Fix cffi_utils for abscence of petsc4py (#3415)

* Fix cffi_utils for non abscence of petsc4py

* Readd KeyError catch

* Apply suggestions

* Ruff

python/dolfinx/utils.py

@@ -10,6 +10,7 @@
 import ctypes as _ctypes
 import os
 import pathlib
+import warnings
 
 import numpy as np
 
@@ -195,25 +196,29 @@ def set_vals(A: int,
                                 ffi.from_buffer(rows(data), mode)
     """
 
-    # Register numba complex types with cffi
+    import cffi as _cffi
+
+    _ffi = _cffi.FFI()
+
     try:
-        import cffi as _cffi
         import numba as _numba
         import numba.core.typing.cffi_utils as _cffi_support
 
-        _ffi = _cffi.FFI()
+        # Register complex types
         _cffi_support.register_type(_ffi.typeof("float _Complex"), _numba.types.complex64)
         _cffi_support.register_type(_ffi.typeof("double _Complex"), _numba.types.complex128)
-    except (ImportError, KeyError):
+
+    except KeyError:
         pass
+    except ImportError:
+        warnings.warn(
+            "Could not import numba, so cffi/numba complex types were not registered.",
+            ImportWarning,
+        )
 
-    # Define CFFI ABI functions for PETSc calls.
     try:
         from petsc4py import PETSc as _PETSc
 
-        import cffi as _cffi
-
-        _ffi = _cffi.FFI()
         _lib_cffi = _ffi.dlopen(str(get_petsc_lib()))
 
         _CTYPES = {
@@ -248,5 +253,10 @@ def set_vals(A: int,
         """See PETSc `MatSetValuesBlockedLocal
         <https://petsc.org/release/manualpages/Mat/MatSetValuesBlockedLocal>`_
         documentation."""
-    except (ImportError, KeyError):
+    except KeyError:
         pass
+    except ImportError:
+        warnings.warn(
+            "Could not import petsc4py, so cffi/PETSc ABI mode interface was not created.",
+            ImportWarning,
+        )