Set default backend dynamically based on availability

src/mici/autodiff.py

@@ -48,18 +48,28 @@ class AutodiffBackend(NamedTuple):
 
 """Available autodifferentiation framework backends."""
 _REGISTERED_BACKENDS = {
-    "autograd": AutodiffBackend(autograd_wrapper, autograd_wrapper.AUTOGRAD_AVAILABLE),
     "jax": AutodiffBackend(
-        jax_wrapper, jax_wrapper.JAX_AVAILABLE, jax_wrapper.jit_and_return_numpy_arrays,
+        jax_wrapper,
+        jax_wrapper.JAX_AVAILABLE,
+        jax_wrapper.jit_and_return_numpy_arrays,
     ),
     "jax_nojit": AutodiffBackend(
-        jax_wrapper, jax_wrapper.JAX_AVAILABLE, jax_wrapper.return_numpy_arrays,
+        jax_wrapper,
+        jax_wrapper.JAX_AVAILABLE,
+        jax_wrapper.return_numpy_arrays,
     ),
+    "autograd": AutodiffBackend(autograd_wrapper, autograd_wrapper.AUTOGRAD_AVAILABLE),
     "symnum": AutodiffBackend(symnum_wrapper, symnum_wrapper.SYMNUM_AVAILABLE),
 }
 
-"""Name of default automatic differentiation backend to use."""
-DEFAULT_BACKEND = "jax"
+"""Name of default automatic differentiation backend to use.
+
+Defaults to first available backend from `jax`, `autograd` and `symnum` (in that order)
+or to `None` if none are available.
+"""
+DEFAULT_BACKEND = next(
+    (name for name, backend in _REGISTERED_BACKENDS.items() if backend.available), None,
+)
 
 
 def _get_backend(name: str):