Add abc for KSPMonitorFunction

petsctools/__init__.py

@@ -1,13 +1,13 @@
-import os
-
-from petsctools.exceptions import PetscToolsException  # noqa: F401
+from .exceptions import PetscToolsException  # noqa: F401
 
 
 class MissingPetscException(PetscToolsException):
     pass
 
 
 def get_config():
+    import os
+
     try:
         import petsc4py
         return petsc4py.get_config()
@@ -34,6 +34,8 @@ def get_petsc_arch():
 
 def get_petscvariables():
     """Return PETSc's configuration information."""
+    import os
+
     path = os.path.join(get_petsc_dir(), get_petsc_arch(), "lib/petsc/conf/petscvariables")
     with open(path) as f:
         pairs = [line.split("=", maxsplit=1) for line in f.readlines()]
@@ -43,13 +45,16 @@ def get_petscvariables():
 try:
     import petsc4py  # noqa: F401
     petsc4py_found = True
+    del petsc4py
 except ImportError:
     petsc4py_found = False
 
-
 if petsc4py_found:
-    from petsctools.init import (  # noqa: F401
+    from .init import (  # noqa: F401
         InvalidEnvironmentException,
         InvalidPetscVersionException,
         init,
     )
+    from .monitor import AbstractKSPMonitorFunction  # noqa: F401
+
+del petsc4py_found

petsctools/monitor.py

@@ -0,0 +1,16 @@
+import abc
+
+from petsc4py import PETSc
+
+
+class AbstractKSPMonitorFunction(abc.ABC):
+    """Abstract base class for a KSP monitor function.
+
+    In order to use it it must be attached to a KSP using the function
+    `PETSc.KSP.setMonitor`.
+
+    """
+
+    @abc.abstractmethod
+    def __call__(self, ksp: PETSc.KSP, iteration: int, residual_norm: float):
+        pass