diff --git a/.gitignore b/.gitignore
index 8875dc4..0fb5791 100644
--- a/.gitignore
+++ b/.gitignore
@@ -11,4 +11,4 @@ __pycache__/
 *.bak
 *.ahv
 *.hv
-*.v
\ No newline at end of file
+*.v
diff --git a/main_ISTA.py b/main_ISTA.py
index eeda8a1..17a577e 100644
--- a/main_ISTA.py
+++ b/main_ISTA.py
@@ -7,7 +7,7 @@
 """
 from cil.optimisation.algorithms import ISTA, Algorithm
 from cil.optimisation.functions import IndicatorBox, SGFunction
-from cil.optimisation.utilities import ConstantStepSize, Sampler, callbacks, Preconditioner
+from cil.optimisation.utilities import ConstantStepSize, Preconditioner, Sampler, callbacks
 from petric import Dataset
 from sirf.contrib.partitioner import partitioner
 
@@ -27,20 +27,24 @@ def __call__(self, algorithm: Algorithm):
         if algorithm.iteration >= self.max_iteration:
             raise StopIteration
 
+
 class MyPreconditioner(Preconditioner):
-    # Use a preconditioner based on the row-sum of the Hessian of the log-likelihood as an example
-    # See:
-    # Tsai, Y.-J., Bousse, A., Ehrhardt, M.J., Stearns, C.W., Ahn, S., Hutton, B., Arridge, S., Thielemans, K., 2017. 
-    # Fast Quasi-Newton Algorithms for Penalized Reconstruction in Emission Tomography and Further Improvements via Preconditioning. 
-    # IEEE Transactions on Medical Imaging 1. https://doi.org/10.1109/tmi.2017.2786865
+    """
+    Example based on the row-sum of the Hessian of the log-likelihood. See: Tsai et al. Fast Quasi-Newton Algorithms
+    for Penalized Reconstruction in Emission Tomography and Further Improvements via Preconditioning,
+    IEEE TMI https://doi.org/10.1109/tmi.2017.2786865
+    """
     def __init__(self, kappa):
-        # add an epsilon to avoid division by zero. This eps value probably should be made dependent on kappa though.
-        self.kappasq = kappa * kappa + 1e-6
-        
+        # add an epsilon to avoid division by zero (probably should make epsilon dependent on kappa)
+        self.kappasq = kappa*kappa + 1e-6
+
     def apply(self, algorithm, gradient, out=None):
         return gradient.divide(self.kappasq, out=out)
-        
+
+
 class Submission(ISTA):
+    """Stochastic subset version of preconditioned ISTA"""
+
     # note that `issubclass(ISTA, Algorithm) == True`
     def __init__(self, data: Dataset, num_subsets: int = 7, step_size: float = 1e-6,
                  update_objective_interval: int = 10):
@@ -56,14 +60,12 @@ def __init__(self, data: Dataset, num_subsets: int = 7, step_size: float = 1e-6,
             f.set_prior(data.prior)
 
         sampler = Sampler.random_without_replacement(len(obj_funs))
-        F = -SGFunction(obj_funs, sampler=sampler)      # negative to turn minimiser into maximiser
-        step_size_rule = ConstantStepSize(step_size)    # ISTA default step_size is 0.99*2.0/F.L
+        f = -SGFunction(obj_funs, sampler=sampler)   # negative to turn minimiser into maximiser
+        step_size_rule = ConstantStepSize(step_size) # ISTA default step_size is 0.99*2.0/F.L
         g = IndicatorBox(lower=0, accelerated=False) # non-negativity constraint
 
-        my_preconditioner = MyPreconditioner(data.kappa)
-        super().__init__(initial=data.OSEM_image,
-                         f=F, g=g, step_size=step_size_rule,
-                         preconditioner=my_preconditioner,
+        preconditioner = MyPreconditioner(data.kappa)
+        super().__init__(initial=data.OSEM_image, f=f, g=g, step_size=step_size_rule, preconditioner=preconditioner,
                          update_objective_interval=update_objective_interval)
 
 
diff --git a/petric.py b/petric.py
index 8a49f6d..be560f0 100755
--- a/petric.py
+++ b/petric.py
@@ -152,8 +152,7 @@ def get_data(srcdir=".", outdir=OUTDIR, sirf_verbosity=0):
     STIR.set_verbosity(sirf_verbosity)                # set to higher value to diagnose problems
     STIR.AcquisitionData.set_storage_scheme('memory') # needed for get_subsets()
 
-    _ = STIR.MessageRedirector(str(outdir / 'info.txt'), str(outdir / 'warnings.txt'),
-                               str(outdir / 'errors.txt'))
+    _ = STIR.MessageRedirector(str(outdir / 'info.txt'), str(outdir / 'warnings.txt'), str(outdir / 'errors.txt'))
     acquired_data = STIR.AcquisitionData(str(srcdir / 'prompts.hs'))
     additive_term = STIR.AcquisitionData(str(srcdir / 'additive_term.hs'))
     mult_factors = STIR.AcquisitionData(str(srcdir / 'mult_factors.hs'))
@@ -169,13 +168,12 @@ def get_data(srcdir=".", outdir=OUTDIR, sirf_verbosity=0):
 
 
 if SRCDIR.is_dir():
-    data_dirs_metrics = [
-        (SRCDIR / "Siemens_mMR_NEMA_IQ", OUTDIR / "mMR_NEMA",
-         [MetricsWithTimeout(outdir=OUTDIR / "mMR_NEMA", transverse_slice=72, coronal_slice=109)]),
-        (SRCDIR / "NeuroLF_Hoffman_Dataset", OUTDIR / "NeuroLF_Hoffman",
-         [MetricsWithTimeout(outdir=OUTDIR / "NeuroLF_Hoffman", transverse_slice=72)]),
-        (SRCDIR / "Siemens_Vision600_thorax", OUTDIR / "Vision600_thorax",
-         [MetricsWithTimeout(outdir=OUTDIR / "Vision600_thorax")])]
+    data_dirs_metrics = [(SRCDIR / "Siemens_mMR_NEMA_IQ", OUTDIR / "mMR_NEMA",
+                          [MetricsWithTimeout(outdir=OUTDIR / "mMR_NEMA", transverse_slice=72, coronal_slice=109)]),
+                         (SRCDIR / "NeuroLF_Hoffman_Dataset", OUTDIR / "NeuroLF_Hoffman",
+                          [MetricsWithTimeout(outdir=OUTDIR / "NeuroLF_Hoffman", transverse_slice=72)]),
+                         (SRCDIR / "Siemens_Vision600_thorax", OUTDIR / "Vision600_thorax",
+                          [MetricsWithTimeout(outdir=OUTDIR / "Vision600_thorax")])]
 else:
     log.warning("Source directory does not exist: %s", SRCDIR)
     data_dirs_metrics = [(None, None, [])]