bufferize`, and `gradient-postprocessing`. `gradient-bufferize` has a new rewrite for `gradient.ReturnOp`. 
+* Bufferization of `gradient.ForwardOp` and `gradient.ReverseOp` now requires 3 steps:
+  `gradient-preprocessing`, `gradient-bufferize`, and `gradient-postprocessing`.
+  `gradient-bufferize` has a new rewrite for `gradient.ReturnOp`.
   [(#1139)](https://github.com/PennyLaneAI/catalyst/pull/1139)
 
 * The decorator `self_inverses` now supports all Hermitian Gates.
@@ -196,9 +202,12 @@
 
 <h3>Bug fixes</h3>
 
-* Resolve a bug in the `vmap` function when passing shapeless values to the target. 
+* Resolve a bug in the `vmap` function when passing shapeless values to the target.
   [(#1150)](https://github.com/PennyLaneAI/catalyst/pull/1150)
 
+* Fix error message displayed when using `qml.cond` on callables with arguments.
+  [(#1151)](https://github.com/PennyLaneAI/catalyst/pull/1151)
+
 <h3>Internal changes</h3>
 
 * Update Enzyme to version `v0.0.149`.

frontend/catalyst/api_extensions/control_flow.py

@@ -19,6 +19,7 @@
 
 # pylint: disable=too-many-lines
 
+import inspect
 from typing import Any, Callable, List
 
 import jax
@@ -236,7 +237,7 @@ def conditional_fn():
     """
 
     def _decorator(true_fn: Callable):
-        if true_fn.__code__.co_argcount != 0:
+        if len(inspect.signature(true_fn).parameters):
             raise TypeError("Conditional 'True' function is not allowed to have any arguments")
         return CondCallable(pred, true_fn)
 
@@ -584,7 +585,7 @@ def else_if(self, pred):
         """
 
         def decorator(branch_fn):
-            if branch_fn.__code__.co_argcount != 0:
+            if len(inspect.signature(branch_fn).parameters):
                 raise TypeError(
                     "Conditional 'else if' function is not allowed to have any arguments"
                 )
@@ -603,7 +604,7 @@ def otherwise(self, otherwise_fn):
         Returns:
             self
         """
-        if otherwise_fn.__code__.co_argcount != 0:
+        if len(inspect.signature(otherwise_fn).parameters):
             raise TypeError("Conditional 'False' function is not allowed to have any arguments")
         self.otherwise_fn = otherwise_fn
         return self

frontend/test/pytest/test_conditionals.py

@@ -415,6 +415,19 @@ def conditional_flip():
         assert circuit(False) == 0
         assert circuit(True) == 1
 
+    def test_argument_error_with_callables(self):
+        """Test for the error when arguments are supplied and the target is not a function."""
+
+        @qml.qnode(qml.device("lightning.qubit", wires=1))
+        def f(x: int):
+
+            qml.cond(x < 5, qml.Hadamard)(0)
+
+            return qml.probs()
+
+        with pytest.raises(TypeError, match="not allowed to have any arguments"):
+            qjit(f)
+
 
 class TestInterpretationConditional:
     """Test that the conditional operation's execution is semantically equivalent