Throw a warning for use of unsupported draw API on hardware targets.

python/runtime/cudaq/algorithms/py_draw.cpp

@@ -122,7 +122,9 @@ path, i.e., the trace, of the provided `kernel`.
   # Output
   #      ╭───╮╭──────────╮
   # q0 : ┤ h ├┤ ry(0.59) ├
-  #      ╰───╯╰──────────╯)#");
+  #      ╰───╯╰──────────╯
+
+Note: This function is only available when using simulator backends.)#");
 }
 
 } // namespace cudaq

python/tests/backends/test_Quantinuum_kernel.py

@@ -243,6 +243,20 @@ def test():
     assert not '10' in counts
 
 
+def test_draw():
+
+    @cudaq.kernel
+    def kernel():
+        q = cudaq.qvector(2)
+        h(q[0])
+        x.ctrl(q[0], q[1])
+        mz(q)
+
+    # Test here is that this does not raise an exception
+    result = cudaq.draw(kernel)
+    assert result is ''
+
+
 # leave for gdb debugging
 if __name__ == "__main__":
     loc = os.path.abspath(__file__)

runtime/cudaq/algorithms/draw.h

@@ -8,10 +8,10 @@
 
 #pragma once
 
-#include <concepts>
-
 #include "common/ExecutionContext.h"
 #include "cudaq/platform.h"
+#include <concepts>
+#include <iostream>
 
 namespace cudaq {
 
@@ -32,9 +32,13 @@ cudaq::Trace traceFromKernel(KernelFunctor &&kernel, Args &&...args) {
   // Get the platform.
   auto &platform = cudaq::get_platform();
 
-  // This can only be done in simulation
-  if (!platform.is_simulator())
-    throw std::runtime_error("Cannot use draw on a physical QPU.");
+  // This is not supported on hardware backends, but we don't want callers to
+  // crash on unhandled exceptions.
+  if (!platform.is_simulator()) {
+    std::cerr << "Warning: `draw` can only be used with a simulator platform. "
+              << "Returning an empty trace." << std::endl;
+    return Trace();
+  }
 
   // Create an execution context, indicate this is for tracing the execution
   // path
@@ -107,6 +111,7 @@ std::string extractTraceLatex(KernelFunctor &&kernel) {
 /// */      
 /// \endcode
 ///
+/// @note This function is only available when using simulator backends.
 // clang-format on
 
 #if CUDAQ_USE_STD20